21-CT-55 power supply failure

[miniman82]

Originally Posted by Penthode

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.

[miniman82]

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.

[Penthode]

Originally Posted by miniman82

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.

[Kevin Kuehn]

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?

[Steve McVoy]

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.

[old_tv_nut]

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+.

[Electronic M]

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.

[miniman82]

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.

[Kevin Kuehn]

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?

[Penthode]

Originally Posted by miniman82

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

[miniman82]

Originally Posted by Penthode

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.

[Penthode]

Originally Posted by miniman82

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).

[miniman82]

Originally Posted by Penthode

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.

[Penthode]

Originally Posted by miniman82

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?

[miniman82]

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