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?

[s-petersen]

It sounds like a power supply / high voltage issue, I would troubleshoot it, check voltages in those areas.

[Yamamaya42]

bandersen is the expert with such things here, having done more of them than anyone.

So he would most likely know.
But I tend to agree with the above, sounds like a loss of power.

[old_coot88]

Troubleshoot first, before thinking of shotgunning anything. First verify that B+ is up to spec.

[bandersen]

Agree - do some basic voltage checks. Looks like you did a pretty thorough job on the VOS board - hate to pull it if you don't need to. You should be able to pull the chassis out a bit with the CRT and such still connected.
Remember to be careful of the clock AC power bracket! What a terrible design kludge. Almost every Siesta I've seen has the motor wires ripped out

[rsasnett]

Thanks for the suggestions and encouragement!

This time I got no picture on startup, but I did hear a faint whine / squeal, probably from the oscillator circuit (it varied in pitch by turning one of the 3 big wheels slightly).

Measured voltages on some easily-accessible pins after one minute, then shut it down after 5 minutes.

Here are the readings, using the Philco service manual lug IDs and target voltages (from pages 14 and 15):

L11 = +276 (should be +270)
L22 = +269 (should be +400)
L38 = +262 (should be +260)
L40 = +.03 (should be -20)
L46 = +.06 (should be +170)

[bandersen]

OK, so your B+ is fine, but everything else is way off. Make sure that big 4K power resistor is good (it drops B+ down to 140). Also check the width control and hor freq controls. If either is dirty or has a dead spot, the hor frequency can go way off

[rsasnett]

The 4K resistor reads 3.98K.

I spritzed some contact cleaner into the width pot and around H freq screw and moved them back and forth. Started up again, this time I got a good picture!

While it was working, L22 measured +379 (supposed to be +400, but much better than +269 last time).

After about 2 minutes the picture started going dim and shrinking and went away, then L22 was back to 269.

[bandersen]

Could be your boost cap if failing. It's a 10uF 450 volt cap

[rsasnett]

That's one of the 3 caps in the first can (E1 on Philco parts list, C3 on Sams). In circuit, it measures 8.7uF with ESR of 3.45 ohms. About 15 seconds after power is turned on it gets up to 302 v on that tab.

Unfortunately I re-stuffed that can and epoxied it back together. But I could open it up to check on what's inside.

[Yamamaya42]

Or just temporarily sub in a new 10-20uf 400-500v cap ( leave the old out of circuit) and see if it makes any difference, it's a filter, it wont hurt to be a little larger.

[Yamamaya42]

It would prob do more harm than good to try to re-open the can, perhaps you could try this cap, if it's failing,

mouser #
661-EKXJ501ELL120MK2
and if it works, just hide it in the chassis someplace?

[rsasnett]

That's a great idea! I'll just bypass the can for that one cap. I'm not positive it's failing and causing the loss of high voltage, but bypassing will make it easy to find out. Thanks for the Mouser part number, will place an order today!

[rsasnett]

I tried bypassing the can for that cap, using a 12uF / 500v replacement. I monitored B+ at L22 on startup. Voltage climbs rapidly to about 295, then over about 10 secs lowers and stabilizes at 269. No hint of picture, and today I'm also not hearing H oscillator whine. Tried 3 times, same results.

Yesterday I had one good startup with picture, getting 379 B+ at L22, before it died down after a few minutes to 269 and picture went away.

Any other electrolytics I should try bypassing?

[bandersen]

NO, I doubt that's it. Sounds like your horizontal oscillator isn't running. Check the 6CG7, 6BG6 and associated components. Check the horizontal B+ supply. +255 on the Sams Schematic.