Today a package arrived, containing a very nice little TV, the RCA 8PT, black, with a swivel stand. It's in excellent physical condition, and my goal is to get it into excellent electrical condition as well.

Opened it up, cleaned out dust, tested the tubes... it needed only one tube, a 6U8, curiously the only non RCA 6U8, a Japanese Channel Master, was bad.

Turning on the TV it gets a nice bright raster, and good sound, but the horizontal hold can't be dialed in.

One symptom worries me a bit, probably due to ignorance. With the Horizontal frequency control at a low frequency (I can hear the flyback), the out of sync image is nice and bright. As I turn up the frequency, it hits a point where the image horizontally collapses into what looks like a modulated vertical stripe, and the screen gradually goes dark. I think I can hear zapping sounds in the flyback, or maybe the frequency is just all over the place, but whatever it is appears to be killing the high voltage. Turn down the horizontal frequency, and the image comes back, full brightness, and the high voltage appears to be restored, but sync is impossible.

I am going to replace all the capacitors in the TV before I worry too much about the lack of horizontal sync, but the possibility of a bad flyback is a bit worrying. I wonder what the experts think.

I have to say, it is a nice contrast to my other vintage tv, in that moving it around can be done with one hand!

Looks like we've got another spammer!

"china wholesale". Yeah,that's just what people on a *vintage* tv site want...Chinese crap!

Spammer banned.

...

Neat little sets, although the innards are tightly packed. This article has a link to the RCA factory service manual, and a few notes that may be helpful.

http://antiqueradio.org/rca16.htm

Phil Nelson
http://antiqueradio.org/index.html

My best guess would be the .047/600v paper cap in the boost circuit...And yes, I have also heard odd clicking noises when the frequency is way off and "squigging" (if that is the proper term). There are only about 4 paper caps in the horiz section (2-.047s, 1-.022 and 1-.22), I think that I would do those first and re-check the performance,
jr

Thanks all for the input!

I think I better go against my desire to see a picture on the screen now, and wait for all the capacitors to show up before I continue on. I'm happy to know this doesn't mean the flyback is suspect, at this point. In addition to the symptom mentioned above, the B+ is about 160, and should be 216, so fresh electrolytics and ditching the selenium rectifiers sure looks like it won't hurt.

Phil - your website is great, I remember reading about your TV several years ago when I used to go on rec.antiques.radio+phono. That's where I first became aware of this model. I printed the service manual from your website about a week before the TV arrived, to have an idea what to expect. I have to say I am surprised by how small everything is... even the controls are tiny.

I cleaned up the case with a toothbrush and warm water... it looks nearly new, although maybe it could be a bit more shiny. The only defect is the antennas are broken.

finally got my order in at mouser - 39 caps and two diodes. If it takes half as long to install these as it did to order them online I'm doing well :)

It's amazing how much grime gets trapped under safety glass...

Yeah, well, you gotta remember that l'il guy is 51-54 yrs old, & I'd about guarantee SOMETIME in its life it was around a smoker or 2, a few pets, & prolly some kids...Too bad these things can't talk-I'd LOVE to hear some of the stories they could tell...

I have one of those I have been using regularly for ten years now. I did a full recap when I got it and it works great. These are some of the best built vintage portables around, transformer powered, no printed circuits, all metal cabinet, etc. Once restored, it will reward you with years of viewing pleasure and it is small enough to use and display anywhere. Take your time with it and it will be well worth it. Let's see some pictures along the way.

As requested - here's some photos. Set is basically as received from member Eric H, aside from some light dusting and cleaning. As you can see it is in excellent internal and external condition!

You can see where I damaged the vertical hold knob a little bit when I was putting the chassis back into the case. Luckily it just cracked off a piece, which I recovered, and it didn't crumble, so I can glue it back together. These knobs are very delicate! When you separate the front and rear halfs of the chassis, getting it lined up again so everything lines up is a bit critical. Mine is one of the later models with safety glass instead of plexiglass, so it is very easy to clean and free of scratches.

Those small knobs are often missing, probably more than half my 8" sets are missing them entirely.
Someone should repro them, shouldn't be too hard provided you can come up with a pristine one to make a mold from.

I have (had) 12 of these little guys, I think every single one of them would at least display a raster without anything being done to them, usually the Horizontal won't quite sync up, that's pretty amazing, only one has a bad CRT , way better odds than most sets.

That is about the condition mine was in when I first got it. Mine produced a weak picture with no sync when I got it. Since restoration it works like new and the CRT is strong and bright. It would have been nice if RCA had used an aluminized tube. I used to lament the lack of UHF on these sets but since the digital switch over it is no longer an issue. They make great daily watchers. It is a lot of work disassembling that cramped chassis and getting to all those paper caps but the results are well worth it. I am surprised RCA put so much into this set considering they were using PC boards in their much more expensive products.

Wow, a portablel with a transformer!

I have not seen one of those in person, but it just amazes me how all that could be crammed together like that!

Wow, a portablel with a transformer!

The 8" Emerson with the radio built in, is also transformer powered.

Those small knobs are often missing, probably more than half my 8" sets are missing them entirely.
Someone should repro them, shouldn't be too hard provided you can come up with a pristine one to make a mold from.


Yep! those little rubber knobs are the first to go, they get brittle and break off in chunks. On my childhood set, I replaced them with short pieces of black insulation in about 1959. Today I just cut short chunks of silicone tubing (model airplane fuel line) and slide onto the knurled shafts. Second to go was the power cord, where it enters the set.

Your set is looking great! good luck with the re-cap job!

jr

3 of 'em have managed to find me-Plus one of the Emerson AM radio/TV combos, which uses the same CRT. Oddly, the best cosmetic-condition one-a sort of grey-beige version-has a CRT that's about shot.

Sandy, Is that emerson am/tv you have got UHF? Ive got one here with no knobs and i need to see pics if its the same ..

Steve

No UHF.

Thanks for all the comments! I've been trying to be extremely careful with the set.. was annoying to crack the little knob, but such is life. The plastic antenna mount is broken on mine, I'm going to try to repair it with epoxy or JB-Weld before I start to search for a replacement. The quality of construction is amazing... even after looking at all the photos on Phil's website, I can't say I was prepared for how small it actually is! The only other place I've seen the same size and type of potentiometer, is in old 50s Motorola two way radios... the type with submini tubes. Makes me wonder what modern electronics would look like had the Japanese never become involved in things.

About 75% done replacing capacitors - wow there's a lot, and wow is it small! Work is going well though! There is an error in the parts list, in the RCA manual. C164 is listed as 0.001uf, but in the TV and on the schematic it is really 0.022. I paralleled two 0.01's, couldn't find any 0.022's locally.

All done except electrolytic can. Gonna restuff it this week, but it isn't warm in operation. TV picture looks awesome, it's sharp, with great geometry. Not perfect, but very close!

Sound has a lot of buzz when text is on the screen, but otherwise is good. I suspect there's not much I can do about this? It's nice to see this little guy alive!

Have a couple more observations about this little TV.

1) The problem Phil encountered with the picture being tilted, seems to be because the yoke is held in place with spring pressure, and rotates on its own a bit when the chassis is installed back into the cabinet. It can be corrected by removing the control door, and inserting an insulated flat blade screwdriver into the tv with the guidance of a flashlight, and using it to move the yoke into correct alignment. It seems like every time I move the TV, the picture gets a bit tilted.

2) The TV is fairly poorly shielded. My cable box puts out a signal on 3, and since there is also a broadcast station on this channel, the interference ends up as lines on the picture. Using a VCR as a video modulator gives me a nice crisp picture.

3) White screen or text puts an unacceptable amount of buzz into the audio. Can anything be done to help this?

I may be remembering wrong, but I think that there is a large Phillips head screw on the outside of the yoke housing near the bottom that pushes a collar with fingers against the back of the yoke to hold it in position.
jr

I may be remembering wrong, but I think that there is a large Phillips head screw on the outside of the yoke housing near the bottom that pushes a collar with fingers against the back of the yoke to hold it in position.
jr

Yes there is, also the plastic yoke covers are nearly always distorted on these, enough to lose the centering magnets in some cases.

When I rebuilt one I discovered the long metal shield around the neck of the CRT had to be in place or it wouldn't get a raster, possibly the Ion trap was weak or just so tiny it wouldn't work without the shielding.

Originally these had a long stiff piece of fish paper attached to the Ion trap and extending out the back far enough to adjust the trap by removing the rear cover, this is often to brittle to use after 50 years and either has to be replaced or you have to adjust the trap with a non ferrous stick of some sort.

BTW Max, I still need to look for the rear cover for you, if you can't fix the antennas you'll need it to cover the hole. Remind me if I don't contact you soon, I forget things easily these days.

I've had the TV on all afternoon... it seems the buzzing has quieted down to such a low level I almost don't notice it since the TV has been really fully warmed up for hours. Figured out my CRT computer monitor is interefering with the picture on the RCA. Easy to solve that, I don't plan on keeping the RCA in my computer room anyway! So far I've been operating it with the antennas down, but Eric if you do dig up a rear cover I'd like to get ahold of it. No big rush, it's working fine as is.

The fish paper is intact on my TV, but haven't had a need to adjust the ion trap yet - picture is good and bright as is. I'm getting used to life in black and white!

I have had many 50s sets that buzz when white lettering is displayed on the screen, and others that don't. It seems that I have read a technical explanation as to why this occurs, but I don't think that it indicates any problem in the set itself. I wonder if slightly adjusting the AGC control might help? It would be worth a try and certainly can't hurt anything. The portable might not even have an AGC adjustment. The main thing is noting approximately where the original setting was for what I call backward reference. LOL!

I may be remembering wrong, but I think that there is a large Phillips head screw on the outside of the yoke housing near the bottom that pushes a collar with fingers against the back of the yoke to hold it in position.
jr
Yes.

http://antiqueradio.org/art/rca1619.jpg

Many old TVs buzz in scenes with very high contrast. Aligning the sound section can help, but on some sets you may never eliminate it entirely. Backing off the contrast will reduce it. Sound buzz is more noticeable now than in Ye Olde Dayes because contemporary TV programs are littered with constant text crawls along the bottom, animated logos, etc., etc.

Phil Nelson

Thanks guys!

Removing the antennas from the rear strongly improved the picture quality. They have an internal mechanism which disconnects the antennas themselves when they are fully retracted, but it wasn't working, which allowed a lot of interference to get into the TV. Removing them and adding a terminal strip made a big difference. Now I am getting very sharp focus and can see all the scan lines.

I've noticed a new problem, a bit more subtle, There's vertical stripes on the left side of the screen. At first I thought it was screen burn, or gas in the picture tube but when you turn the contrast all the way down, and brightness all the way up to get a full raster, the horizontal hold control will shift the stripes back and forth. Attached are a couple pictures. I still haven't restuffed the can filter cap, but all other paper and electrolytic caps have been replaced.

Check the filter cap for the G2. Doogie

You mean G2 of the CRT ?

I think this is a 5uf cap, part of filter can. Will try this tonight.

It is probably a 0.1 – 0.5 mfd cap. At between 400 – 1000 vdc. It will not be a can. Lookup up your CRT type number on Google and follow the wire back to the chassis and find the first cap to ground. Doogie

Ah.. in that case I would have already changed it. Will verify that the value is correct.
Schematic says G2 not tied to ground through a cap?

That could be true. Rectifiers were very expensive in the old days. Check caps in the yoke circuit. Just because you replaced them does not mean there good. Do you have an ESR meter? Good luck. Doogie

It could be a dynamic thing.

Rebuilt filter capacitor can. First one I've ever done, it was a lot easier than I was expecting. No impact on this problem, but now all electrolytics are new.

Attached is the schematic so you can see what I mean about G2

Looks like ringing in the Damper circuit, have you tried adjusting the horizontal drive coil or the width?

Just for kicks tried subbing the 6CG7, 6BQ6 and damper one by one, to no effect - all were good.

Horizontal drive doesn't seem to do much at all. Width I need to find a more appropriate screwdriver to adjust... it's got a slot in the end of a long threaded shaft, and all of my miniature screwdrivers seem to immediately slip out of the slot.

Finally got some time with the TV.
Drive/width don't do anything for the vertical bars, aside from move them around a bit. It's a shame, the picture is otherwise nearly perfect.

You might try a different Damper or Horiz Output tube, sometimes a set can be fussy about which tube works better even if they are good.

Nope, not a difference. Have tried 3 different of each, and a couple 6CG7s too. I think I'm going to have to start checking resistors, etc.

Ah yes, I looked back through this thread and see you did already do that.

This is one of the more aggravating problems I run into on old sets, probably something out of spec, a resistor seems likely at this point.

There are a couple of caps and resistors inside the yoke but the caps are likely Mica and unlikely (but not impossible) to be bad, the resistors might be however.

I was pretty careful to do neat work, only replaced about 8 or 10 caps in a sitting, tried to put the new caps in exactly the same place as the old ones, but I may have moved something, it's not totally out of the question.

Is it reasonable to think the problem of vertical stripes is more likely in the horizontal than the vertical section? Right now is a bit late to get the TV on the bench, but over the next few days I plan to give it some time. Problem is fairly subtle, but for sure want to get it worked out before I put the TV upstairs and into regular service.

Yes, it is a problem with the horizontal sweep section, it's not sweeping across evenly and that causes the bars.

I'm not sure if it's the Damper circuit or the Sweep circuit that causes this, or if it's a mismatch between the Yoke and the Flyback (possibly caused by bad resistors and caps inside the yoke)

I doubt it's related to anything you replaced, more likely to be something you didn't replace yet.

Checked resistors in yoke, and most of horizontal circuit. Nothing is way off, but not dead on either.

Read -30V on G1 of HOT, versus -36 on schematic, and 105 versus 126 on screen of HOT, with 225V B+ versus spec of 216 (probably due to silicon rectifiers),

Attached is photo of set tuned to inactive channel, with contrast and brightness tweaked to emphasize problem.

Noticed something new - maybe I am looking in the wrong place.

When contrast (pix) is turned up all the way, the lines are very visible (first attached pic)

When contrast turned all the way down, the lines nearly dissapear.

See attached

Still at this - I think I'm starting to understand why Phil didn't recommend this TV for beginners :)

Anothing thing I've noticed is the problem seems to get worse the longer the tv is on, stabilizing once the tv hits operating temperature.

I tried shorting the cathode and G2 of the CRT to ground through a capacitor, to no effect, so it really must be in the horizontal section. The old book "Television Simplified" said says that problems on the left side of the screen are damper related, the right side of the screen more likely horizontal output tube or oscillator related. The only components related to the damper I haven't subsitituted are L156 and L157, but not sure how to test this. I did short L156 with a jumper, and it seems to affect nothing. It reads 0.8 ohms, but I'm not sure what it's supposed to be.

Have tried a few brands and even values for C173, also to no effect.

Regarding attached - green stuff has been checked and/or subsitituted, red has not.

Noticed something new - maybe I am looking in the wrong place.

When contrast (pix) is turned up all the way, the lines are very visible (first attached pic)

When contrast turned all the way down, the lines nearly dissapear.

See attached

So perhaps a pulse from the Horiz retrace is causing some "ringing" in the power supply to the video amp? :scratch2:

jr

When I put a clip lead at X, and use it as an antenna, near the horizontal output section, the following picture results.

The old C121 had very long leads, and the new one shorter leads... I have a feeling placement in the area must be critical, so will spend some effort here.

EDIT - even with the 6AW8 pulled, and the plate grounded through a capacitor, problem still there as always, just not emphasized as below. Also, the problem is still there when the front and rear chassis are separated. I'm starting to wonder if a NOS 6AX4 might be the answer... all of the ones I tried test exactly at the minimum value on my TV-7 tube tester.

I think swapping in another 6AX4 damper tube would be a good thing to try. Also C174, the 330pF silver mica cap could be the problem. It's got the full plate voltage across it and may suffer from silver migration.

Good luck,

Clark

One of my old TV books illustrates insufficient damping with a photo of vertical bars in the left half of the screen. These would roughly equate to ripples in the ascending part of the horizontal waveform. The purpose of a damper tube is to smooth out the ripples.

So, it sounds sensible to try a new damper tube and examine whatever else is in circuit with it (typically not much, IIRC).

Phil Nelson

Thanks much for the input! Sorry to drag this out so long, but hopefully the result will prove useful to someone else at one point.

Found a 6AX4 GTB which tests a bit higher than the original, and all others I have on hand. Bars still present as ever - no change. C174 is a tiny ceramic disc, not a mica, but jumpered it with a 0.01uf disc also to no result.

Looks like in this circuit, the damping is taking place through C173, since the boost voltage is in series with the B+. All the same, I have changed it out already without results.

The bars are present either with a program, or with no signal and a blank raster.

I wrapped a couple turns of wire around the 6BQ6, to try to get the horizontal waveform (I have no high voltage probe). Sure enough, it does reflect what's on the screen of the TV. (Note: Polarity was set to reverse on scope, so waveform is actually flipped upside down)

Damper plate waveform (this was jumping around on the scope a bit. Scope is an old Stark from 1962 with all original parts, so it's hard to get stable sometimes)

Horizontal output tube G1 waveform

RCA Manual waveforms for comparison (Thanks Phil!)

Looks like bad ringing in the output stage, I think the damper can cause issues like this.

have tried probably 5 damper tubes and 3 or 4 outputs, including a 6DQ6 just for kicks. Even tried the 1V2 from my scope just to be sure. I am starting to suspect either the width coil, or the flyback itself as all other major parts have been checked.

I have seen a bad Flyback cause this, on a Silvertone, it was obviously bad though, burnt to a crisp but still making HV. Replacing it got rid of the problem.

I'll see if I have a spare Fly somewhere.

mine leaked out a ton of wax at some point. Hardly a surprise given its location directly over the power tubes! I could (and likely am) overlooking something simple here, but the flyback looks more and more suspicious . Have you got the sams for this tv? I think it might be prudent to check the resistance of the windings at least, and of the yoke too for that matter.

Here ya go.

http://antiqueradio.org/art/temp/RCA8PT7012HVPartial.jpg

http://antiqueradio.org/art/temp/RCA8PT7012Yoke.jpg

If you send me an email via http://antiqueradio.org/contact.htm , I can email you the whole thing.

Phil Nelson

They all leak a lot of wax out of the Fly, it would have made more sense to put it on the bottom and the tubes on top!

A resistance check may not do much good, a shorted turn or two wouldn't show up that way.

Phil:

Thank you, will send email soon! Mine is the KCS100K, so the flyback is different than that one - I started comparing resistances, and then realized that diagram has C78 = 0.047uF, which corresponds to the earlier KCS100B chassis.

Eric:

Yep, I thought it looked upside down too - when I took apart the chassis to clean it, the first thing I thought when I encountered the wax covered damper and HOT was hmmm heat rises, what's up with this idea?

I spent about an hour or two with the TV again tonight, without any real progress. I did however get a much cleaner photo of the horizontal waveform, as below.

Schematic sent (336-14, chassis KCS100B, the only match I found in the index).

Which waveform is that last photo showing?

If you want to go beyond resistance testing, this thread from the "other forum" has an article describing how to test for ringing with an oscilloscope (haven't tried it myself).

http://antiqueradios.com/forums/viewtopic.php?t=154217&postdays=0&postorder=asc&start=0

Phil Nelson

Hi Phil,

Thank you for the sams! Been reading through it, and it has some perspective which the RCA manual does not. It also has a much better drawn schematic, from an understanding how the circuit operates perspective, even if the sweep circuit has some subtle differences.

Last waveform is the a few turns of wire around the horizontal output tube's plate lead, into my scope's probe.

Thanks for the article - Looks like I've got something to do tomorrow night now!

heres one that i might need some guidance on i,am looking to get and old b/w tv and convert it to radar the tx and rx of it will be around 300mhz now i know that the tubes won,t be able to handle the high freq so should i stay with ic,s or go with tube?

Your radar question seems completely unrelated to this topic. You can click the New Thread button to start a separate discussion in this forum.

Phil Nelson

Ok, had some time this morning to look at the TV. I think I want to try a new fly, and maybe width coil. The only waveform which looks wrong, according to the RCA service manual, is the horizontal.

The testing method for ringing flybacks in the above article counts on having a "known good" to compare with.

One thing about this problem, is I am sure getting well acquainted with the inside of this wonderful little tv - it's actually more fun to work on than my early 70s Zenith whose performance was my previous interest, with its massive chassis, hot chassis and overwhelming number of parts.

On another "note" I think I figured out the problem with audio buzz too.... this is a design defect caused by the 6AW8 being nonlinear when the contrast is set too high. Once I get the TV working perfectly, I am going to think about ways to have the audio not go through the 6AW8

Noticed a new 'defect', or possibly characteristic. When contrast is cranked up, text can get fuzzy. Plate voltage on video amp is off spec, quite a bit too high. Since contrast control works by varying bias on 6aw8 pentode video amplifier, and current is too low, i'm guessing either tube is bad or resistors have drifted. This design doesn't appear to have a lot of headroom built in.... Effect not terrible, it just means one has to balance contrast with sharpness. Will work on this one once new flyback installed. In the mean time have been using this as my only tv... Works great... Wonder how long until i start to dream in black and white? :)

Noticed a new 'defect', or possibly characteristic. When contrast is cranked up, text can get fuzzy. Plate voltage on video amp is off spec, quite a bit too high. Since contrast control works by varying bias on 6aw8 pentode video amplifier, and current is too low, i'm guessing either tube is bad or resistors have drifted. This design doesn't appear to have a lot of headroom built in.... Effect not terrible, it just means one has to balance contrast with sharpness. Will work on this one once new flyback installed. In the mean time have been using this as my only tv... Works great... Wonder how long until i start to dream in black and white? :)

When I turn the contrast all the way up on mine it almost looks like overloaded agc. I think what you are seeing is just a characteristic of the design and not a defect. The picture and fine tuning controls need to be adjusted for the best overall performance which on a set like this is invariably a compromise. I expect this was typical of 1950s sets. The one thing these little RCA sets would have benefited from is an aluminized tube.

When I turn the contrast all the way up on mine it almost looks like overloaded agc. I think what you are seeing is just a characteristic of the design and not a defect. The picture and fine tuning controls need to be adjusted for the best overall performance which on a set like this is invariably a compromise. I expect this was typical of 1950s sets. The one thing these little RCA sets would have benefited from is an aluminized tube.

I agree completely, just for the sake of improved contrast. If attached advertising (radio & tv news, July 1954) is credible, it would have been a pretty big improvement. I wonder if any aluminized replacement CRTs were made for this set.