The other day I installed the last of the new caps into the '55, so it's a good reliable runner now... or so I thought.

A couple seconds after it came on, the picture got all fuzzy with hum bars slowly drifting up the image. Voltage checks revealed B+ was 360 when it should be 400, and there's 36VAC of hum now. I suspected one of the new filters I installed might have bit the dust since they are only 80uf when they should be 200 (doubler circuit), but replacing them didn't fix it. I suspect the seleniums might have gone bad, so I'm going to replace them with new diodes. Trouble is new silicon outputs more voltage, so I need to know what value resistor to use. I could always just run the variac, but I don't want lower filament supplies. Can anyone with a CT55 tell me what they have done with the power supply?

Nick,

I used a 7.5 ohm 25W tubular ceramic with the adjustable slider on my set. I was able to tweek the B+ right on the money. I used 1A 1000PIV diodes. I hid the resistor inside the rectifier cage mounted with a screw and fiber insulating washers through the center of the resistor bolted to the wall of the rectifier cage.

7.5 ohm? That's it? I would have thought more, since the CTC-4 needed 75...

IIRC, Seleniums have a forward voltage drop of about 5 to 9 volts, and a silicon rectifier has a forward voltage drop of .7 to .9 volts.

Georg Simon Ohm's little contribution says that E=IxR, and we need to drop approximately 7 volts (somewhere in the middle of the selenium voltage drop range).

I'm assuming a 21-CT-55 would draw about 450 watts from the power line at 115/117V, so the current draw would be about 3 Amperes. Not all of that is B+ current, so let's say (worst case) the B+ current is 1 Ampere. It is more likely in the 550-800mA range, I suppose.

Now that we know the current and voltage, getting to the resistance needed to drop that voltage is simple Algebra 0.5:

R=E/I, so 7/1 gives you about 7 ohms, well within the adjustable range of Bob's 7.5 ohm resistor.

My calculations/explanations are full of assumptions, and are VERY open to correction.....I just did the same math for an old 6L6-based amp, and it worked well. Your mileage may vary. Fuses are a must, I might add.

Cheers,

Nick,

The ctc4 has a 10 ohm 25W resistor in the ac side between the common lead of the diodes and 4.5A power supply fuse. The other power resistor you saw on my CTC4 chassis is original to the chassis and is somewhere else in the power supply ckt.

IIRC, Seleniums have a forward voltage drop of about 5 to 9 volts, and a silicon rectifier has a forward voltage drop of .7 to .9 volts.

Georg Simon Ohm's little contribution says that E=IxR, and we need to drop approximately 7 volts (somewhere in the middle of the selenium voltage drop range).

I'm assuming a 21-CT-55 would draw about 450 watts from the power line at 115/117V, so the current draw would be about 3 Amperes. Not all of that is B+ current, so let's say (worst case) the B+ current is 1 Ampere. It is more likely in the 550-800mA range, I suppose.

Now that we know the current and voltage, getting to the resistance needed to drop that voltage is simple Algebra 0.5:

R=E/I, so 7/1 gives you about 7 ohms, well within the adjustable range of Bob's 7.5 ohm resistor.

My calculations/explanations are full of assumptions, and are VERY open to correction.....I just did the same math for an old 6L6-based amp, and it worked well. Your mileage may vary. Fuses are a must, I might add.

Cheers,

Not a good idea to calculate the resistance needed based on the DC (average) current. It might be right but probably won't be. The voltage drop actually depends on the instantaneous charging current, and that depends on the size of the filter caps and how much ripple is on those caps.

Why not just try different values until the B+ is what you want?

Phil Nelson

Not a good idea to calculate the resistance needed based on the DC (average) current. It might be right but probably won't be. The voltage drop actually depends on the instantaneous charging current, and that depends on the size of the filter caps and how much ripple is on those caps.


Why not just try different values until the B+ is what you want?

Phil Nelson

I believe that is what Wayne was implying. The overall forward resistance of the silicon diode plus added resistance must match the forward resistance of the original selenium rectifier. This should maintain the instantaneous charging current.

The net result of the properly chosen extra resistor will ensure the same ripple. Hence the resistor is chosen to match the original B+ voltage.

Terry

those seleniums seem never to last long. I just used newer diodes in mine too and kept sels. for looks

OK, so I did some poking around tonight and it's definitely not the power supply like I thought. I see no ripple when the deflection circuits are disabled, but the metal ballast is still getting pretty warm. It seems something from somewhere else in the set is causing the issue, but I don't know where to start.

Does this set have a boost cap? If so, I suspect it might be leaky. There's other caps in the HV garage that still have not been replaced, so I'll start there. Any other suggestions?

I have the orginal seleniums in my Model 5, CT-100 and Westinghouse 15 inch. They've held up well.

Now it's obvious that some circuit is taxing the power supply big time, even with the sweep tubes out the ballast is getting really hot and B+ is still 15 volts low. The drain appears to be on the 285 volt rail, so I guess I'm just gonna have to start disconnecting things till the load goes away. I think maybe a cap somewhere has (partially?) shorted or something.

Also tried replacing the seleniums with diodes, didn't change a thing.

Nick, you could try measuring the resistance of the 285V circuit (with the power off and any voltage bled off). It may measure low since you have a high-current problem. The next trick is to test the "other side" of each part that branches off from that line. One of them would likely measure lower resistance than the main circuit itself, and that would be where your shorted part is.

Something just occured to me. My CT-55 chassis is sitting in the dining room so it's easy to access. If you need a resistance measurment, let me know what you'd like to compare; that may be helpful.

Regards,
Mark

Sure, what's the resistance reading you get on the 285v line with the ballast pulled? More importantly, how much current does the 285 rail pull when working correctly?

Please give me a few days; due to the upcoming holiday weekend I have commitments that will take me away from home. I can easily measure the resistance; however, because the chassis is not in the cabinet, I am not presently able to fully power-up it up with a deflection load to measure total current. I may have that information from the manufacturer though. Alternatively, we should be able to calculate the nominal current based upon voltage drops (if indicated on the schematic) across the ballast. I will study the diagram if other readers cannot answer this empirically. I prefer to make these measurements with my Simpson 260 rather than a digital meter. I will provide measurements with reversed polarity as well, since in theory, they could differ.

With regards, to your recent acquisition, congratulations. I seem to recall this prototype was well described in the Second Color Television Issue of the Proceedings of the IRE, 1953. If you have not already studied this report, let me know and I will dig it out for you.

Regards,
Mark

Anything for the proto set should go in its respective thread: http://videokarma.org/showthread.php?t=251369

I too have commitments this weekend, I'll get back to the CT-55 next week.

K, so I finally had more time to look at this.

I have now ruled out the power supply as a failure source. I have disconnected the following items from the power supply, to try and isolate the current drain:

Everything on the 285v 'IF' rail. Nothing in the audio or IF sections is getting power anymore, I accomplished this by disconnecting pin 2 on the ballast.

I disconnected the tuner from the other 'normal' 285 rail so the only things getting power are the sweep circuits, video (Y), chroma paths and whatever convergence circuits may still be hooked up.

I am putting a composite signal from my gen into the grid of the 6CL6 to get video and the set again has a nice color picture, but read on.

Still getting the hot ballast, so I started looking hard at the horizontal section. Oscillator still had some wax caps in it, but replacement did nothing. Adjusted the wave shaping coil for equal peaks (it wasn't far off to begin with), no change. HOT is drawing 225ma (cathode), too much for my comfort. Tube data sheet for the 6CB5 says 220 is the safe limit as a deflection amplifier, so I have to get that down somehow. Messing with the drive adjustment won't get it low enough, and the lin coil is set correctly.

Grid bias on the HOT is high by 16 VDC: should be -56, but I get only -40 on the meter. Also, with the HOT pulled, I get a perfect waveform on the scope. Putting the HOT back in causes the waveform to become sort of flat-topped, which I don't think is good. I could be wrong, but I believe this means it's being driven as hard as possible and may be drawing excessive grid current. This and the high grid bias are likely related, but I can't figure out how.

At this point, I have subbed out the HOT, damper, shunt reg, horizontal osc, and vert output. None had any impact at all. The fly windings are in the ballpark with the resistance values called out in Sams, so I have a hard time believing it's got a shorted turn.

Only tubes I can't sub are the 3 rectifiers, since I don't have any 3A2's. Only thing I have left is maybe the shunt reg is set incorrectly, sucking away too much power from the HV. Wayne says he will stop by with his HV probe tomorrow evening, so we can check. Beyond that I'm not sure where to go next, short of cap shotgunning and a test of all the tubes left in the chassis.

I apologize for not making the resistance measurements in a more expedient manner. Weather and family issues are, or were, weighing on me heavily.

I will look at my RCA service literature again for this receiver to see if I can offer any useful guidance. It is doubtful, in my opinion, that there exists a shorted winding on the horizontal output transformer. In my experience, even one shorted turn results in catastrophic effects; nonlinearity, foldover, or complete failure of the transformer after a few minutes of operation. A “ring-down” test is simple to execute and provides a pretty comprehensive assessment of the transformers health. Certainly you are familiar with this, but for readers who may not be: http://www.repairfaq.org/sam/flytest.htm

While a transformer failure isn’t out of the question, those employed in the CTC-2B chassis were far more durable than say ones in later models like the 16’s or 17’s.

The drive waveform asymmetry would be a place I’d start investigating.

Regards.

The drive waveform asymmetry would be a place I’d start investigating.

Agreed. So I understand that the waveform applied to the tube is what develops the bias, but what could cause it to be too high? It's possible the high voltage doorknobs are getting leaky, causing HV to sag. I already repaired one that had fallen apart, but I'm not sure it's possible to get an exact replacement anymore.

Any chance the HOT grid coupling capacitor might be leaky, raising the grid voltage?

Any chance the HOT grid coupling capacitor might be leaky, raising the grid voltage?

That was the first thing I replaced.


Wayne stopped by tonight, so we had some fun. First we used the HV probe, which revealed a perfectly healthy 24.5kv. The set makes a nice picture when driving a composite signal directly into the video tube, but going through the tuner/IF produces snow. There's still some kind of load on one of the B++ lines, it's still measuring 40 odd volts low. We tested all the tubes I haven't already subbed, no surprise we didn't find a single one bad.

This thing is kicking everyone's ass all over the place, we just can't figure out why it's doing this. I have my suspicions about a leaky cap still, but the only thing I have left as far as troubleshooting is a shotgun recap of the leftover micas.

CT-100: is there any way you can give me a current reading from both legs coming out of the ballast tube? Just insert the meter between pins 2 and 3 on the ballast and let me know what you find. At this point I have nothing else to go on.

Refering back to your first post: When you replaced the two voltage double caps in the LV PS (C103 & 104), did you replace with correct 200 uf, or did you put new 80 uf back in?

If your HOT grid bias is too positive, that could be caused by not enough grid drive. Do you have the correct 200 and 270v on the plates of the Horz OSC tube? Does the drive signal amplitude change much with the HOT pulled? How's the voltage on the screen grid of the HOT?

If it can be of help I have a heathkit eye type cap checker that I recaped reciently which I'd be willing to lend to you. It seems to work well... new caps test good and old ones in various TVs test anywhere from good to completely dead.

Tom C.

I suppose I should have my own by this point, but I've always taken the position that if in doubt replace it.

Another day, another 12 caps replaced, still no change. There's definitely hum bars rolling vertically up through the picture when using an RF signal through the tuner, perfect picture when injecting composite into the video amp. I'm at a loss.

Have you considered a possible heater to cathode short in a stage prior to your baseband video injection point? That is the Tuner through the Video IF. A heater cathode short could inject the hum and could create a higher current draw if a stage is dependent upon a cathode resistor bias arrangement. And a partial short is sometimes not obvious in a tester.

Terry

I thought the tester would catch something like that, but I suppose anything is possible. I don't have any 6DC6 tubes, so I can't sub anything in the IF strip. I can probably borrow a complete set from Bob G though, I'll ask next time I see him.

A 6CB6 or 6BZ6 is a reasonable substitute for for the 6DC6 for testing purposes. The tranconductance is a little higher for these tubes (8000umho as opposed to 5500umho for the 6DC6).

Of course you would not want to mix up the existing tubes or necessarily permanently exchange the IF tubes as realignment would be advised. But I would try a little step by step substitution to see if the hum clears up.

One other thing: when you inject the baseband composit signal to the video amplifier, do you deactivate the RF/IF section? Or does the hum only appear when the signal is applied to the antenna?

Also, is it 60Hz hum or 120Hz hum?

Bought some IF tubes today, didn't make much difference. The noise got a little lighter, but it's still there. I'm not sure how to tell what frequency this is, I only know that there are bars floating up the screen. I'd shoot a video, but I'm not sure the camera would pick up on it.

The noise is only there when passing a signal through the tuner/IF, when I put composite on it the tuner/IF were not powered and there was no noise.

I also have an unrestored heathkit cap checker I'd be willing to sell that is burried in storage, but can probably be located next time I go there, if you want to own one.

As long as you take that Silvertone in trade, I'm game. lol

120Hz is ripple from the B+ supply. With 120Hz you will see two distinct bars floating up the screen.

60Hz will originate from the mains directly and you will see a single bar.

How many bars are there? Can you post a photo of the screen with the bars present?

And have you substituted the tuner tubes?

If you are refering to the console that I did not take then the only way that will happen is if I sell some TVs or toss some out. My folks have ordered me to stop collecting anything bigger than large table radios for an indefinite period of time.

Tom C.

Yes I can make the measurement, but this again, is going to take a few days. My primary delay is that I have not powered my chassis up since I am working on the CT-100. I am aware of your request and will do what I can as quickly as I can. If you lived in my neighborhood, I'd give you my chassis and you could take it to your lab for precise comparisons.

I have a lot of background with this particular set as my dad brought 5 or 6 of them home as they became obsolete; one even had an IF-RF problem like you have described. We're talking 1968, so I have to review the schematic and try to reverse engineer the solution he discussed with me. (I couldn't work on sets then, he did and I only watched the process)

If the grid bias is too high, then the grid current must be too low [OR] the tube is being over-driven. I need to get my diagram; is there a cathode resistor in the deflection output; if so, is it by-passed? I will look this weekend for sure.

I'm max'ed at work and have not fairly studied the responses ahead of this, so forgive me if I am repeating suggestions. I admit, I have not done my homework yet so please take these comments with that in mind.

Regards.

120Hz is ripple from the B+ supply. With 120Hz you will see two distinct bars floating up the screen.

60Hz will originate from the mains directly and you will see a single bar.

How many bars are there? Can you post a photo of the screen with the bars present?

And have you substituted the tuner tubes?


To me it looks like a pair of bars on screen at one time travelling up through the pisture, but the power supply has no ripple so I'm not sure what to think. I've already subbed every tube in the tuner/IF and even the first video (6CL6), nothing made an improvement. If I had to pick a word, I'm flummoxed. :sigh:


If the grid bias is too high, then the grid current must be too low [OR] the tube is being over-driven. I need to get my diagram; is there a cathode resistor in the deflection output; if so, is it by-passed?

I think the tube is being overdriven, because it's DC grid bias is not what it should be. As I said before, I'm going on the assumtipn that the cause of the flat topped waveform at the grid is being caused because the tube cannot be driven any harder than it already is. Result: power comes from the only place it can, in the form of grid current.

It seems you explored the most obvious plus some of the most obscure prblems. maybe signal tracing with a scope is the best way to track this down.

Can to apply to the antenna a test signal of a flat field of grey. Ensure you can see the bar clearly on the CRT. Then 'scope back from the CRT stage-by-stage. I'd expect you should see the hum when scoped at the CRT and I would trust at some stage the hum will disappear from the scoped waveform.

At this stage I am as flummoxed as you...

Terry

I'd suggest trying to duplicate the SAMS resistance readings at the tube pins in the IF and tuner - might lead you to an anomaly.

You know what's interesting? My SAMS shows 60 Hz Hum even though this is, in fact, a full wave PS configuration.

My RCA data is quite explicit, the drive waveform geometry is critical to the proper operation of this circuit.......

Are you talking about that lower rail, with the .6 volt p-p wave on it? I think that comes from the vertical centering circuit, but I could be wrong.

I should have said ripple, not hum. I am referring to the waveform shown on the SAMS schematic at pin 5 of the ballast tube. It is shown as 60 Hz ripple. That is, two cycles at the 30 Hz rate. This must be an error since a full wave doubler should exhibit 120 Hz ripple, correct?

To me it looks like a pair of bars on screen at one time travelling up through the pisture, but the power supply has no ripple so I'm not sure what to think. I've already subbed every tube in the tuner/IF and even the first video (6CL6), nothing made an improvement.

I am still not clear about the hum frequency on the screen. The distiction is important because 60Hz points to a heater cathode problem and the 120Hz points to a power supply problem.

Is it possible for you to post a picture or two of the screen with the hum problem?

I am still not clear about the hum frequency on the screen. The distiction is important because 60Hz points to a heater cathode problem and the 120Hz points to a power supply problem. Is it possible for you to post a picture or two of the screen with the hum problem?


Not easy to get a picture of what I'm talking about, this is the best I can do.

http://videokarma.org/attachment.php?attachmentid=171663&d=1316235015

It's almost like ghosting. I can barely see it, but I know it's there. Whatever this is, it's related to the ballast overheating because both issues showed up at the same exact time. I figure something is loading down the power supply, which throws everything out of whack.

Some more questions:

Your right: it is hard to see whether there are two distinct bars or one. But the bar seems to appear sharply.

1. Which +285V is being pulled down: the +285V IF or the other +285V supply feeding the vertical centering control?

2. Did you change the rectifiers to silicon?

3. Have you tried putting back the old electrolytics for a test?

4. What did you last do before the problem appeared? (Did you see the problem before changing the electrolytics?)

5. What basis do you think the ballast too hot? (Between the two resistors, the ballast is going to dissipate close to 50 watts which will make it pretty darn hot!)

I wonder if by either changing the electrolytic capacitors in the doubler and/or changing the rectifiers you may have inadvertently introduced rectifier switching transients which are effectively being coupled to the RF/IF stages? A simple test for this is to place 0.01ufd 600v ceramic capacitors across each of the rectifiers.

My long shot guess is that if the capacitor impedance has changed maybe you are seeing the rectifiers switch on the screen?

1. Which +285V is being pulled down: the +285V IF or the other +285V supply feeding the vertical centering control?


Here's a rundown of voltages:

Mains input set at 116 with RCA Isotap.

Main 400v: 380
IF (coming from pin 2 of the ballast): 300
285 supply going to vert centering control: 260

As read at the filter cap of each respective line.

2. Did you change the rectifiers to silicon?

Yes I swapped some in briefly for a test, made no difference so I changed it back. Stock seleniums are still connected.

3. Have you tried putting back the old electrolytics for a test?

You mean the chassis originals? They are totally shot (open circuit), so no point.

4. What did you last do before the problem appeared? (Did you see the problem before changing the electrolytics?)

I didn't change or do anything really, just moved it from one room to another and it worked fine in the new room for a spell then this popped up for no reason.

5. What basis do you think the ballast too hot? (Between the two resistors, the ballast is going to dissipate close to 50 watts which will make it pretty darn hot!)

Hot enough to produce smoke, I'm not willing to let it go any longer than that...

My long shot guess is that if the capacitor impedance has changed maybe you are seeing the rectifiers switch on the screen?

I don't think it works like that with seleniums, but I could be wrong. Something about this set changed in a bad way to make this start happening, it was working fine one minute, the next it's all jacked up. I just haven't been able to track down the source yet.

I'm beginning to notice there are some issues with the horizontal oscillator. Pin 5 is right where it should be at 270, but the grid is sitting at -65 when it should be around -85. This in theory would cause the tube to conduct harder, leading to an increase in output waveform, which could in turn overdrive the HOT. This makes sense, since I'm no longer able to get rid of the drive lines in the screen even with the drive control all the way at minimum. There is a cap (C236 in the RCA manual) connected between terminal F on the hor osc sweep trans and tube pin 4 (grid) that if leaky, could cause this rise in grid voltage. Tomorrow, I'll see if I can find me a sub. This is exactly why I never trust these old micas, nothing but trouble and headaches if you leave them in.

I'm beginning to notice there are some issues with the horizontal oscillator. Pin 5 is right where it should be at 270, but the grid is sitting at -65 when it should be around -85. This in theory would cause the tube to conduct harder, leading to an increase in output waveform, which could in turn overdrive the HOT.

I have a gut feeling the horizontal oscillator fault is not related. The Horizontal and vertical sweep sections get their power directly from the 400v line.

The +285v line appears to feed only the luma, chroma, sync and RF unit (tuner). I assume you injected a baseband composite signal at TP101?

As a curious subnote, I noticed on my RCA Service bulletin for this set (First Edition-First Printing 11-24-54) that the connections to the RF Unit to the main chassis do not match. Connections 2 and 3 are reversed! Funny, it has to do with the +285v supply and the other is the 6.3vac filament connection. Do you see the same on your schematic?

I do not see a particular single point of failure on the 285v line which stands out. Can you reaffirm that you see a full color image when it is injected the baseband video?

I would guess that maybe another electrolytic has bitten the dust? Some more voltage measurements are in order. Good luck and we will look forward to the next installment.

I agree, if the HOT is pulling too much current, that could be pulling down the 400v line, but I don't see how a horizontal issue could be overheating the ballast. Is there any way the vertical centering could be pulling down the 285v, possibly a short in the yoke?

Here is what I'd try to localize the hum bars. Insert a signal generator into the antenna terminals, and set it to a VHF channel visual carrier frequency. Connect a bias box to the AGC line. Connect a scope to the video amp. Turn on the AM modulation on the signal generator and adjust the bias to produce a somewhat undistorted sine wave on the scope (I say somewhat because the AM modulation of most signal generators is not that linear). Turn off the modulation. Now connect a detector probe to the scope and work your way back, looking at the plate of each IF tube until you see hum on the scope.

A possible source of hum is the AGC line, by the way.

I would not describe this as hum from what I saw when I was at Nick's - more like noise bars, which could be rectifiers.

Edit: also, there's some noise that seems video related, like the video or IF is radiating back into the antenna. This all goes away when the input is baseband into the first video.

Also, the voltages Nick reads show a much greater drop across the ballast resistors than what SAMs says, if I recall correctly, which would explain the excess heat. That says there's some kind of short, but finding it has been difficult. If it's in the IF or tuner, it might be possible to isolate it by resistance readings on the tube pins (as compared to the SAMs resistance chart). It's too bad SAMs doesn't have resistance readings from the pins of the ballast socket, which would isolate it to one branch or the other of the B+.

I had a bad short in a B+ line in my 71' Zenith reciently, and I tracked it down by unhooking the different branches of the rail, and checking the individual resistances to ground and then going after the one with the lowest resistance till I found the exact spot that was shorted. Since you can still run it you can probably find fault by disconnecting everything from the overloaded rail, and slowly reconnecting things untill the supply voltage drops drastically.

Just a thought.

Tom C.

I took a bunch of readings tonight, all done with an live picture on the screen and all circuits functioning.

Pin 3 of the ballast is a 430 ohm resistor, which feeds all 285v circuits that are not IF/tuner related. The voltage drop accross it is 96 volts, which according to ohms law means it's dissipating 21 watts. Inserting the current meter indicated that the circuits are drawing about 88ma from that pin.

Pin 2 of the ballast is an 800 ohm resistor, feeding IF/tuner. Voltage accross it is 70 volts, indicating a 6 watt dissipation. The circuits of this leg only draw 34ma, but that makes sense to me since there are a lot more plates to feed on the other leg.

Horizontal is still goofy. Something has GOT to be causing the HOT to draw excessive current, but I cannot figure out what it is. The loaded down horizontal section has got to be what's causing all the other issues, it's the only thing in common with every part of the set. Whatever it is, it doesn't seem to be affecting HV though.

You could always try adding a 100 ohm 10 watt cathode resistor on the HOT, just to see how much the current drops, and to what effect it has on the other symtoms. Is there any possibility there is leakage between the HOT socket pins?

Horizontal is still goofy. Something has GOT to be causing the HOT to draw excessive current, but I cannot figure out what it is. The loaded down horizontal section has got to be what's causing all the other issues, it's the only thing in common with every part of the set. Whatever it is, it doesn't seem to be affecting HV though.

Excessive horizontal output tube current can be due to a number of things. Is the horizontal drive correct? Excessive drive will cause excessive current.

What are the voltages around the tube? What does the horontal linearity look like? Depending how the linearity looks, that will provide a clue(s). How about a picture of a crosshatch pattern on the screen?

Have you checked and substituted the H O Tube? (Sorry, HOT to me means horizontal output transformer not tube). It could be gassy.

I am curious you say the EHT (+25,000v) is okay. How much curent is the 6BK4 shunt regulator drawing? It could be the EHT is low or high depending on the shunt current. Without the shunt, the EHT voltage should jump to +35000 to +40000. (Reminds me of the time my Grandma was watching her CTC11 and the shunt tube was bad. Her cat would like to sleep on the top of the set and she wondered why the cats hairs stood out. The EHT with my divider probe and VTVM measured +40000V. I suppose the cat had an especially good dose of Xrays when passing in front of the screen!

Terry

Excessive horizontal output tube current can be due to a number of things. Is the horizontal drive correct? Excessive drive will cause excessive current.

Drive is excessive, but I can't figure out why. I can't even get the drive lines to go away with the control and the oscillator tube has incorrect grid voltage as well, not sure where to go with this.

What are the voltages around the tube? What does the horontal linearity look like? Depending how the linearity looks, that will provide a clue(s). How about a picture of a crosshatch pattern on the screen?

Lin is fine, nothing amiss there.

Have you checked and substituted the H O Tube?

2 different ones, it's not the tube.

I am curious you say the EHT (+25,000v) is okay. How much curent is the 6BK4 shunt regulator drawing?

Don't know, when Wayne was here we checked the HV and it was fine. If the shunt were set incorrectly, I would expect it to be much higher, or if it were redplating much lower.

Drive is excessive, but I can't figure out why. I can't even get the drive lines to go away with the control and the oscillator tube has incorrect grid voltage as well, not sure where to go with this.

Since we can assume the oscillator is on frequency, have you set up the synchroguide with the horizontal locking range as per the service info?

Is the horizontal waveform amplitude applied to the grid of the 6CB5 correct?
Have you replaced C232 (0.01ufd)?

Does the horizontal drive control work properly eg do you see a change in the picture?



Lin is fine, nothing amiss there.

Are you sure they are drive lines? (I am suspicious because drive lines = bad linearity and you said the linearity is okay). If you rotate the horz hold so the picture moves to the right, do the lines remain stationary (drive lines) or do they move opposite to the picture movement (retrace line image)?



Don't know, when Wayne was here we checked the HV and it was fine. If the shunt were set incorrectly, I would expect it to be much higher, or if it were red plating much lower.

I was wondering if the HV reserve is there. I guess if you are getting lots of drive, the HV is okay. But if the brightness is advanced, does the picture bloom or does the width increase? (This would indicate the regulator is dropping out of regulation and the HV being generated is insufficient).

Since we can assume the oscillator is on frequency, have you set up the synchroguide with the horizontal locking range as per the service info?


Yep, everything is set up right and I have equal peaks on the wave coil.

Is the horizontal waveform amplitude applied to the grid of the 6CB5 correct?

Yes, but the waveform has a flat top.


Have you replaced C232 (0.01ufd)?

First thing I did.

Does the horizontal drive control work properly eg do you see a change in the picture?

It works, but because the oscillator is making such a large waveform it doesn't get rid of the drive lines.

Are you sure they are drive lines? (I am suspicious because drive lines = bad linearity and you said the linearity is okay). If you rotate the horz hold so the picture moves to the right, do the lines remain stationary (drive lines) or do they move opposite to the picture movement (retrace line image)?


The line moves opposite of the picture. That means it's not a drive thing?


I was wondering if the HV reserve is there. I guess if you are getting lots of drive, the HV is okay. But if the brightness is advanced, does the picture bloom or does the width increase? (This would indicate the regulator is dropping out of regulation and the HV being generated is insufficient).

No blooming, seems to have plenty of brightness. BTW, Wayne is probably right about the picture. It seems to be noise, not hum or anything else.

The line moves opposite of the picture. That means it's not a drive thing?

Yes, that means that either the retrace is a bit slow or the CRT is not blanked off during retrace. It isn't a drive line that you see.

The one white line is probably a video overshoot meaning that the perhaps a sync pulse edge is causing the line on retrace. The multiple lines to the left of it are the individual cycles of burst. The overshoot will generally be due to IF alignment or if the fine tuning moves the video carrier too far down the Nyquist slope. Do you see the line with the video injected at baseband?

I have seen the line made more obvious if the CRT RGB screen adjustments are set too high: they have to be set to ensure the beam current extinguishes on retrace.


No blooming, seems to have plenty of brightness. BTW, Wayne is probably right about the picture. It seems to be noise, not hum or anything else.

Then that sounds okay to me. If it is picked up at the receiver front end, could it be your workshop fluorescent light?

Since it's not a vertical blanking issue, how exactly does this chassis blank horizontally? I don't see a blanking tube.

Since it's not a vertical blanking issue, how exactly does this chassis blank horizontally? I don't see a blanking tube.

There is not blanking circuit as such. Blanking is done by the video waveform. Hence if the if there is a bad VIF response or misadjusted fine tuning, high video frequencies will overshoot producing a white line.

Try adjusting the fine tuning and reduce the RGB CRT screen adjustments.

Here's a scope trace, maybe you can get something from it.

http://videokarma.org/attachment.php?attachmentid=171676&d=1316388575

Bottom is the composite output from the generator, top is from the grid of the second video amp tube. It does look like there's some fuzz on the blanking pulse, like the whole signal slid over a little. If so, are we talking sync issues or IF issues?

Here's a scope trace, maybe you can get something from it.

http://videokarma.org/attachment.php?attachmentid=171676&d=1316388575

Bottom is the composite output from the generator, top is from the grid of the second video amp tube. It does look like there's some fuzz on the blanking pulse, like the whole signal slid over a little. If so, are we talking sync issues or IF issues?

Maybe IF and maybe sync. Maybe both. What I am trying to say is that if the video frequency response is not flat or the phase response linear, you can get an overshoot at the sync pulse rise and fall. Getting a good wide bandwith with the tube technology of the past was a bit of a challenge. The problem is exaserbated with the use of vestigial sideband trasmission: broadcast transmitter design went to great lengths to try to mitigate the problem.

Also the broadcast sync waveform had a carefully controlled rise and fall time to help reduce this effect. Your test generator might have too fast a rise and fall time which may cause problems. Your modulator, like most consumer modulators, is double sideband so that should not cause a problem.

If your IF response is not flat or if the video carrier is too low on the Nyquist slope of the VIF response, this could lead to the white line appearing. I have attached a jpg photo I found on the web to illustrate what I had expected the sync to look like as applied to the CRT. Your waveform seems to have very fast rise/fall time sync pulses. Try expanding the waveform around the sync fall as an overshoot there may be seen in the video as the white vertical line.

Are you injecting at baseband or RF input? If at RF, if you adjust the fine tuning control to raise the video carrier to the top of the Nyquist VIF slope (that is adjust until you begin to lose picture detail and color and picture starts to smear) see if the line diminishes.

Lastly, consumer DTV boxes have proper broadcast sync rise and fall times so see if another video source causes problems.

It's getting worse, now an RF signal with a staircase won't even get through. All I get on screen is a jumbled mess, horizontal tearing, ect. I think it's high time to pull this chassis for a good while and put the B&K 415 on it, real detective work is in order.

I'm on mobile and cannot easily follow all responses. With regard to your hum, did you bypass your Si diodes with .001's? That looks like switching noise to me.

I decided to bypass the IF for the time being, injecting video into the 1st vid amp and disconnecting power to the IF strip. Found something odd: the bandpass amp grid is registering 0 volts, when it should be -15 or so. I can't figure out why it's not biased right, all the resistors in the circuit measure correctly and the caps are new. With the grid at 0, that tube would draw more current that it's supposed to which might explain the power supply drain. It might also explain the dot crawl I'm getting now.

For continuity, this problem was finally fixed here: http://videokarma.org/showthread.php?t=252849


The voltages on the bandpass tube ended up being a Sams typo, the CT-100 chassis had the same readings. After fixing the problems detailed in the thread above, dot crawl is much better as well. I can probably make it disappear by tweaking the 3.58 trap in the luma line. So glad this is finally over. :smoke: