Sync symptoms

[W9RAN]

Doing my first TV restoration - a Philco Predicta Princess. The good news is, after replacing some electrolytics and a few other caps, I was rewarded with a good picture and sound. Problem is, it doesn't stay that way for long - after about 15-20 minutes of operation the sync starts to go. The problem seems to affect both horizontal and vertical sync, It starts with tearing along the side of the screen but soon just becomes unwatchable and neither horiz or vert controls help. However the video brightness remains unchanged and the sound is still fine, which reinforces my theory that the problem is in the sync circuit.

If I turn the set off and back on again after a short time, the problem remains just as bad as it was. But if I let everything cool off for 30 minutes or so, it'll work fine again, until the problem recurs after 15-20 minutes.

Tubes all checked good but that doesn't mean anything. I haven't found any overheated resistors and don't see any likely suspect caps in this part of the circuit. I thought perhaps the symptoms would sound familiar to someone with more experience - thanks in advance.

Bob

[DaveWM]

check R43 monitor voltage at pin 3 of the syn sep tube 6AW8. sub that tube

sub the 6EA8 tube.



Try injecting a composite video signal at Test point C, you can use the video out of a DVD player.

try switching the normal and distance switch a few times, monitor the AGC voltage at test point A. You did replace C11 a .22 cap correct?
a long shot on that cap.


Try clamping the agc with neg 6v at test point B and neg 3v at test point A, you can use some dry cell batts for this if you dont have a variable supply.
again if AGC voltage is the problem


check the AFC diode with a diode checker (actually if you are losing vert as well this is prob not the problem).


spray the K6 with cool juice, actually do this 1st.

[DaveWM]

btw the AGC will effect the signal enough to clip the sync pulse out, but allowing the audio and video to get thru, causing tearing ect.

by injecting the composite video you can determine where the defect is, either before or after that point. if it still goes bad with the composit video, then the IF and AGC and Tuner are prob ok.

since its heat related I would think the cooling spray on the sync sep network would most likely prove that out.

[DaveWM]

any progress?

[Eric H]

If you haven't already I would just replace all the caps on the PC board, and the all the electrolytics in the set as well.

The set is so difficult to work on it doesn't make sense to try and figure out which individual cap is the problem.

I do think this particular problem will be in the area of the sync separator tube though.

[W9RAN]

Thanks Dave and Eric. Here's what I've found so far:

I'd replaced C11, and the AGC seems to be working OK, going from -.4 volts with no signal to -2.4 volts with. Changing the AGC switch only causes it to vary by .1 volt or so.

I've injected composite video in to test point "C" and get the same symptoms. So this at least should eliminate the RF, IF, and AGC circuits.

I was optimistic after seeing "component cooler" on the Radio Shack catalog that they might have it in the store, but to paraphrase the Eagles, the manager told me in essence that they "haven't had that spirit here since 1969". So I'll have to order a can I guess. In the interim I tried using an air compressor, and proved that indeed I could restore proper operation by cooling *something*, but wasn't able to pinpoint which component. But it was in the vicinity of the 6AW8, so I'm going to substitute that.

I'm also wondering if it might be a mechanical issue like a tube socket pin that warps open when hot. I've had trouble with poor connections on this and the horiz. osc. tube - to the point where filament voltage is lost, despite cleaning and trying to tighten up the sockets.

Based on my radio repair experience I tend to look at a capacitors function in the circuit and replace those where it makes sense in most cases. But you're right Eric, this is one where if I end up pulling the PC board out, I'll do 'em all.

Anyhow - thanks again, and I'm sure I'll get to the bottom of this. I remember modifying old TVs in the 70s for use with early home computers and should have remembered the composite video idea.

Bob

[DaveWM]

I just use the reg dust off, hold it upside down and you will get some cools stuff.

yes sockets are often a problem, you could try just wiggling that tube around.

I had a couple bad sockes on my princess. Also the pcb ground stakes can cause issues, I had a intermittant damper filament that was traced back to a poor solder joint on a ground stake.

FYI I was able to replace all but one or two of the caps without pulling the pcb, so if your sockets are ok you may want to try that approach and not make more work for yourself. Good luck with it. I love my predicta, watch hitch on it every night.

[ctc17]

These symptoms always seem to be caused by caps for me. Not electrolytic but paper and domino style and freeze it never seems to correct the difference.

Its almost like the chemicals move around in the cap or something from having power applied, not heat.

[freakaftr8]

Possibly a resistor near a tube socket that heats up? I ran into this in a maggie once in the horiz AFC section. Also I found a bad cap, orangeish brown square with color dots (don't know the technical term for this component). But it was bad and exibited the same symptom.

[W9RAN]

I think I've finally isolated this problem - would appreciate thoughts from the group before proceeding.

Cooling didn't produce any results, so I started doing voltage and waveform measurements. I have proper waveforms at the AFC diode (W11 and W12) on the Sams) - but virtually no bias or signal on the grid of the 6CG7 horizontal multivibrator. I've checked the values of the resistors that I can access from the pins of the K6 Couplate, and they're within tolerance, and subbing for the 220 mmf doesn't help. So my best guess is that the sync signal is being shunted to ground by a failed 50000 mmf capacitor inside the Couplate.

There's a little bit of really noisy signal (<100 mv) visible on the grid, but nowhere near the 3.5 volts the Sams specifies. Maybe it's enough to sync the horizontal for a few minutes, then the horizontal just goes open-loop which would explain the symptoms I have.

Unless I'm going wrong somewhere I reckon the next step is to make a replacement network using discrete components and see if I'm right.

Thanks again,

Bob

[DaveWM]

before digging into the K6 did you check C45 the cap at the grid of the 6Cg7?

I cant recall if thats a disc or not, but would not hurt to check it out. Pin 2 resistance measruement check out?

But I agree the k6 is looking like the suspect.

one last thing tube socket ok? I would check the resistance measurement with a tubt socked extender. I have had micor fractures along the top of the tube socket where the pin bridges betweent the PCB pin and the socket hole.

[Don Lindsly]

Not clear if the problem is horizontal sync or both vertical and horizontal. If H only, do a quick oscillator alignment and see how it works. Don't overlook the dual diode. The circuit is always trying to correct to near zero. A bad diode will not affect waveforms, only the DC correction voltage.

The networks are fairly reliable, particularly the dark red Eric Pacs. Those are also repairable as long as you know which component you are replacing.

[W9RAN]

I knew this TV thing would be a learning experience ;-) Somewhere along the line the problem stopped being intermittent - now it will not sync even from a cold start. So that speeds the troubleshooting process up at least.

By measurement and/or substitution (including a homebrewed K6 couplate), I think I've eliminated the AFC diodes and K6 as the problem. The Sams shows 3.5 volts (DC) on the grid but I still measure zero (although the AC voltage is somewhat greater). I think now the problem lies upstream from the AFC diodes.

I'm not real familiar with what the waveforms should look like, but I'm pretty sure that what I have is not right. Rather than trying to describe what I'm seeing verbally, I shot a short video off the oscilloscope screen and am hoping perhaps some of the experienced techs here might be willing to take a look. I didn't expect to see this much video and "jitter" at the output of the sync separator, yet the socket voltages aren't too far off. It seems clear that the horiz. oscillator can't sync, but I'm not sure where to look next.

The video runs only one minute and shows in sequence, 1) grid of sync sep tube, 2) plate of sync sep tube, 3) pin 2 of AFC diodes, 4) pin 1 of AFC diodes, and 5) a short clip off the screen showing the "tearing" and inability to achieve horiz. sync. All traces are with the scope set at 10 V/div and 200us/div.

Video: http://smg.photobucket.com/albums/v6...ckel/Predicta/

I really appreciate the help. I haven't run into most of these circuits before but am trying to understand how they work.

Bob

[Don Lindsly]

Good screen shots.

The sync separator plate is not right. It should show clean sharp pulses. Check for leaky coupling caps in the sync and video circuits.

Check resistor values in the sync separator grid and plate and video amp plate. Anything within 20% is OK for now. Voltage measurements at sync separator plate, grid and tuner AGC would be useful.

Those tube sockets can be trouble. They break internally between the pin and PC contact. Check for voltage differences at the tube pin and the PC contact pin. Differences point to a socket bad pin.

When/if changing a tube socket on a PC board, break the socket apart and unsolder one pin at a time. Heat only the pin, not the board, and push it through. Trying to remove an old socket in one piece will usually damage the board.

[W9RAN]

Fixed!!

R-53. A 22K 1 ohm resistor that develops the bias for the noise inverter stage was open. I didn't (still don't) understand quite how the noise inverter works, but once I remembered how to use the Line trigger mode on my scope to get decent traces, I walked through the sync circuit and confirmed this is where I was losing the sync pulses. I found that the cathode voltage was nowhere near where it was when I first checked it a few days ago - but that was when the problem was intermittent, so the resistor must have been changing value at first, and then finally failed completely. This explains why it initially would work for 15 minutes or so, and as the resistor value went up it finally reached the point where the sync was lost. Sync is solid as a rock after an hour, so I'm declaring victory!

Thanks for the helpful suggestions from everyone here. I definitely understand B&W TV circuitry better now, which is a good thing as I think I've got another set coming my way.

I'd also mention that I found a copy of "Television Servicing" by Alex Levy and Murray Frankel on my bookshelf, and it seems to be a very good resource. While the Predicta circuitry seems to be a bit unique, I found a lot of useful information and tips in the book