Video can't get through IF on Portacolor
 

Recently acquired a mid 70's HE chassis PC. After the first step of replacing ceramic white tubulars in the H section in order to obtain HV, I set out to address the absence of audio. I was able to get video, but only on channel 4 (very weak signal on ch 3 and fine tuning doesn't seem to help.

Pic of set running before video went out (there was no audio at this point)

https://i.ibb.co/Rc0BWnx/1.jpg

I had removed a couple of burnt resistors in the audio section and had to pull the board out somewhat to do this. after replacing the board HV became intermittent until I fixed a couple of broken/cold solder joints as a result of moving the board (apparently the solder joints had bumped a metal board screw housing)

HV returned, but now video was completely gone.

Injecting into the 2nd, 3rd and video amp produced a picture, but if I injected into the 1st IF, no pic nor sound. Why? well, probably because the voltage on pin 10 of V3 reads -17v (should be .1v). Something is dragging that pin down, but I couldn't see any source of negative voltage.

https://i.ibb.co/QXv96L9/Screen-Shot...0-44-27-AM.png

Further probing showed that on pin 9 of V6 (AGC) there was -70v! There should be .2v, according to the schematic. Keying waveform is correct here, and the correct amplitude. The 11BT11 tube tests fine.

Examining the board, I see no cold joints or bad connections in the area of the AGC, but things are very cramped there as we know, so it's hard to be sure.

I experimented by clamping the agc down (TP2) to where it should be, while I fed RF into the antenna terminals, and was able to get a weak picture and decent level audio. Just putting a Sencore bias box on that terminal clamped sufficiently, I didn't have to apply more than a volt. Something about that box that shunts voltage to ground, which is another issue. Shorting TP2 to ground, btw, has no effect on pic or sound.

By the way, resistance on pin 9, V6 is well below what it should be - read 436k, should be 1.1M, according to Sams.

Probably looking at a bad resistor, but what I don't get is the large NEGATIVE voltages on pin 9, V6 and pin 10, V3. The other voltages on pins 3 and 4, V6, are correct. Next step is removal of the chassis to have a closer look to lift a few resistor legs and to check again for cold/open joints.

I'm probably about to save you a bunch of time....Clean the tin whiskers out of the AGC pot.

I had this exact problem on a portacolor I used to have and it was whiskers in the AGC pot, others have had the same issue and cause. I've also heard of other rear pots on these sets getting whiskers. It's a known problem.

Thanks Tom.

That's the one pot assembly I haven't cleaned yet. I did find tin whiskers in the other two, but I didn't see anything from the outside on that one, so I let it be.

Edit: I did clean this assembly, but no improvement. I didn't spot any whiskers in it, either...

Did you have the negative voltage readings on the 1st IF input and AGC plate as well?

I recall the 1st IF and AGC voltages were badly negative. I believe the grid voltages on the 2nd and 3rd IF were too negative on mine as well. IIRC I remember looking over the circuit and concluding the the plate current in the 1st IF biased the 2nd stage and cascades the same way to the 3rd.

Granted it has probably been more than 4 years since I dealt with that issue.

Are you saying though, that just cleaning the AGC pot fixed this condition?

More like just adjusting it in my case... Though I definitely followed up with a proper cleaning.

IIRC it was a case to element whisker and I was able to verify it's presence before and absence after adjusting the pot.

While adjustment of the AGC control does vary the voltage on pin 4, V6, there is no effect on picture. V6 tests ok on tester for emissions.

Biased pin 9 on the agc keyer down to -18 and got a reasonably good pic and sound. So, something’s up with the keying of that system. It’s a straight shot all the way from the flyback to the plate of V6B, so no caps or resistors in the path. Since V6B derives its video sync pulses from the sound sync amp (also V6, section C) there may be a faulty component in this path, possibly a shorted one delivering too much signal to pin 3 and thus biasing the agc negative. Further investigation needed.

Quick video of set playing with bias on pin 9 of AGC keyer.

https://m.youtube.com/watch?v=nrhhw7jPFok

Been reading old AK threads trying to get any info about this issue.
In this thread an issue is mentioned about the focus pot being in close proximity to the AGC and current from that leaking into the AGC circuit and producing problems.

http://www.videokarma.org/archive/in.../t-128371.html

I did voltage readings on the AGC control pins but didn't see any radically high voltages, just a bit above and below the 60v indicated on the plate of V6B.

I'm thinking it has more to do with the received video sync pulses on V6B, pin 3, which if too strong, is my understanding, will create a negative potential on the plate of V6B. Will get some scope readings today.

Can you point us to a full schematic? Hard to tell what could be causing -70 volts from the part you posted.

Sure, I think this should work...

https://drive.google.com/file/d/1TGh...ew?usp=sharing

Thanks!

Update: R192, 193 check ok out of circuit.

This the waveform I got on pin 3, V6. It looks like about just under 10v pp. AGC control does change the amplitude of the waveform, but no effect to picture. If you compare it to the waveform in the schematic, it looks a bit off? I'm not sure if I should see any of the video waveform here at all?

(image removed for update)

I would not usually jump to suspecting a ceramic cap, but there is a path through C508 in the burst gate circuit to a point that normally has -85 volts. If C508 is shorted, it would also reduce the resistance measurement to ground at V6B pin 9.

Worth checking as another elimination step, anyway.

EDIT: Any possibility of an external short across C508?

Check resistance at V7 pin 4.

Any possibility of an external short across C508?

Oh my gawd...Old TV Nut guessed it...

Sure enough, C508's leads got bent into a nice bridge, from pulling the board out and hitting that metal screw housing. It was in a tough spot, hidden by the plastic cabinet, but sure enough, there it was!

https://i.ibb.co/qkjJV9X/1-2.jpg

Still some work to be done, but I have pic and sound again!

Thanks goes out to Old TV Nut :beerchug: