finishing a Predicta Siesta restoration

[rsasnett]

With a lot of help from Phil Nelson's post about this model on the antiqueradio web site, I was able to restore a Siesta with 10L43 chassis in pretty bad shape back in 2013, and miraculously got it working as a first-time TV project. Some pix attached. Now I've got an intermittent problem to chase down and could use a sanity check.

I'm a tinkerer, software developer by trade, with no training in electrical stuff. I started out by repairing and re-wiring guitars, then graduated to restoring old Fender tube guitar amps. Did about 3 tube radios, then for some reason thought I could try a TV, and somehow got VERY lucky -- I had a working although imperfect picture on my first power-up after replacing a ton of components (all the rectifiers and capacitors and about 75% of the resistors, but not the original couplates). I have to say, that was an AMAZING feeling -- as a programmer, it's like writing code all day, and the first time you try to compile and run it, everything "just works".

Next I replaced all the couplates, which I made on breadboard. Lots of detective work required, so I may have made mistakes. But they worked, at the end I had a nice picture. Sound was faint, but I tweaked the bottom core of T5 and got it dialed in pretty good.

I restored the cabinet, which was in terrible shape. Sanded down, primed and repainted in original gold and cream. Found replacement knobs for the alarm clock and got it working again by reattaching its transformer leads. Replaced the broken volume / power / contrast combo switch. Polished the clear plastic CRT cover, although it has a couple of short deep cracks in it -- surprisingly they don't show up much when watching TV. Installed a replacement antenna, works fine with local transmission. Had the brass arms re-plated, and also brass-plated some steel hole covers to match the missing ones (I have a TON of extras if anybody wants these, they fit both 17" and 21" models).

I then used the TV for about 6 months, watching movies and TV shows from a DVD source for hours at a time, no problem. One day the picture started shrinking, horiz and vert, until it was just a dot at the center. The failure was consistent -- after running about 5 minutes from a cold start, the picture would start shrinking until after 15-20 seconds it was gone. I believe I still had sound after the picture went away. I verified the behavior was still the same a couple of years ago, even after swapping out all tubes with a backup set.

Now I want to finally fix it and use it.

My plan was to extract the VOS board once again (sigh), replace the couplates with reproductions I bought from Crist Rigotti, replace all remaining original resistors, clean all the tube sockets with DeOxit and brushes, re-flow solder connections on the tube bases, and then start troubleshooting if there is still a problem after I get it back together. I feel like I should do this work no matter what for long-term reliability.

But now I'm wondering... should I try to do some basic troubleshooting first before I pull out the board and replace all those components?