Zenith 19L25 Chassis Restoration - Advice?

[jr_tech]

I would also double check the capacitors in the video output circuit for correct value and wiring...it looks to me as if you get a decent black at the edges of a transition to a black area but it does not last, as if there is a cap somewhere in the circuit is too small a value. Does this set have DC restore circuit?
jr

[RitchieMars]

Here's something I was wondering about ever since I recapped my set...



To mount these two filter capacitors I used the lugs of two of the vertical output tube's unused pin sockets. It was convenient this way because I could still arrange that 470 ohm resistor between them and send them both to a common ground, which is also being shared with a 47uf from the audio output tube. The big 80uF filters from the low voltage rectifier, and the 30uF filters pin 8 of the audio-out. I just thought I'd mention this in case there's a chance that there might be a problem with this arrangement. It concerned me that there might be something about the close proximity of these unrelated tubes/circuits that could cause any of the problems I've had.

None of the capacitors I replaced are of the wrong values. A couple of the 40uF have been replaced with 47uF, which should be fine. If I'm reading this right, however, this .002 should be a .001?



Oh, unless this was the .001 I was looking for...?



There's three red "domino" caps in this set. Never dealt with them before.

Anyways, I was able to watch some tv on my set last night and today with some buzz here and there. If I played old movies from the DVD player, it worked fine. But if I put in something more modern, there were sudden scenes of bright to dark and everything got buzzy. I just pulled the chassis back out again and I found a 100k resistor between the contrast control and the video amp tube that was reading about 40% out of spec, so I replaced that. I'll let you guys know if I find anything else.

Oh, and I'm not sure about the DC restore circuit. I can scan some of the schematics if it would be of any help.

[RitchieMars]

Here's a clip I recorded earlier today after making some adjustments to the yoke and centering rings. I still don't have the contrast I want, and the buzz is still there on some DVD's and the cable channels, but I can certainly tune it out well enough to play these black and white movies!

http://www.youtube.com/watch?v=ofUCT57sfRM

[RitchieMars]

Here's what my Ryder's manual for this set has to say about sound alignment:

"1. Tune in a tone modulated TV signal and adjust the step attenuator until the signal is reduced to a level where "hiss" is heard with the sound.

2. Adjust the sound take-off coil L19 ( top and bottom slugs ), intercarrier coil L22, quadrature coil L2 and buzz control R39 for the cleanest sound and minimum buzz. It must be remembered that any of these adjustments may cause the "hiss" to disappear and fruther reduction of the signal will be necessary so that the "hiss" does not disappear during alignment.

If intercarrier buzz is in evidence, after all normal sound adjustments have been made, the cause may be attributed to one or more of the following:

1. Improper adjustment of the AGC delay control.

2. Defective 6AU6 sound limiter

3. Extremely high signal levels which require attenuation of the antenna circuit.

4. Transmitter over modulation."

I think I should locate these adjustments and have a look at it. The most believable fault here to me would be the 6AU6. I just cant imagine that these adjustments were ever in need of changing from the factory, so I get the impression there is a component failure that's pushed this out of alignment.

The contrast issues are another story...

[jr_tech]

Originally Posted by RitchieMars

Oh, and I'm not sure about the DC restore circuit. I can scan some of the schematics if it would be of any help.

It might not hurt to scan in the circuits between the last video IF stage and the CRT... perhaps we can spot some potential trouble spots, and the DC restore circuit (if any).

jr

[RitchieMars]

Alrighty, here's a scan showing the last stage of the video section, and the vertical section is visible below it. I left all that in there because the AGC is controlled by half of the 12AX7.



Again, the symptoms include a washed out picture and virtually no contrast when the picture is adjusted to maximize sound output. Turning down the AGC improves contrast somewhat at the cost of sound, and eventually an unstable image. Brightness on the whole is fair, I suppose. It wouldn't look so great in a well-lit room.

[DaveWM]

comp signal applied at video test point, determine if proble is before or after that point. (pin 2 V6)