1958 RCA 14-PD8057 Horizontal Issues/Circuit Board Nightmares

[Yamamaya42]

This is where it's good to have a LCR meter.
It's said that ceramic disc capacitors for the most part never go bad, but they also say never say never!
You would be more likely to find a dodgy resistor than a disc cap, but you never know!

[Chris K]

What do you think would be the implications of C99 being a 10,000pf cap instead of 1000pf? If it wouldn't make a difference, I'd like to minimize how much work I need to do under this board. I'd just as well leave it there if it makes no difference.

[Yamamaya42]

it may have something to do with the sawtooth shaping, not sure, but if that's what they got in there.

[Chris K]

I think there's a chance RCA didn't put it in there. The Sams shows a 1000pf in the capacitor list, schematic and it looks like the board photo is of a mica cap.

[Yamamaya42]

At this point there is no way to know, there should be clear evidence that someone has touched it before, disturbed solder joints vs OEM, joints, if it does look like it was replaced, you have nothing to lose by trying what's on the SAMS, but this also can be a case of an update after the SAMS was made, if it makes it worse, it's not like it will go FOOM right away, it just won't work any better.

[kf4rca]

I had trouble with a synchroguide circuit once and it turned out to be a 330K resistor had gone sky high.
Here's a tutorial on the synchroguide circuit:
http://rca.vobj.org/RCA%20Engineer/R...ignHistory.pdf

[Chris K]

Thanks! The resistors in the circuit I've tested so far have been close to the stated value. The tearing in the upper part of the picture in the RCA tutorial is exactly what I'm seeing.

[Chris K]

Still no joy on this project. I’ve checked all the horizontal waveforms at the test points and they all look the way they’re supposed to look. Frequency still low at 14KHz. I changed a 120pf original capacitor in the circuit and it’s better but not right. There’s a couplet in the AGC circuit. Could that be affecting the circuit?

[Yamamaya42]

From what I understand, the horizontal oscillator should be able to free run at 15.7khz approximately without any external influences, IE, no rf signal or anything, the incoming video only keeps it locked in step as needed to hold the picture to be seen, so that circuit before the horizontal oscillator (couplet), really should not be having any effect for you to tune it to 15.7khz, something else weird is going on.

[Chris K]

I agree. I think I'm going to have to go down the rabbit hole and at the least, partially remove this circuit board and look at the traces and grounds. Ugggggh! I've been watching CuriousMarc's channel obsessively for the past month and maybe I need to take their approach...detailed analysis and then dive in. If they can reverse engineer Apollo core rope memory literally stored on bundled wires, maybe I can troubleshoot this board.

[Chris K]

Originally Posted by Yamamaya42

From what I understand, the horizontal oscillator should be able to free run at 15.7khz approximately without any external influences, IE, no rf signal or anything, the incoming video only keeps it locked in step as needed to hold the picture to be seen, so that circuit before the horizontal oscillator (couplet), really should not be having any effect for you to tune it to 15.7khz, something else weird is going on.

The truth is in the whacked picture but maybe I need to evaluate the frequency on another scope. I'm using my HP digital which has always been accurate when assessing frequencies.




The waveform at this point is easily manipulated by B1 and I have it dialed in to the exact shape seen here. I also have a sawtooth waveform at W12 but, again, the same off frequency signal. I'll play around with it tonight and double check for idiot based errors. The tubes test good and I have switched them with known good tubes, the tube sockets are OK and the 5% and 10% resistors are in tolerance.

But again, I'll see if I did something stupid.

[Penthode]

Coming to this thread late. Have you tried for trouble shooting shorting out the stabilization coil? It appears to be called Horizontal Waveform here and a short can be placed across the 48 ohm coil. Then you should be able to first adjust the horizontal hold coil to sync.

If with the other coil shorted you cannot get sync then something in the circuit is at fault. But that would be my first step tackling this.

[Penthode]

I want to add that I left the stab coil shorted in my RCA Victor portable. It improves VHS tape playback performance (less bending at the top of picture) and it allows me to easily adjust the oscillator to 10,125Hz for UK 405 line display.

The stab coil was only introduced to marginally improve noise performance on very weak signals. Feeding modulators with strong signals it accomplishes nothing today

[Yamamaya42]

So, one should be able to get a 15.75khz shark fin waveform at point W12 with the frequency being adjusted significantly by L24, above and below 15.75khz, and having Horizontal Waveform shorted or not should have little effect on this.
However, influence from the sync amp given that it's tuned to a good video input, will adjust the oscillator to some degree.
I just find it a bit odd that you say that it's running at 14k, cause if it was that low, would there not be any lock at all and just a jumble on the screen horizontally?

[Penthode]

Some screenshots would be useful. I would like to see the stab coil shorted and screenshots as it is tuned to sync.