Sync Problem

[old_coot88]

Originally Posted by vts1134

This capacitor was the problem. I thought I had gotten all of the caps the first shot around but this one snuck past me.

Well sunnuvugun. Was it shorted? Leaky? Congrads on nailing it.
I'm tryin' to figure the mechanism by which it caused the symptom in your vid.

[vts1134]

Originally Posted by old_coot88

Well sunnuvugun. Was it shorted? Leaky? Congrads on nailing it.
I'm tryin' to figure the mechanism by which it caused the symptom in your vid.

It was not shorted, I don't have a capacitor tester so I don't know if it's leaky. If you figure out how a failure in that part would cause the symptom I had please let me know, I would really like to understand it more.

[Geist]

Hi All;
Congratulations on you Find.. It seems like its the little things that can mess things up.. My best guess, is since the Sync Discr Trans is coupled by this cap to the Horizontal Osc. Control, that this is where the Sync pulse comes in, and so it had trouble keeping the oscilator synced to the sync pulse.. Or maybe to the wrong pulse, like noise instead.. Its just a thought.. More than likely some one like Old Coot could speak better to this and tell you what is happening, alot better than I could.. Good find !!!
THANK YOU Marty

[old_coot88]

Dang, that's a puzzler for sure. In the vid, there was definitely a 60hz component mimicking B+ ripple (hum) from a bad filter cap. But it wasn't that, obviously.
Lacking a cap tester, have you checked the old cap for leakage with an ohmeter? A good cap will show infinite resistance on the highest R range (after a brief charge-up blip).
Any leakage might be a clue to what was going on. If the old cap's good, maybe the problem was a tin whisker.
(This is not a good test for electrolytic caps though, as good ones will always show a small amount of leakage.)

Edit. .. Since one end of C169 goes to the cathode of the 6AC7, if something was allowing 6V AC heater voltage to get onto the cathode, it could certainly cause the problem

[vts1134]

It's a mystery for sure. When I went through replacing the caps in the set there was a wiring error in this part of the set. C173 and C169 were reversed. What I hadn't noticed was this red tubular cap was also wired in C169's place. So in essence there were two capacitors in C169's position, I replaced the one I could see (and put them in the correct place) and didn't notice this one hiding until later. The junction of T108A, C172, C173, and C169 was hacked up pretty bad from a previous repair job, lots of leads just clipped off and left in the set waiting to short something else out. There was also a bumble bee cap hiding in that junction (mistake these for resistors sometimes) that tripped me up for a little while.
All part of the fun.
Can't wait to get this one finished and posted up in the B&W section.

[Geist]

Hi All;
VTS1134, I noticed that in my copy of the schematic, that it was slightly different than your copy, on the posting.. Yours showed the cap 169 at .015 all by it self.. Whereas, the one I have has two caps in parallel.. C169 at .015 and C164 at 1200.. It would be interesting to see what various versions of schematics that exists.. And what or in what way they differ.. And also look at the schematic for the 730 and the 8T30 and if there is a 9T30, and see what changes to the basic circuit they made, thru the years.. It may be that they had trouble with that part of the circuit and made more than one fix, since you say that there were numerous wires that had been cut.. and/or parts replaced in that area..
THANK YOU Marty

[old_coot88]

VTS1134, for the short time you've been doing this stuff starting from zero experience, you're certainly demonstrating an outstanding aptitude for it. Many kudos to you.

[vts1134]

Originally Posted by Geist

Hi All;
VTS1134, I noticed that in my copy of the schematic, that it was slightly different than your copy, on the posting.. Yours showed the cap 169 at .015 all by it self.. Whereas, the one I have has two caps in parallel.. C169 at .015 and C164 at 1200.. It would be interesting to see what various versions of schematics that exists.. And what or in what way they differ.. And also look at the schematic for the 730 and the 8T30 and if there is a 9T30, and see what changes to the basic circuit they made, thru the years.. It may be that they had trouble with that part of the circuit and made more than one fix, since you say that there were numerous wires that had been cut.. and/or parts replaced in that area..
THANK YOU Marty

I just went back and looked at the Sams version and you're right there are two caps in parallel in that area. That solves the mystery as to why there were two in my set. I still wonder why removing the smaller cap solved my problem.