Predicta pedestal vertical hold - Videokarma.org TV - Video

judge · #1 10-10-2019, 09:28 AM

Maybe I should have started a single thread on this restoration. Anyway I thought I would start a thread about an issue I am having with the vertical hold not being stable. It will lock on for a while then roll, then lock etc. etc. Perhaps not coincidentally, the horizontal hold is finicky. The picture will tear before it locks on, and then the picture may or may not snake.

This predicta basically 'worked' prior to me starting work on it. I have replaced all the electrolytics, paper caps, black beauties etc., plus all the resistors on the main board, the big resistors on the chassis, the diodes, and the thermistors. In particular, although it had several other issues, it did not have this one specific issue.

Anyway - fiddling with the tubes shows that the 10DE7 was loose. I tightened up the socket so it wasn't as loose, but it had no effect other than fiddling with it no longer caused the picture to collapse.

As a next step, I ordered replacement tubes, including the 10DE7. A few have arrived and been fitted (not that one yet) and it looked like that fixed the problem, but after a few more rounds of messing about with the tubes, the instability was back. I had to mess with the tubes some more because one of them was DOA, so it took a while to isolate which tube was bad. BTW it tested well, but just doesn't work when inserted - the 12DQ6A.

I will wait until I have tried a new 10DE7 before going any further, but scouring this forum and antiqueradios.com shows there are quote a few possible causes of this instability, so this may take a while. I'll update this thread with progress.

Electronic M · #2 10-10-2019, 10:34 AM

It is better to keep to one thread per set. It makes it easier to glean service history and related issues that the poster may not realize relate.

Since it is effecting both H and V sync it most likely is a sync separator stage issue (though if if the sync stage is fine and both sweep stages have issues that can cause the same symptoms.

If a tube tests good but is dead in set there is a solid chance the socket may have an open pin....the sockets in predictas have a well earned reputation for failing.

I've got the same chassis on the bench with good horizontal hold but no vertical sync signal reaching the vertical osc. New caps and couplates in mine. I haven't had much bench time lately...

judge · #3 10-10-2019, 03:33 PM

I'll keep this thread the main one. So two more tubes arrived, 10DE7 and 9BR7. Swapped them both in and it was just a mess. Swapped them back out and it was still just a mess. One thread I saw mentioned the tubes in the IF section, so I jiggled those around a bit and the contrast suddenly improved dramatically when I jiggled the middle 3BZ6. After which I was able to get the picture stable too.

Then I noticed a burning smell and pin #5 of the 10DE7 socket was making sparks and burning the PCB. This socket has been giving me issues so I suspect I need to disconnect the PCB again and see what's going on with it. Not sure what would make that pin generate sparks like that, since it is just one of the heater pins, but this socket has been suspect for a while. Where can I get replacements, if I need one (and it isn't just dry joints)?

Electronic M · #4 10-10-2019, 04:00 PM

I've just been collecting NOS sockets at swapmeets and pulling them from junked equipment. There may be a source for new...AES and some of the places that cater to tube amp builders would be where I'd start my search.

judge · #5 10-10-2019, 05:21 PM

Vertical hold problem sorted. The socket had two dry joints, pins 4 and 5. Now for the horizontal hold. The picture tends to be wavy until it actually locks in, then it is very stable. The adjustment is very touchy.

old_coot88 · #6 10-10-2019, 06:56 PM

Quote:

Originally Posted by judge

...Now for the horizontal hold. The picture tends to be wavy until it actually locks in, then it is very stable. The adjustment is very touchy.

The horz AFC dual diode is the most frequent culprit for this.

judge · #7 10-10-2019, 10:53 PM

Quote:

Originally Posted by old_coot88

The horz AFC dual diode is the most frequent culprit for this.

I’ve seen that on the 17” models, but I don’t see it on the 9L37?

judge · #8 10-11-2019, 06:23 AM

Threw a test pattern up and saw that the vertical linearity is way off. Turns out that the adjustment pot is open for most of its run, so I guess I'll be putting a new one in. Anyone have experience opening up the old ones?

judge · #9 02-01-2020, 07:29 AM

I replaced the vertical linearity pot with a new one, then the vertical hold just wouldn't settle. Turns out that R53 (SAMS) had opened up. Replacing that fixed that issue.

Horizontal hold is still very flaky, with the picture being wavy.

***Eric H*** · #10 02-01-2020, 01:41 PM

Looks like the 9BR7 is the Horizontal AFC Diode in this set, could be something flaky in that circuit.

Did you do the procedure to set up the Horiz sweep circuit adjustments outlined in the Sams?

judge · #11 02-02-2020, 11:11 PM

Yes, it was the anti-hunting resistor (R61 in SAMS). It had somehow become a short.