21-CT-55 power supply failure

[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?