21-CT-55 power supply failure

[miniman82]

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.

[miniman82]

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.

[CT-100]

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.

[miniman82]

Originally Posted by CT-100

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.

[Zenith6S321]

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

[miniman82]

Originally Posted by Zenith6S321

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.

[Kevin Kuehn]

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?

[Electronic M]

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.

[miniman82]

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

[miniman82]

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.

[Penthode]

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

[miniman82]

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.

[Penthode]

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.

[Penthode]

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?

[miniman82]

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.