View Full Version : Westinghouse H886K21


FrankieKat
07-31-2017, 09:27 PM
Hey everyone,

I've come into a 1955 Westinghouse H886K21 which is my first tube TV set. I've done restoration and repair of tube radios and HiFi, but no TVs. Having watched many hours of YouTubers' TV restoration videos I felt foolhardy enough take this on.

I found it with a brightener (which I removed) and roughly 2 out of 3 tubes tested weak and a few with minor shorts. Before pulling the chassis I've first tried replacing tubes to see if I could at least get a baseline. Came up right away with full horizontal, some vertical but no visible image and after replacing a few tubes and working controls free, I was able to get sound, but still no image. There's a low level of constant static in the background of the audio, but otherwise it's full and has good volume so we know front end is functioning. I've also replaced the sync amp, the sync separator, AGC keying, vertical mult, vertical output and horizontal AFC tubes, which has yielded little change.

Here's a video of where it's at:

https://youtu.be/vGhXHNI5qdE

In the video, I'm first adjusting the brightness, then the vertical hold then the horizontal hold (clearly the control needs a good cleaning). Adjusting the rear sync, height, width, linearity, AGC controls affects the height (though never goes below the top half of the screen vertically) but still no solid picture.

As I haven't yet even pulled the chassis I'm obviously not expecting perfection as it's likely on a good amount of original components. I'm more inclined to want to troubleshoot to learn as much as I can, instead of just doing a wholesale/shotgun replacement of caps, etc.

So I guess I'm looking for some advice for where I might start, and next obvious steps to try to sort out my little vertical problem, first?

Thanks!

FK

benman94
08-01-2017, 11:40 AM
I hate to be the bearer of bad news, but with a set of that vintage, a shotgunning of the original paper caps is strictly necessary. A set in that state is essentially impossible to troubleshoot as you suggest. There's no video, weak sound, insufficient vertical sweep, horrid hum (though it might be 6BN6 buzz; Westinghouse and Zenith were fond of the 6BN6 in that era). And that's just what we can see that's wrong with the set. You could be running at extremely high horizontal output cathode current thus putting your flyback at risk, etc.

Plan on replacing all the paper caps and electrolytics, but you can do what I do (as suggested by Phil Nelson): replace a handful of paper caps, say six or seven, then plug the set in and make sure you haven't made anything worse.

Start recapping in the sweep sections and power supply first to avoid nuking an unobtainable part.

Findm-Keepm
08-01-2017, 02:47 PM
...and if you need the Sams Photofact, it's Set 294, folder 11. I have an original, yours for postage. PM me if interested.

FrankieKat
08-02-2017, 03:17 PM
I hate to be the bearer of bad news, but with a set of that vintage, a shotgunning of the original paper caps is strictly necessary. A set in that state is essentially impossible to troubleshoot as you suggest. There's no video, weak sound, insufficient vertical sweep, horrid hum (though it might be 6BN6 buzz; Westinghouse and Zenith were fond of the 6BN6 in that era). And that's just what we can see that's wrong with the set. You could be running at extremely high horizontal output cathode current thus putting your flyback at risk, etc.

Plan on replacing all the paper caps and electrolytics, but you can do what I do (as suggested by Phil Nelson): replace a handful of paper caps, say six or seven, then plug the set in and make sure you haven't made anything worse.

Start recapping in the sweep sections and power supply first to avoid nuking an unobtainable part.

Well, shucks. Was hoping it'd be like one of those videos where replacing a few tubes and it plays "fairly well". Beginner's luck I suppose!

It's possible the sound issues you mentioned could be caused by the cheap DTV box I was using for that, so if that persists after doing recap I'll try other sources and see.

I've ordered a full set of caps and agree with your technique of recapping in small batches to make it easier to spot and roll back mistakes. The good thing is that this cabinet has a lot of space to work and seems very simple to get the chassis in and out so hopefully this will be a good learning TV!

Thanks for the advice!

FK

FrankieKat
08-18-2017, 02:18 PM
Update -- finished replacing paper and electrolytic caps and with a lot of fiddling of the controls, I finally have a picture of sorts.

https://www.youtube.com/watch?v=-nMNRP56N1Y

The vertical hold is very touchy and this is the best I can get for brightness and contrast. As this CRT had a brightener on it when I found it very possible this is the end of the line for the picture tube. Think there are any more gains to be made by working on it, or best to shelf it and start the search for a 21ALP4A? :(

Thanks all!

FK

Electronic M
08-18-2017, 02:56 PM
If it had a brightener on it, it is that dim, and the CRT bias voltages and HV are in spec, then may as well put the brightener back on and see how decent of a pic it can make and for how long.

B&W CRTs prior to ~1960 were often not nearly as bright as newer CRTs we are used to seeing....That said it does look dim for a tube of it's era.

I may have a good spare 21ALP4. IIRC I sold one this spring, and I think I had 2 before then.

Since the vertical is rolling and the horizontal is stable I'd suggest checking the vert and sync separator tubes, checking all resistors in those stages and replacing any that are out of tolerance.

old_coot88
08-18-2017, 03:13 PM
Get the set to working solidly first, then you'll know for sure that it's worth hunting a new CRT.

FrankieKat
08-18-2017, 07:50 PM
If it had a brightener on it, it is that dim, and the CRT bias voltages and HV are in spec, then may as well put the brightener back on and see how decent of a pic it can make and for how long.

B&W CRTs prior to ~1960 were often not nearly as bright as newer CRTs we are used to seeing....That said it does look dim for a tube of it's era.

I may have a good spare 21ALP4. IIRC I sold one this spring, and I think I had 2 before then.

Since the vertical is rolling and the horizontal is stable I'd suggest checking the vert and sync separator tubes, checking all resistors in those stages and replacing any that are out of tolerance.

So with the contrast at about 10%, the CRT voltages are on spec, though there's no picture at all until contrast is past about 90%. The B+ supply voltages are right on too. I don't have a HV test probe though so can't measure the HV unfortunately. I've been checking resistors and only have had to replace two or three, and the worst was maybe 25% off so clearly they used good parts.

Have tried a few different vert, sync and AGC tubes and have the best ones in there now, though there wasn't a huge difference. Both horiz and vertical lock are fairly poor and only get solid horiz lock with the control is the extreme end. It also seems to get worse as the set heats up.

Nothing I've done has improved or changed the brightness. Put the brightener back on for a little bit but improvement was minimal if any. That video was recorded in a pitch black room and it actually seems a bit brighter on the video than in person.

I'm going to keep grinding and checking voltages and components and see what happens.

Thanks for the tips!

FK

DavGoodlin
08-18-2017, 08:41 PM
That CRT will not be hard to find. The 21ALP4, 21CBP4, 21AUP4, 21BSP4 and others were pretty much used in all the mid-50 sets. I know for sure that Motorola, Silvertone and Philco used them in almost all their 21" models 1956-58.

Can you try a good CRT checker to be sure you can't give it a good bump?

Eric H
08-18-2017, 10:09 PM
Have you tried adjusting the Ion Trap? A slight misadjustment will kill the brightness.

benman94
08-19-2017, 06:00 AM
It could also be low HV... I've seen sets, especially B/W, where some incompetent repairman put a brightener on the CRT to try to fix a dim image caused by droopy HV.

dieseljeep
08-19-2017, 10:36 AM
That was one of the last good sets Westinghouse built! All the following models were cheap corner-cutting, hot chassis, PC board jobs. Some of the lower-end 21" models, didn't even have a voltage doubler, a portable chassis in a 21" console. They were minimal performers.
On Decades TV, they were running some old Westinghouse Studio One. On one Westinghouse advert, Betty Furness was extolling the virtues of the Gated Beam Audio Tube, that always insured perfect sound.

old_coot88
08-19-2017, 11:09 AM
Betty Furness was extolling the virtues of the Gated Beam Audio Tube, that always insured perfect sound.
Hoo Ha!:no: It was an elegant idea that lowered parts count and worked great until a bit of component drift set in.

benman94
08-19-2017, 11:59 AM
Hoo Ha!:no: It was an elegant idea that lowered parts count and worked great until a bit of component drift set in.

At least some Westinghouse sets had the "quieting" or "buzz" control on the front of the set. If they were going to use the 6BN6, at the very least they should have made the quieting control a standard for the front apron. It needed frequent enough adjusting to justify it, that's for sure...

old_coot88
08-19-2017, 12:23 PM
At least some Westinghouse sets had the "quieting" or "buzz" control on the front of the set.
Ha. Didn't know that. L'arn something new every day.

FrankieKat
08-19-2017, 02:09 PM
Have you tried adjusting the Ion Trap? A slight misadjustment will kill the brightness.

Well, I did adjust it a bit and was able to get a little bit more brightness out of it -- didn't think to try that, thanks!

Vertical and horizontal lock are still very poor, and definitely degrades as the set heats up. Have been swapping out tubes, but none seem to make a significant difference. Even changed HZ output and damper just for grins and no change at all.

Brightness is getting better, but hold is bad. It loses vertical when changing channels on the DTV box and even when a show cuts between scenes it can lose vertical.

https://youtu.be/Pgm-8XGRZq0

I unfortunately don't have a HV or CRT tester or pattern generator. I'll check eBay and see if there's something available that won't break the bank.

I've been checking voltages and the third B+ (should be 130V) measures around 117 though the higher two (240 and 210) are dead on. I've changed the power resistors and all of the filter caps are brand new, so something is dragging it down. Also noticed that the AGC control on the back has absolutely no effect at all on the picture. I guess I'll start focusing there and see if maybe there's a bad resistor hiding out or something, since at this point all of the paper caps have been changed.

Big progress since day 1 though! Thanks so much for all of the suggestions!

FK

Eric H
08-19-2017, 03:37 PM
If both vert and horiz stability are bad, and the caps have been replaced, I'd look for a drifted or open resistor in the sync separator circuit.

FrankieKat
08-20-2017, 05:00 PM
If both vert and horiz stability are bad, and the caps have been replaced, I'd look for a drifted or open resistor in the sync separator circuit.

I think I figured this one out actually. Turns out the tuning and fine tuning on the same shaft were frozen solid together and didn't even realize there were two. Was finally able to work it free and with the fine tuning dialed in the lock is solid both ways. Now of course, sound is best when tuned away from best picture, but that seems like more of an alignment issue -- problem for another day!

I was noticing the certain voltages were still low, such as the CRT screens and the boost voltage was about 520V where it should be 580. Checking the schematic again I realized there were two caps that I didn't mark as replaced which turned out were in the HV cage (duh). I carefully took it all apart, replaced the caps and carefully put it back together and.... no HV. Took it all the way apart again, triple checked the wiring and joints, carefully put it all back together and... still no HV. Verified DC resistance all the way across the flyback, resistance between the grid caps of horiz output and HV rect were both spot on schematic. Ultimately, it appears that in the jostling around, the filament of the HV rect decided to go open - no continuity between pins 2 and 7. Tried touching up the pins, removed the base and tested directly at the leads, but still open. Unfortunately this is the only tube I do not have a spare of, so at a small pause.

Before all of this I did experiment with the brightener and it did make a difference in terms of overall brightness however the contrast was still very washed out. As in, adjusting brightness/contrast it went from dim gray on gray to saturated white on white. Possible if the caps I did replace could have been causing this? (attaching snippet of schematic)

Thx all!

FK

FrankieKat
08-20-2017, 11:58 PM
Well, I've learned that apparently you can't test a 1B3 tube's filaments with an ohmmeter, and apparently my tube tester also cannot test a 1B3 tube so entirely possible that my diagnosis could have been inaccurate.

I went back and did a few more checks and found a few more out of tolerance resistors and one that was open and decided to give it another try. This time, HV did come up briefly but it started arcing after a few seconds. I took the HV cage apart again, even coated the leads of the new caps with corona dope, quadruple checked everything put it back and same thing.

https://youtu.be/mxDbkEfzzWA

The camera doesn't pick it up very well but the buzzing is accompanied by blue-ish flashes in the HV tube. From what I've read, blue means the tube is gassy, but everything was working fine before.

Ideas?

Thanks!

FK

Eric H
08-21-2017, 01:10 AM
Pull the cap off the top of the 1B3 and see if you can draw a nice blue arc to the tip of an ungrounded (plastic handle) screwdriver, if you get a good arc and the buzz is gone then the problem is probably after the tube, perhaps under the base of the 1B3. It could be a gassy 1B3 but those usually just glow purple without any noise.
It's possible you cracked it somehow and completely let the vacuum out, in that instance it might arc and buzz internally.

jr_tech
08-21-2017, 03:02 AM
"Well, I've learned that apparently you can't test a 1B3 tube's filaments with an ohmmeter"

Huh? I just went out to the shop and measures about a dozen of them ... all were right around 2 ohms, pin 2 to 7.

jr

FrankieKat
08-21-2017, 10:52 AM
"Well, I've learned that apparently you can't test a 1B3 tube's filaments with an ohmmeter"

Huh? I just went out to the shop and measures about a dozen of them ... all were right around 2 ohms, pin 2 to 7.


I read somewhere else that it requires the 15KHz AC from the flyback to actually light the thing... so that's not the case? Well, that lends itself well to the problem being the actual tube then. It must have just been barely holding on and taking it out and putting it back in just did it in. Have purchased two more so should have them by end of week. Edit: found a guy not too far from here selling a lot of tubes for possible use as an "art installation" that includes two 1B3GT's... hope they're good! Gonna go pick them up tonight.

FK

FrankieKat
08-21-2017, 04:33 PM
Pull the cap off the top of the 1B3 and see if you can draw a nice blue arc to the tip of an ungrounded (plastic handle) screwdriver, if you get a good arc and the buzz is gone then the problem is probably after the tube, perhaps under the base of the 1B3. It could be a gassy 1B3 but those usually just glow purple without any noise.
It's possible you cracked it somehow and completely let the vacuum out, in that instance it might arc and buzz internally.

Pulled the cap, buzz is gone. No arc from the screwdriver... tiny little blue spark if you basically touch the cap but that's it.

https://youtu.be/Ms4351ADleA

Bill Cahill
08-21-2017, 10:27 PM
What number, or, size is the picture tube? That arc sounds low, but, you never know.
Does your set have a width control, Horizontal Phase, Horizontal oscillator, or, width?
My guess is horizontal oscillator troubles. If not, something in horizontal output, of, Damper. Does your set use a hv capacitor on the hv rectifier?
Bill Cahill

FrankieKat
08-22-2017, 10:38 AM
Pulled the cap, buzz is gone. No arc from the screwdriver... tiny little blue spark if you basically touch the cap but that's it.


Good news -- back in business, it was the 1B3 tube after all.

https://youtu.be/fsLuy2y1dy4

Brightness is still a problem, the above video is taken in a pitch black room with brightness and contrast up almost all the way before the image completely saturates. The brightener increases the overall amount of light produced, but the poor contrast is unchanged. This video is without the brightener. Would also say it looks brighter in the video than in person.

The picture is a bit noisy too, but that could be more related to the $20 DTV box that I'm using. Depending on how this all turns out, I might try to pick up a BT Agile for a better signal source.

The horiz and vertical lock are solid as long as the station is tuned in properly and both controls are pretty much at their extreme clockwise position. I've subbed out every tube at this point and tested every resistor, and all caps are replaced. The horizontal control is a little bit fritzy so I'm going to try to replace it and see what that does.

I purchased an VA62 pattern generator, a CR70 CRT tester and a 1000-1 HV test probe so should soon be able to determine CRT state.

FK

FrankieKat
08-22-2017, 12:00 PM
What number, or, size is the picture tube? That arc sounds low, but, you never know.
Does your set have a width control, Horizontal Phase, Horizontal oscillator, or, width?
My guess is horizontal oscillator troubles. If not, something in horizontal output, of, Damper. Does your set use a hv capacitor on the hv rectifier?
Bill Cahill

It's a 21ALP4 and has a 1956 date code sticker and Westinghouse branded with no evidence of repair so looks like the original. It doesn't use a HV cap -- attaching the HV portion of the schematic.

Also, does anyone know what is symbolized on the schematic by the x's and the dashed lines? I had thought that meant a disconnectable connector or the result a switch of some kind, but don't see either of these present.

The width control does have an effect on sync, and I have not yet tried adjusting the horizontal oscillator nor the horiz mult trim. The AGC control has absolutely no effect on the picture and I've checked everything I can find between the tuner and the 1st IF and all seems to check out okay. Perhaps it just doesn't have an obvious effect due to substandard signal from the $20 DTV box?

FK

Tom9589
08-22-2017, 12:08 PM
The x's and dashed lines are SAMs notation for production changes. The solid line circuit is what was the original or predominant circuit. The dashed lines represents a circuit change. So you either have the circuit of the solid line or the dashed line (with the solid line circuit broken at the x's).

No switches or connectors are represented in this manner.

FrankieKat
08-22-2017, 01:19 PM
The x's and dashed lines are SAMs notation for production changes. The solid line circuit is what was the original or predominant circuit. The dashed lines represents a circuit change. So you either have the circuit of the solid line or the dashed line (with the solid line circuit broken at the x's).

No switches or connectors are represented in this manner.

Got it - thanks. That had occurred to me, but none of those are listed in the Production Changes table they have. So for example, in later revisions, the second 330K resistor (R39 the blurry illegible one) was removed and both G2 and the focus control were fed off the same 330K resistor... those tightwads.

Based on that, I appear to have the earlier revision. Good to know!

FK

DavGoodlin
08-22-2017, 01:24 PM
The AGC control should allow you to go from blank raster (no signal) to overloaded (negative picture with poor sync and buzz in sound) at full clockwise.

It may be an open control but what are the chances the set is producing a marginal picture and not one of those extremes?

That CR70 CRT tester should permit you to see the "cutoff" performance of the CRT, that is how well and linearly the control grid voltage G1 modulates the brightness, and by default, contrast.

Those coils between the 12BY7 and the CRT should be checked to see if one is open.

FrankieKat
08-22-2017, 03:48 PM
The AGC control should allow you to go from blank raster (no signal) to overloaded (negative picture with poor sync and buzz in sound) at full clockwise.

It may be an open control but what are the chances the set is producing a marginal picture and not one of those extremes?

That CR70 CRT tester should permit you to see the "cutoff" performance of the CRT, that is how well and linearly the control grid voltage G1 modulates the brightness, and by default, contrast.

Those coils between the 12BY7 and the CRT should be checked to see if one is open.

The voltages there vary based on the contrast (aka picture) control, and with it in the right position I can get the 210V on the plate of the 12BY7 and 160V on the CRT cathode, so the coils aren't open. I'll double check that the total DC resistance matches schematic.

I'm going to go over all of the AGC circuit again with a fine-tooth comb. I agree it doesn't sound right so it seems like it's the next thing to tackle.

FK

irext
08-22-2017, 06:36 PM
From the youtube video it looks like a low emission CRT to me. It has that silvery look on highlights. If the CRT tester you are getting has a rejuvenate function it might be worth a go. I've found that older B&W tubes rejuvenate quite well and usually last for a good amount of time. Colour tubes not so good.

Cheers.

FrankieKat
08-22-2017, 11:24 PM
From the youtube video it looks like a low emission CRT to me. It has that silvery look on highlights. If the CRT tester you are getting has a rejuvenate function it might be worth a go. I've found that older B&W tubes rejuvenate quite well and usually last for a good amount of time. Colour tubes not so good.


I went through and checked all of the resistors in the AGC and video output circuits and found the 390K was measuring about 430K and the 15M was measuring closer to 30M. I replaced the 390K, and put back the 15M (since I don't have anything that high on hand), and in so doing it seems to have developed a new problem. There's now this pulsing effect in both the video and audio that isn't affected by tuning or any of the controls.

Still absolutely no effect from the AGC control on the picture. The schematic says the control is 750K, but it tests closer to 475K. However the wiper travel is smooth and resistance is correct all the way around and I'm not finding anything out of the ordinary at all.

Now for the good news. I tested the CRT on the CR70 and sure enough, no cutoff and tested at about 2 in the bad range. Irext, I took your advice and did a rejuvenation, and just like that I have cutoff and emissions in the 10-11 in the green. And now the picture is indeed brighter, though the life test drops like a stone. Is this effect typically permanent or have I just given it an adrenaline shot that will wear off?

This was how it tested after the rejuv, last night. I left it connected to the CR70 and tried it again this morning and while cold it started at about 2 in the bad and slowly creeped up to about 9-10 in the good range over about 3-4 minutes. Still had cutoff control, but not as good as in the video. Is there ever any benefit to hitting it again with rejuv/auto restore or is that just asking for trouble?

https://youtu.be/tGNYEsj6CGw

Not sure what's going on now with this pulsing/noise. Here's the new, brighter picture with that annoying new problem.

https://youtu.be/GSc_GAQxrsI

FK

Electronic M
08-23-2017, 09:44 AM
That noise could be HV arcing. One of my sets did a milder version of that for a few weeks before the HV rect. died.

FrankieKat
08-23-2017, 11:26 AM
That noise could be HV arcing. One of my sets did a milder version of that for a few weeks before the HV rect. died.

Was afraid of that. It was working fine two days ago but this just started last night before I did anything to the tube. I looked in the HV box and didn't see any visible arcing, and have tried two different 1B3's and no change. I'll try different horiz output tubes and see if that makes any difference.

I did notice that rotating the AGC control does have an audible effect on the normal HV sync pulsing/buzzing sound (whatever you call it), mostly from a volume perspective. So it's doing something, just absolutely no change on the picture. If the 15M resistor between B+ and the AGC control was bad or flakey (it tested at about 35M on my VTVM, and was beyond my DVM's measurement) could that cause the AGC issue and perhaps this new noise issue? The vertical seems to be affected most by this and occasionally the screen will go bright and saturated for a second. Just a thought...

FWIW tried injecting IF signal from the VA62 and still had same "arcing" and AGC problem. So at least we can rule out the tuner contributing to any of the problems... not like that was really a culprit, but oh well.

Thanks!

FK

jr_tech
08-23-2017, 12:50 PM
That 15 meg resistor might be the cause of your problems, causing the noise and poor agc control.
15 meg sounds high, are you sure that is the correct part?
Have you cleaned the pins of the tubes in the IF amp?

jr

FrankieKat
08-23-2017, 01:25 PM
That 15 meg resistor might be the cause of your problems, causing the noise and poor agc control.
15 meg sounds high, are you sure that is the correct part?
Have you cleaned the pins of the tubes in the IF amp?


Yeah, 15 meg: brown, green, blue - see attached pic (yep that solder joint is factory) and schematic. How critical do you think that value is? If I put say two 2.2's in series would that be sufficient? It's not like there's going to be any useful amount of current through a 15M, it's just to complete the circuit but not actually really do anything right?

I've swapped around the IF tubes several times and wiggled them and doesn't affect the noise. I wiggled all of the tubes too and even tapping on the chassis does nothing so really doesn't seem like something loose. Going to hit an electronics store on the way home and if I have to stick six 2.2M's together I will, lol!

FK

jr_tech
08-23-2017, 01:58 PM
Since the gold band indicates a 5 % tolerance, I would think that it is fairly critical. It is part of the voltage divider network that sets the AGC voltage. Likely 3-4.7 or 5.1meg resistors in series would be close enough (since the pot gives you some adjustment range).

jr

Electronic M
08-23-2017, 02:10 PM
Too little of the schematic is shown to fully evaluate it's purpose, but it's value is likely critical. See that gold band on the end?...That signifies a 5% tolerance resistor. Most resistors in most TVs were 10%, or 20% tolerance. %5 resistors usually cost more and typically were not specified by engineering unless needed. You can make a series set of resistors if you need, but you probably want to check that it measures within 5% of the design value. If it is off a bit more it should not kill the set, but AGC may not set or react right...If you have signal and no more flicker that would confirm the resistor.


Speaking of resistors I've seen ones in series with the HV lead develop internal arcing...They may or may not test fine with 9V from your multi-meter, but at 10-30KV will arc and make RF noise that the set can tune and feed back to the picture and sound.

AGC stages especially non-keyed ones often had long time constant RC circuits with large value paper caps and or resistors.

FrankieKat
08-23-2017, 02:23 PM
Too little of the schematic is shown to fully evaluate it's purpose, but it's value is likely critical. See that gold band on the end?...That signifies a 5% tolerance resistor. Most resistors in most TVs were 10%, or 20% tolerance. %5 resistors usually cost more and typically were not specified by engineering unless needed. You can make a series set of resistors if you need, but you probably want to check that it measures within 5% of the design value. If it is off a bit more it should not kill the set, but AGC may not set or react right...If you have signal and no more flicker that would confirm the resistor.

Speaking of resistors I've seen ones in series with the HV lead develop internal arcing...They may or may not test fine with 9V from your multi-meter, but at 10-30KV will arc and make RF noise that the set can tune and feed back to the picture and sound.

AGC stages especially non-keyed ones often had long time constant RC circuits with large value paper caps and or resistors.

Got it. The parts list doesn't call out a tolerance on that resistor specifically, but of course makes sense to replace with as close to correct value as possible. I'll see what I can come up with.

Attaching a larger version of the relevant schematic (really hard to get these legible and stay under the upload file size limit!)

FK

FrankieKat
08-23-2017, 03:44 PM
Since the gold band indicates a 5 % tolerance, I would think that it is fairly critical. It is part of the voltage divider network that sets the AGC voltage. Likely 3-4.7 or 5.1meg resistors in series would be close enough (since the pot gives you some adjustment range).


Good ol' Radio Shack!

http://www.videokarma.org/attachment.php?attachmentid=195393&stc=1&d=1503517290

Just 6 of these together and I've got myself one 15 meg 1/2 watt resistor... :rolleyes:

Tom9589
08-23-2017, 04:35 PM
Why don't you try three 10 Meg resistors? One 10 Meg (10 Meg) in series with two 10 Meg resistors in parallel (5 Meg). That gives you your needed 15 Meg total resistance.

I take it you still have a Radio Shack store in operation? They pulled out of Atlanta awhile ago.

FrankieKat
08-23-2017, 04:48 PM
Why don't you try three 10 Meg resistors? One 10 Meg (10 Meg) in series with two 10 Meg resistors in parallel (5 Meg). That gives you your needed 15 Meg total resistance.

I take it you still have a Radio Shack store in operation? They pulled out of Atlanta awhile ago.

These nice RS 10 meg resistors are actually 1/4 watt and the schematic calls for 1/2 watt so I'd need to double up two strings of three to increase the wattage if I wanted to do it "right". Though for a test, I'm sure what you suggest would work just fine until I put in my next mouser order and get the right one.

FK

Tom9589
08-23-2017, 05:29 PM
Manufacturers didn't use 1/4 watt resistors back then. 1/2 watt was the smallest. Using the formula P=Vsquared/R. P=250*250/15,000,000= .00384 watts. 1/4 watt resistors should do just fine.

FrankieKat
08-24-2017, 01:29 PM
Well, I replaced the 15M resistor (R20) and made no difference -- the picture still has that noise and AGC control has no effect. I've checked and replaced all resistors and capacitors between the 1st IF grid, through the AGC control and to the plate of the AGC keying tube. I bypassed the AGC control with a low fixed value resistor and nothing different.

The DC voltages on the AGC tube and IF tubes are close enough to spec. Also verified all of the scope traces drawn on the Sams and frequencies are all correct, though P-P voltages vary a bit from the schematic but are within the right ballpark.

I don't see any visible HV arcing, and while there's an audible "pulse" that occurs with the noise, it sounds softer, more like a hold/sync issue than a sharper HV noise.

I swapped out every tube in the set last night including the HV rectifier and still no change. I can try to order up some more tubes to try (though I have at this point at least three of every one) if it's still possible it might be tube-related.

I injected an IF video signal from the VA62 after the third IF tube (with it removed) and still has noise and no AGC control.

I also saw no visible AGC control on the first power up of the set too, so I'm pretty confident that it's not related to something I did.

The only thing I've noticed is that the noise seems to go away when it's tuned a bit away from the station or the signal level from the VA62 is low.

Where could I be looking next?

Thanks!

FK

(schematic attached)

Electronic M
08-24-2017, 02:08 PM
Is the 100K resistor in series with the CRT HV lead still original? If so try replacing it. I mentioned earlier that resistors in the HV can develop internal arcing, the arcing acts like a spark gap transmitter and feeds back through the RF chain as video and audio interference.

Was the 3RD IF tube installed when you injected after it? If so large RF noise spikes could still be getting through the IF and mix with the injected signal resulting in the noise remaining present. If you pulled the 3RD IF for the signal injection then it suggests a noisy component, B+ line, or connection in the stages between the injection point and the element of the CRT being driven with video.

Do you have a working o-scope? If so it may be good to ground out the signal going to the detector (a large cap ~.1-10uF from it to ground should do the trick) and if the noise persists search for it in the stages between there and the CRT. Once you find it trace it in in the signal chain to the strongest point, then look for it on any power supply lines in the area to see if it is stronger in those lines.

FrankieKat
08-24-2017, 04:24 PM
Is the 100K resistor in series with the CRT HV lead still original? If so try replacing it. I mentioned earlier that resistors in the HV can develop internal arcing, the arcing acts like a spark gap transmitter and feeds back through the RF chain as video and audio interference.

No, that one is original, though I tested it and value was good. I'll replace it next time I pull out the HV box (it's not a lot of fun to do on this).

Was the 3RD IF tube installed when you injected after it? If so large RF noise spikes could still be getting through the IF and mix with the injected signal resulting in the noise remaining present. If you pulled the 3RD IF for the signal injection then it suggests a noisy component, B+ line, or connection in the stages between the injection point and the element of the CRT being driven with video.

It was removed, and I also removed the 1st IF and injected signal right after it. I will repeat the experiment again tonight and make detailed notes about what happens.

Do you have a working o-scope? If so it may be good to ground out the signal going to the detector (a large cap ~.1-10uF from it to ground should do the trick) and if the noise persists search for it in the stages between there and the CRT. Once you find it trace it in in the signal chain to the strongest point, then look for it on any power supply lines in the area to see if it is stronger in those lines.

I do, and I'll give that a try. It's tricky because the noise either goes away or becomes imperceptible with low or no signal. With the antenna leads disconnected or IF tube pulled the screen is solid white/blank.

Do you think this is related to the AGC or do I have two unrelated problems?

Thanks for the suggestions -- will get to work on those next and let you know.

FK

Electronic M
08-24-2017, 04:54 PM
If the noise goes away completely with the IF tubes pulled then the problem is either up stream of the 1st IF tube you can pull to kill the noise or it is in the AGC.

If the noise goes away with the 1st IF tube pulled, then odds are it is in the tuner, or it is local interference. If you have a signal source that operates on other channels than the one your currently using it may be wise to try different channels. There is a large gap (FM band + other stuff) between channels 2-6 and channels 7-13. So switching from low band VHF to high band or vice-versa may be a wise test (to get away from wide band noise). Also if a local DTV channel is using the same RF carrier freq as your signal source, you will likely get noise from that DTV carrier leaking into your set.

One way you can eliminate AGC noise is to connect the AGC buss to an external adjustable DC supply 0-25V and use that supply to clamp the AGC line to a proper fixed voltage.

FrankieKat
08-25-2017, 11:56 AM
If the noise goes away completely with the IF tubes pulled then the problem is either up stream of the 1st IF tube you can pull to kill the noise or it is in the AGC.

With all three IF tubes removed and the IF signal injected afterwards, same issue.

If the noise goes away with the 1st IF tube pulled, then odds are it is in the tuner, or it is local interference. If you have a signal source that operates on other channels than the one your currently using it may be wise to try different channels. There is a large gap (FM band + other stuff) between channels 2-6 and channels 7-13. So switching from low band VHF to high band or vice-versa may be a wise test (to get away from wide band noise). Also if a local DTV channel is using the same RF carrier freq as your signal source, you will likely get noise from that DTV carrier leaking into your set.


I used the VA62 to try on channels 7+ and still there. The DTV box is only 3 and 4, but same on both.

Here's what I'm seeing now with VA62 feeding channel 3 RF: https://youtu.be/q8nD70vokkE

It's odd that the audio takes hits too because it's picked off immediately after the detector which is basically right after the IF injection point that I used. Would this seem to suggest the issue is in fact HV-related since that would be about the only thing remaining that could cause this effect on both audio and video?

One way you can eliminate AGC noise is to connect the AGC buss to an external adjustable DC supply 0-25V and use that supply to clamp the AGC line to a proper fixed voltage.

I used the VA62's DC output to vary the AGC voltage and that actually did what the AGC control is supposed to do. Also, with at least -5V applied, the AGC control does actually have an effect on the picture( for the first time) since the injection point is at the wiper of the AGC control. Without it, at that point, I measure a small negative voltage only in mV that varies maybe 8mV with the rotation of the AGC control. With just the VA62 connected with the voltage set all the way down to nearly nothing, the picture does become slightly more saturated - like the AGC control being turned slightly up.

There are otherwise no voltage readings called out on the schematic for that portion of the circuit so I'm confused as to where this negative voltage would be normally developed? I should expect to see that same between -9 (or so) and 0V at that point if everything was working correctly yes?

Thanks!

FK

jr_tech
08-25-2017, 12:45 PM
Are the power supplies to the video output and audio output stable and quiet?
Is the external dag coating on the crt in good shape and grounded well?

jr

old_coot88
08-25-2017, 01:59 PM
...There are otherwise no voltage readings called out on the schematic for that portion of the circuit so I'm confused as to where this negative voltage would be normally developed? I should expect to see that same between -9 (or so) and 0V at that point if everything was working correctly yes?

Thanks!

FK

Excellent discussion here on how keyed (or gated) AGC works. Especially the 4th post down..

http://antiqueradios.com/forums/viewtopic.php?f=3&t=201711

Electronic M
08-25-2017, 02:06 PM
I second what JR said.^

Given that with the 3RD IF pulled and the signal injection after that, that the interference remains. The problem is most likely noise in the B+ or HV arcing. There could be issues in the stages between the video detector and the CRT, but it is not as likely.

Since you are getting sound interference too, here is an idea to save you some time....Pull the H output tube or damper (to stop supply of HV) and see if the sound interference goes away after it has had a couple of minutes to run without HV (you obviously can't check picture with no HV).
If disabling the HV stops the interference then the issue should be in the HV or CRT grounding. If the noise remains without HV then check the B+ lines with a scope, and if those are clean use the scope to look at the stages between the injection point you were using and the CRT video feed.

You may have AGC issues, but if as you say the IF is passing signals properly (with the exception of the mystery noise), then I'd ignore the AGC until the noise issue is fixed. The AGC appears to only affect the tuner, 1ST and 2ND IF...So given you can inject after the 3RD IF with the 3RD IF tube pulled and still get noise, I think it is safe to say the AGC is not your noise source, and can be ignored for now.

FrankieKat
08-25-2017, 02:08 PM
Are the power supplies to the video output and audio output stable and quiet?

Yeah, all new electrolytics - Nichicon 105 deg, 500V caps replacing the original 450 and 350 volt can sections. Replaced the original candohm with two 10W power resistors as well and the B+ lines are all within 3 volts of schematic. My DVM locks right on and they are stable, but I'll scope them all and look for any ripple or odd noise too.


Is the external dag coating on the crt in good shape and grounded well?


Interesting question! The grounding spring is tightly in place and overall the coating looks in good shape, however it is a little scratched where it makes contact with the tube - see attached pic. Would that be enough to cause a problem? Recommend trying to bend the spring so that it contacts in a slightly different place?

FK

Electronic M
08-25-2017, 02:22 PM
Interesting question! The grounding spring is tightly in place and overall the coating looks in good shape, however it is a little scratched where it makes contact with the tube - see attached pic. Would that be enough to cause a problem? Recommend trying to bend the spring so that it contacts in a slightly different place?

FK

I'd try changing the contact point. I f the dag is missing (even just a whisker) under the point of contact or making poor electrical contact with the spring then you will get HV arcing (and spark gap transmitter action) at that contact.

old_coot88
08-25-2017, 02:47 PM
One way to test for questionable dag grounding is tape on a grounding wire, and see if the noise disappears. Strip several inches of insulation off a piece of hookup wire and tape it to the dag, and the other end to ground.

That's assuming there's no isolated patches or 'islands' of dag lurking around, which happens occasionally.

Tom9589
08-25-2017, 03:57 PM
You might want to be careful using tape to hold a wire against the dag. The dag is not known to have very good adhesive characteristics and you might pull off a big amount of dag when you remove the tape.

FrankieKat
08-25-2017, 05:10 PM
I'd try changing the contact point. I f the dag is missing (even just a whisker) under the point of contact or making poor electrical contact with the spring then you will get HV arcing (and spark gap transmitter action) at that contact.

Okay, so pulled away the metal contact and cleaned it up a bit and then bent it to another place on the tube, and looks to be making good contact. I don't see any "islands" or places where the dag is separated from the rest. The coating looks pretty complete... just a little scratched off where the metal has been resting for the last 60 years.

Now here's the thing... before that, after playing a bit with the suggestion of grounding out the 3rd IF with a cap, and working with DC on the AGC circuit, the noise seems to have gone away... along with decent picture contrast. It's possible that it could have been poor contact with that rubbed-off dag and in working on the chassis the contact was intermittent.

So without a clear explanation, I seem to have a more or less clean but poorly contrasted image. As with before, it looks brighter in the video than in person.
https://youtu.be/xWOoI252aZY

There is definitely some wobble to the picture as well, which you can see with the DTV but can see it much better with the test pattern. I'm adjusting first the fine tuning then the brightness then the contrast and you can see there's not really a sweet spot.
https://youtu.be/XO5ciFJxHiM

Since the CR70 rejuv, the tube now produces a lot of light with brightness all the way up. I re-checked it again and still has responsive cutoff and gets up to about a 10.5 on the CR70 emissions when it's warm.

Still nothing on the AGC control, but injecting negative or positive DC doesn't make any large improvement to the picture's contrast so maybe that's just not going to be a big help.

I would say contrast and vertical hold tends to get worse as the set warms up. The sound (as can be heard in the test pattern video above) get very soft when the channel is fine tuned in for best picture. I know this is typical with old B&W sets and cheap DTV boxes though.

Just got the HV probe and it's measuring 15kV, while the schematic calls for 17.5k. I'll try a few different horiz damper and output tubes and see if that makes any difference.

FK

tom.j.fla
08-25-2017, 06:01 PM
Remember, 17.5kv is at zero brightness set brite control all the way off. HV will be lower as the screen gets brighter. All the best,Tom.J

FrankieKat
08-25-2017, 08:08 PM
Remember, 17.5kv is at zero brightness set brite control all the way off. HV will be lower as the screen gets brighter.

Ah, got it. Okay, that brought it up to 15.35kV. I tried all three output tubes and the one that was in there is the best HV. Changing damper had no effect.

Edit: Turning width control all the way up and brightness/contrast all the way down, I've got 17.6kV so HV seems solid.

FK

old_coot88
08-25-2017, 08:23 PM
You might want to be careful using tape to hold a wire against the dag. The dag is not known to have very good adhesive characteristics and you might pull off a big amount of dag when you remove the tape.
Good point. But we used to leave the grounding wire in place as a "belt & suspenders" supplementation when there was a sketchy spring contact.

FrankieKat
08-26-2017, 06:05 PM
So here's the update in quick bullet points.

What's working:


Horizontal sync - strong, stays locked in at center of control rotation
Audio - full and loud (when using BT Agile modulator)
High Voltage (measures 17.5kV) :thmbsp:
CRT has responsive cutoff and good emissions as tested by CR70. Output is bright
Odd video/audio "noise" has stopped (unclear why) :scratch2:

What's not working:

AGC control non-responsive
Contrast very poor
Vertical hold poor -- worse as set warms up
Picture has a "wobble" -- especially visible with test pattern (https://youtu.be/XO5ciFJxHiM)

The Sams troubleshooting notes keep pointing at V7-6AN8 (Sync Amp), V8-6AN8 (AGC keying/Vert Mult) and V6-12BY7 (video output). Have tried four different 6AN8 tubes, and two 12BY7's -- however I've just ordered some more of both.

At this point, is there anything else this could be other than those tubes or anything else I could try?

Thanks again everyone, for all of your help and great suggestions!

FK

Electronic M
08-28-2017, 03:55 PM
Given the rate of wobble I'd reckon that the power supply filtering is insufficient, your AC outlet voltage is fluctuating, or some component is shorting/leaking/arcing intermittently at a regular interval.

I've seen problems with AC utility voltage fluctuations causing picture size to change 1 a minute to ~3 times a sec. There is a business park about 1 mile away with manufacturing operations there, it was worst for the first 2 years I lived there, but still happens occasionally....I knew it was the power line since I could have 2-3 working tube era sets and one new set fed the same signal source on at once, the 3 tube sets would 'wobble' identically, but the new set would not wobble (due to having actively regulated B+ rails)...The cure was to run the tube sets through a SOLA VRT (AC voltage regulating line isolation transformer).

As for contrast check the video amplitude with a scope and compare to the schematic. If it is low there are a number of things that could be affecting it.

Your AGC may be tied to the low contrast issue....For instance the tube IF grids should be biased negative enough relative to the cathodes to prevent conducting more than their max rated plate current....If the IF tubes are weak and not amplifying enough then the AGC could be be pushing the grids as positive as the biasing design wall allows...When it hits the wall it can't make any more gain, but still it trying to.

It might be interesting to connect that external bias supply back to the AGC line, see how much more contrast you get and how the IF and tuner voltages (especially the AGC controlled ones) compare to the schematic with the external AGC voltage.

May also be good to check the sync amplitude entering the vert mult/osc system. Bad vert hold can be weak sync or osc off freq.
If you can get the vertical to roll both ways (up and then down) slowly, even 'float' without rolling but not fully synced for a while, then the problem is in the sync pulse level/cleanliness and or the osc circuits handling of it.
If vertical only rolls one way then it is off freq and the vert osc/mult/output stages are to be suspected (and sync signal can be ignored till float/dual direction roll is achievable).

FrankieKat
09-10-2017, 08:31 PM
I've gone through and checked through all scope traces and voltages several times and everything is generally within spec, except for a few places.

The biggest anomaly is that the grid on the sound IF / sync amp should be, according to the schematic, -1.8V however I measure between -0.3 and -0.5V. The plate voltage should be 60V but I see about 54V when the set is cold, and it falls down to about 45V. When first powered on cold, I get actually watchable contrast and fairly strong sound, however... after it warms up for about 5 minutes the contrast, sound and to a lesser extent sync gets worse (as that plate voltage goes down).

I have tried no less than six different 6AN8 tubes and all the good ones are the same. Have verified and/or replaced all nearby resistors (nearly of the originals were still within 20% spec) and replaced all but the mica and ceramic caps. And still absolutely nothing from the AGC control. The DC voltages on the IF stages are pretty much right on.

What could be the temperature sensitive components that would cause that?

FK

Electronic M
09-11-2017, 09:48 AM
Resistors are usually among the more temperature sensitive parts. Usually the best way to find a thermal sensitive part is to run the set till symptoms manifest and hit sections with freeze spray (or something similar like canned air) to see what makes it clear up. Start with general coverage of suspicious areas and once you get a good reaction to the spray, target specific components in the area of sensitivity to narrow it down.

Looking at the schematic you attached earlier I'm not seeing the -1.8 volts grid or 60V plate specs on either section of the 6AN8 sound IF/sync separator tube...Are you referencing a different listing of voltages? Also on multi section tubes like that one it is good to specify which section's plate and which sections grid when you say one is not right so we know which stage to look at.

FrankieKat
09-12-2017, 01:16 PM
Resistors are usually among the more temperature sensitive parts. Usually the best way to find a thermal sensitive part is to run the set till symptoms manifest and hit sections with freeze spray (or something similar like canned air) to see what makes it clear up. Start with general coverage of suspicious areas and once you get a good reaction to the spray, target specific components in the area of sensitivity to narrow it down.

Looking at the schematic you attached earlier I'm not seeing the -1.8 volts grid or 60V plate specs on either section of the 6AN8 sound IF/sync separator tube...Are you referencing a different listing of voltages? Also on multi section tubes like that one it is good to specify which section's plate and which sections grid when you say one is not right so we know which stage to look at.

Sorry about that, I meant V7B. Attaching a close up (http://www.videokarma.org/attachment.php?attachmentid=195528&d=1505232920) of that part of the schematic (http://www.videokarma.org/attachment.php?attachmentid=195411&d=1503594661). I picked up a fresh can of duster and will try cooling off some components to see what might be heat affected.

A few discoveries I've found:


By fiddling with all of the controls I can pretty much dial in most of the DC voltage measurements called out in the schematic (thought it only produces pretty much a bright white raster). The exceptions are the grid of V7B that measures -1.8V and no matter what I do the range is about -0.1 to -0.5. V11 (6CS6 SYNC SEP) pin 7 shows -10V and closest I can ever get it to is about -2V.
The scope trace shown for the grid is somewhat confusing to but I assume I'm looking for some kind of beat on a 60Hz pulse there? (See attached (http://www.videokarma.org/attachment.php?attachmentid=195529&stc=1&d=1505236773))
By feeding in an incredibly strong signal (from the Blonder Tongue) I can get the plate voltage on V7B up to 60V, however at that point the signal is incredibly overloaded and has a negative image and poor sync (both horiz and vert)
The scope traces on the AGC keying (V8A) look consistent with the traces on the schematic. I cannot measure the plate on it since the voltage is beyond the range of my scope and can't measure DC since those huge pulses confuse my DVM, however visually it seems to be correct. G1 (pin 8) measures pretty much the same as the plate of V7B as the schematic suggests (see attached (http://www.videokarma.org/attachment.php?attachmentid=195530&stc=1&d=1505236773))
Adding -9V bias to the wiper of the AGC control has what would be the effect of the AGC control, and at that point the AGC control actually affects the picture. However, the control has no effect without it.
Sound, contrast and sync continue to degrade as set warms (will try freezing components tonight)

At this point, is it possible that some of this could be alignment-related? The multiburst bandwidth test from the VA-62 looks pretty close to the trace from the manual (see attached (http://www.videokarma.org/attachment.php?attachmentid=195532&stc=1&d=1505236773)) and there are distinct vertical lines on screen up through 3.0MHz, so would that imply alignment is healthy?

Thx!!

FK

Electronic M
09-12-2017, 02:12 PM
On V7B: Tube biasing 101 says the less negative the grid the more plate current is drawn, the more plate current the less plate voltage (at least in a common cathode circuit like V7B)....If your grid bias is off it will effect the plate.
I'd like to see more of the schematic to the right of V7B. The grid circuit wanders off the edge of your attached scans...Seeing where it goes could give some insight into why it is not negative enough.

There is a slight chance the video detector diode is going bad (I've seen it in some TVs from time to time) and reducing video amplitude.

The AGC wiper appears to basically be the start of the AGC bus that biases the tuner and IF grids. If the control is dead till a voltage is injected it probably means the AGC circuit is not working. If all the tube voltages look right I'd be looking that fly winding between the plate of the AGC tube and the AGC control.

Unless you can get proper/much better contrast by injecting -9V at the AGC control (and adjusting the control) I think the AGC is not the main issue....Even if you can I'm still a bit more concerned about V7B.

With what I see I'm not inclined to think it is an IF alignment issue....And even if it is part of the problem, you need functional AGC and to work out the issue with V7B first before any alignment can be attempted. If you try to align circuits with defects the results often are not an improvement over the defective circuit as it was.

I'm running on ~3 hours of sleep presently so take this advice with a grain of salt...

FrankieKat
09-12-2017, 02:59 PM
On V7B: Tube biasing 101 says the less negative the grid the more plate current is drawn, the more plate current the less plate voltage (at least in a common cathode circuit like V7B)....If your grid bias is off it will effect the plate.
I'd like to see more of the schematic to the right of V7B. The grid circuit wanders off the edge of your attached scans...Seeing where it goes could give some insight into why it is not negative enough.

There is a slight chance the video detector diode is going bad (I've seen it in some TVs from time to time) and reducing video amplitude.
Attached the second page of the schematic. Makes sense that the issue could be in the detector since the DC voltages on the IF stages are spot on. I assume I'm going to need to dig up some germanium diode or equivalent, but worth trying.

The AGC wiper appears to basically be the start of the AGC bus that biases the tuner and IF grids. If the control is dead till a voltage is injected it probably means the AGC circuit is not working. If all the tube voltages look right I'd be looking that fly winding between the plate of the AGC tube and the AGC control.
Yeah, checked that -- the flyback winding is 5 ohms, right on the schematic value. Glad it's not that at least! Since I am seeing those 15,750KHz pulses on the plate of V8A, would that imply the AGC is at least partly working?

I've replaced every passive component on the AGC circuit between the grid of the 1st IF and the flyback. The control works perfectly too... no jumps and smooth resistance all the way across. I suppose I could try changing out some of the small mica and disc caps in the adjacent circuits.


Unless you can get proper/much better contrast by injecting -9V at the AGC control (and adjusting the control) I think the AGC is not the main issue....Even if you can I'm still a bit more concerned about V7B.

With what I see I'm not inclined to think it is an IF alignment issue....And even if it is part of the problem, you need functional AGC and to work out the issue with V7B first before any alignment can be attempted. If you try to align circuits with defects the results often are not an improvement over the defective circuit as it was.
I've tried at least six different 6AN8 tubes that have all tested way-off-the-charts-good and all produce the same results, so not a tube. Adding the bias certainly affects the contrast but not really better. It's more like if you turn the AGC too far where it whites out the picture.


I'm running on ~3 hours of sleep presently so take this advice with a grain of salt...
No problem at all! I really appreciate all of your help and thoughts!

I'll try changing out that diode and see what that does. I think I have some low forward voltage drop diodes... somewhere...

FK

FrankieKat
09-13-2017, 10:35 AM
There is a slight chance the video detector diode is going bad (I've seen it in some TVs from time to time) and reducing video amplitude.

I pulled out the old diode and tested it and it was showing a 0.7V voltage drop one way and 0.1V drop the other way, so it was definitely suspect. I changed it out with a new 1N60 and it seems to have made a difference in terms of how the signal degrades as the set warms. Contrast and sound seem more stable as the set warms. No change to the grid or plate voltages on the V7B tube though.

What I do see is that there is a slow brightness increase over 5 mins or so when starting from cold. As in, having adjusted the bright/contrast the night before, it will start very dim - almost not visible. Could this just be the age of the CRT and it's poor life test?

I can set all of the controls so that all of the voltages on the CRT are exactly on spec, and this is how the picture appears https://youtu.be/jzV57jRqrcI. I can adjust for better picture, but it changes the cathode and g1 voltage by 50-60V away from the schematic spec. Does it make logical sense that I would want to keep the controls set to produce the schematic voltages on the CRT because that will always be it's optimum operating point? As in, this should be the best picture, and there's something else causing it to be whited out like this that still needs to be found.

FK

Electronic M
09-13-2017, 11:25 AM
When a CRT is weak or old it can have a long warmup time to it's stable operating point. I'd guess that if you put that CRT on a tester and watched it's emission/cutoff readings it would probably change over that same 5 minute interval.

I've got a color set with a weak CRT with poor life. All guns will start dim and the weakest one will take the longest to warmup and stabilize (which makes the colors look weird for a bit).

Generally speaking the CRT gun voltages listed in the schematic are the correct settings for a new CRT (because that is what they would have had when recording that data). The optimum CRT grid and cathode voltages are the ones that correspond to the contrast and brightness settings that look best. Those settings will drift as the CRT ages from use.
Keep in mind that you want to NOT* adjust the picture for the brightest possible image with good contrast (doing that drives the CRT hard and given it seems to be a tired CRT that will kill it faster than normal). You want to adjust for the dimmest picture (with good contrast) you deem comfortably watchable (at least if your concerned about CRT life).

*Unless you plan to ride the CRT hard till it dies, and then replace it...Some people do that when they have a replacement on hand.

FrankieKat
09-13-2017, 01:24 PM
When a CRT is weak or old it can have a long warmup time to it's stable operating point. I'd guess that if you put that CRT on a tester and watched it's emission/cutoff readings it would probably change over that same 5 minute interval.

I've got a color set with a weak CRT with poor life. All guns will start dim and the weakest one will take the longest to warmup and stabilize (which makes the colors look weird for a bit).

Generally speaking the CRT gun voltages listed in the schematic are the correct settings for a new CRT (because that is what they would have had when recording that data). The optimum CRT grid and cathode voltages are the ones that correspond to the contrast and brightness settings that look best. Those settings will drift as the CRT ages from use.
Keep in mind that you want to NOT* adjust the picture for the brightest possible image with good contrast (doing that drives the CRT hard and given it seems to be a tired CRT that will kill it faster than normal). You want to adjust for the dimmest picture (with good contrast) you deem comfortably watchable (at least if your concerned about CRT life).

*Unless you plan to ride the CRT hard till it dies, and then replace it...Some people do that when they have a replacement on hand.

Interesting and got it. This CRT is clearly high hours since it is the original and was pretty dead when I got it. I zapped it with the rejuv on the CR-70 and it brought it back to life with good emissions and cut off, but yes the life test was poor. I consider it borrowed time and I don't plan to make this a daily use set. It sounds like this can explain some of the remaining warming up issues though.

FK

(attaching bonus pic of original Ratheon diode)

FrankieKat
09-14-2017, 10:47 AM
Well, this set is just full of surprises. I took the cover off of the tuner to double check all of the resistors and voltage measurements, but it's really tight in there and wasn't able to safely probe everything. Resistors were all good and voltages seemed a bit high and I wasn't really watching the picture and so put it back together and turned it back on.

Good news:

So... what happens next is that for no explainable reason, the picture has massive contrast, sound is loud and clear and has flawless sync all the way across both controls. What?? And... the AGC control now actually does something. It doesn't go all the way from no picture to white picture/no sound, but it does have a noticeable affect on brightness.

Odd news:

The voltages that were previously low or in question are now massively high. The 130V B+ supply is now measuring about 175V -- despite the fact that the two other B+ voltages right above it in the power supply are still right. The V7B plate that was supposed to be 60V and was previously measuring as 45V is now measuring 95V, and the plate and grid voltages on the IF tubes which are supposed to be 115-120 are now measuring 160V. The grid 3 on V11, that's supposed to be -10V, that was previously measuring -1V is now measuring -10.5V. Oh, and now adjusting all of the CRT voltages to the schematic is right about where the best, brightest picture is.

This is how it looks, though I was able to dial it in even better with the focus, fine tuning and width controls later. The video doesn't even do it justice -- this thing looks like a brand new modern TV in person.
https://youtu.be/dKXksyLwGqs

Note the sharpness and clarity of the text:
https://youtu.be/6pDbjJgIk1Q

Bad news:

Of course it doesn't like to be run with those voltages. After about 5-10 minutes, the picture starts to take hits like this. And yes, I turned it off as soon as I recorded this:
https://youtu.be/rYIgDQ-g3ZE

The only thing that was actually changed recently was that detector diode, and this was not happening right after I did. Previously the B+ supply voltages were all dead-on correct with the schematic and the IF's being fed from the 130V line were all correct while the sync circuits fed from the same 130V were all too low. I've never seen a situation where only one single part of the power supply divider goes from being right to being 30% high, unless a significant load was removed. If that were the case, and a tube that was drawing a lot of current got disconnected why in the heck would the set all of the sudden start playing WAY better?

Has anyone ever seen something like this before?

Very befuddled,
FK

Electronic M
09-14-2017, 11:02 AM
Have you cleaned the contacts in the tuner? I've seen dirty tuner contacts do stuff like that.

If you have not cleaned the tuner contacts make sure to get both the ones involved in changing channel and the tube socket pins.

I recommend Deoxit, or the cheaper comparably effective De-ox-id as the cleaning solution. If you can rub the contacts with a pencil eraser (the soft white ones are especially good) while they have the cleaner on them it will be better still. Those erasers are almost good enough contact cleaners by them selves to not need the spray.

FrankieKat
09-14-2017, 11:21 AM
Have you cleaned the contacts in the tuner? I've seen dirty tuner contacts do stuff like that.

If you have not cleaned the tuner contacts make sure to get both the ones involved in changing channel and the tube socket pins.

I recommend Deoxit, or the cheaper comparably effective De-ox-id as the cleaning solution. If you can rub the contacts with a pencil eraser (the soft white ones are especially good) while they have the cleaner on them it will be better still. Those erasers are almost good enough contact cleaners by them selves to not need the spray.

I had cleaned the channel changing contacts earlier with WD40-branded spray electrical contact cleaner (I ran out of deoxit), and have worked and rotated around the channel clunker quite a bit since and never saw anything like this happen. I've wiggled the OSC and RF tubes in the tuner and didn't see any reaction, but who knows. I can try cleaning the tube pin contacts next, but now I'm afraid if I touch it that it might go back to the lower voltages and sub-par performance!

Clearly getting the sync and AGC circuit voltages up directly affected all of the remaining problems so at least there's some confirmation there. I'll trace out everything from the 130V on and see what's up...

FK

Electronic M
09-14-2017, 12:37 PM
My guess is dirty contact or loose connection in the tuner. The issue with AGC and such is probably being caused by that connection and the issues in the 3rd video are probably it trying to relapse.

irext
09-15-2017, 12:50 AM
I've had noisy carbon resistors cause intermittent grief. They check o/k with a meter but break down when passing current. Since it was the tuner that you disturbed before it came good it might be worth replacing the carbon resistors in it. There shouldn't be many in it but access will be tight. Be careful not to disturb coils and feed through caps etc. The turret will need to be removed to gain access. I'm not a fan of WD40 for electronic work. Use Deoxit as it leaves minimum residue. If you have a spare tuner from another similar vintage set even better. Doesn't have to be physically the same just electrically close enough.

FrankieKat
09-15-2017, 10:01 AM
I've had noisy carbon resistors cause intermittent grief. They check o/k with a meter but break down when passing current. Since it was the tuner that you disturbed before it came good it might be worth replacing the carbon resistors in it. There shouldn't be many in it but access will be tight. Be careful not to disturb coils and feed through caps etc. The turret will need to be removed to gain access. I'm not a fan of WD40 for electronic work. Use Deoxit as it leaves minimum residue. If you have a spare tuner from another similar vintage set even better. Doesn't have to be physically the same just electrically close enough.

Wasn't regular WD40 lubricant, it was their electrical contact cleaner (http://a.co/dAhszxT) that's just typical electrical contact cleaner with a fancy nozzle. And yeah, I'm very careful around those wire coils that had been hand tuned by carefully forming and pinching in the factory.

I spent some time last night cleaning the tuner contacts and tube pins and so far what I'm seeing is that when all wafer switches are making good contact I'm getting great performance but with the over spec voltages. I tested tube voltages and everything that is fed of the 240V B+ is right about on, and everything fed by the 130V B+ (the one measuring 170V now) is proportionately high. It doesn't seem like it's a bad contact on the tube socket since all voltages are present and no glitching at all if I wiggle the tubes as would normally be associated with dirty connections.

Meanwhile though, the thing is still producing a beautiful picture and sound (though I know at those voltages I'd be cooking tubes alive). This is with CRT pin and high voltages dead-on schematic measurement, so it's not even being pushed out of spec. Not bad for a tube that was found with a brightener (which is of course no longer on) and had virtually no emissions and zero cutoff. That CR70 rejuv zap is incredible!

https://youtu.be/MYtxY-Eanrk

I'll keep cleaning and maybe try replacing some resistors best I can without having to pull the tuner totally apart.

FK

FrankieKat
09-15-2017, 01:16 PM
So I might have just hit a "duh" moment here. Reading the barely-legible scanned print at the bottom of the schematic it says "channel selected shown in channel 13 position" and "measurements taken with no signal applied". And well, I had previously been testing voltages with a signal connected. Now, with no signal I get pretty close to all of the voltages on the schematic -- of course with a signal they jump up to those much higher ones.

Based on that, would it be safe to say then that what I'm seeing is in fact the correct, normal working voltages (despite being quite a bit different) when tuned in?

FK

tom.j.fla
09-15-2017, 01:39 PM
As a rule voltages are +/- 20%. Service data at one time would have that noted in the info bar/box. All the best,Tom.J

FrankieKat
09-15-2017, 01:44 PM
As a rule voltages are +/- 20%. Service data at one time would have that noted in the info bar/box. All the best,Tom.J

Yeah, with no signal I'm getting pretty darn close to the schematic which says they were taken with no signal. With signal, many of those B+ voltages jump up 40% or more, but it sounds like that's actually fine because what was published wasn't measured under actual use conditions. The set works really darn well and so if this is in fact how it is supposed to run and I won't cook the tubes then I'm more than glad to move on from it!

Thanks!

FK

Electronic M
09-15-2017, 02:13 PM
Generally speaking the only tubes you ought to be concerned with are power rectifier, H/V/Audio outputs and maybe a stray 6CG7/6FQ7 or 12AU7/12AX7....The rest are usually $1.50 bottles that only TV folks want (and unlike audiophiles/radio collectors) we're cheapskates. :D
I've got ~8 caddies full of mostly TV tubes in my work shop (a good portion used pulls in unknown condition), and I've been known to grab a used one smash it open and dissect it on the spot to explain how a tube works to an interested guest..."Hey Tom, might that have been a good tube worth keeping intact?"
ME:"who cares? It's a $1.50 6xx#....The visual aid/savings in explanation time we got out of it is worth it." :D

FrankieKat
09-15-2017, 02:35 PM
Generally speaking the only tubes you ought to be concerned with are power rectifier, H/V/Audio outputs and maybe a stray 6CG7/6FQ7 or 12AU7/12AX7....The rest are usually $1.50 bottles that only TV folks want (and unlike audiophiles/radio collectors) we're cheapskates. :D
I've got ~8 caddies full of mostly TV tubes in my work shop (a good portion used pulls in unknown condition), and I've been known to grab a used one smash it open and dissect it on the spot to explain how a tube works to an interested guest..."Hey Tom, might that have been a good tube worth keeping intact?"
ME:"who cares? It's a $1.50 6xx#....The visual aid/savings in explanation time we got out of it is worth it." :D

Ha ha, yeah! Well, I've bought so many tubes for troubleshooting that I've easily got enough for two more sets just like this, so I should be in good shape just in case. This whole voltage issue seems like a rookie misunderstanding on my part, so really a non-issue. The data sheets on the tubes in question (6CB6 and 6AN8) have a max plate voltage of 330V so running them at 160V instead of 125V doesn't sound like it would be any problem either way.

Otherwise my last issue here is that pesky picture glitching (https://youtu.be/rYIgDQ-g3ZE) (and this (https://youtu.be/GSc_GAQxrsI)), possible HV arcing noise that was bothering me a while ago went away, and now has come back which seems to happen after the set has warmed up about 5 mins or so. I did have to repair the grid cap wire on the horiz output tube because it had frayed at the point where it exited the cap and eventually broke off. Perhaps my solder job wasn't quite perfect enough. If not, will go back to dag. Will keep pluggin' away.

Also... let me just say huge thanks to everyone who helped out on this. I feel like this project a little bit of all kinds of things that could go wrong with an old, unrestored set and there's probably enough useful information in here to help many more restorers in the future.

Thanks everyone!

FK

FrankieKat
10-12-2017, 11:20 AM
Hey everyone,

Just an update on this. All back together in the cabinet and have figured out that the remaining problems (including picture glitching) are all contact issues in the tuner as was suggested earlier. I cleaned it once, but apparently still needs more work.

At some point I'll pull it back out, thoroughly re-clean tuner contacts and attempt an alignment with the VA62, as there is a little bit of smearing and ringing in the picture but still very watchable. Both syncs are rock solid -- controls in the center and has never rolled once, and brightness and contrast are good. See pic.

Thanks again for all of your help on this. I learned a lot and this TV is so fun to watch!

FK

tom.j.fla
10-12-2017, 01:26 PM
Frankie glad it is working fairly well for you. All the best,Tom.J