21-CT-55 power supply failure - Page 4 - Videokarma.org TV - Video

miniman82 · #1 09-17-2011, 11:27 PM

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 · #2 09-18-2011, 01:27 AM

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 · #3 09-18-2011, 01:44 PM

Quote:

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 · #4 09-18-2011, 02:22 PM

Quote:

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.

Quote:

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.

Quote:

Have you checked and substituted the H O Tube?

2 different ones, it's not the tube.

Quote:

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 · #5 09-18-2011, 02:58 PM

Quote:

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?

Quote:

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)?

Quote:

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).

Electronic M · #6 09-17-2011, 11:22 PM

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 · #7 09-19-2011, 12:35 AM

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.

CT-100 · #8 09-19-2011, 06:36 PM

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.

miniman82 · #9 12-10-2011, 05:27 PM

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.

miniman82 · #10 12-16-2011, 05:09 PM

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.