finishing a Predicta Siesta restoration - Page 2 - Videokarma.org TV - Video

rsasnett · #16 11-10-2023, 06:42 PM

I get +254 at L38, which Philco calls "Hor Osc B+" on page 14 of their manual. That pin is next to the Hor Out tube 6DQ6A. This voltage is consistent whether or not I have picture. The 6DQ6A tube tests good.

On about 20% of startups I see +370 on L22 and hear the oscillator and get picture briefly before that voltage drops back down to +269.

bandersen · #17 11-10-2023, 07:27 PM

Try shorting out the ringing coil. It may have an intermittent fault.

rsasnett · #18 11-10-2023, 08:49 PM

By "shorting out" I assumed you meant connecting between pins 1 and 3 of the T7 coil, called "Hor Stab" in Philco component diagram, and "Hor Ringing Coil" on their schematic. I attached alligator clips between tabs 1 and 3 on the exterior of T7 (couldn't reach the pins underneath, hope that was sufficient to short it out) and restarted.

This time I had picture for at least 10 minutes!

It started out as a narrow horiz line but with some picture detail. Looked like no vertical deflection, so I tapped / wiggled the 6DR7 tube (Vert Osc & Out) and got a full raster. Was great for a while, but after 10 mins it dimmed and shrank and went away like before.

Two more restarts with no picture, then the third time I had picture for a little over 5 minutes.

So... shorting out T7 (if I did it correctly) does help -- when I get picture it stays on much longer than before. But it doesn't fix the problem.

old_coot88 · #19 11-10-2023, 11:00 PM

Does the pic stay locked in horz sync during the fade-out? Or does it go out of sync?

rsasnett · #20 11-11-2023, 10:14 AM

I believe it stays locked in horz sync. I can still see the full raster, but its dimensions change.

I thought it used to shrink to a dot. But I just captured this morning's fade-out (after 7 minutes) with my phone, and actually it only shrinks to about half the normal horiz width before the image fades out.

Here's what it looks like:
https://www.dropbox.com/scl/fi/3qnl8...u20f9oyxc&dl=0

Yamamaya42 · #21 11-11-2023, 11:26 AM

I swear I have seen that exact type of behavior before, when I have very young on an RCA hybrid set, and it was caused by the damper being bad, not sure if it's the same this time, but it damn sure looks exactly the same as I saw way back then, damper circuit failing, robbing the FBT of power, causing the horz field to shrink, and thus lose HV but no loss of sync.

bandersen · #22 11-11-2023, 01:43 PM

Yes, that's definitely a posiblity. So is the 6CG7 horizontal oscillator circuit. Id check both tube sockets. The 9 pins sockets are especially prone to developing stress cracks.

jr_tech · #23 11-11-2023, 02:43 PM

After “fade out”, do the heaters of the damper, horiz osc and output remain lit?
Dirty pins and/or socket problems are common.

jr

rsasnett · #24 11-11-2023, 04:35 PM

@Yamamaya42, great to hear that damper circuit failure causes similar symptoms! I'll check what I can in that area of the board. But almost everything has been replaced there, including the two couplates, except for two original carbon comp resistors that measure correctly in circuit.

@bandersen, I just checked and cleaned all 3 of those tube sockets with a tiny wire brush and DeOxit Gold inside, no difference. There are no obvious cracks in the sockets, and they don't wiggle.

@jr_tech, I watched carefully, and the Horiz Out tube (6DQ6B) heater DOES go out just as the picture starts fading -- the other two tubes continue to have lit heaters. Interesting!

Seems like a thermal issue. If I try startup a minute or two after picture loss it never works -- the Horiz Out filament does not light up (never noticed that before), and I usually hear some oscillator noise but no picture. But if I wait 5 to 10 minutes after shutdown it will usually work again, and I'll have picture for 3 to 5 mins, with a longer "off" time leading to a longer "on" time.

timmy · #25 11-11-2023, 04:45 PM

Heater goes out in only the one tube bad tube bad socket bad trace changing with heat

bandersen · #26 11-11-2023, 06:04 PM

Good news. Yes, could be a cracked PCB trace. Or an issue with the socket or tube.

timmy · #27 11-11-2023, 06:15 PM

Quote:

Originally Posted by bandersen

Good news. Yes, could be a cracked PCB trace. Or an issue with the socket or tube.

Same with all the early ctc series chassis even if the tube sockets seem to have been stable.

rsasnett · #28 11-11-2023, 09:17 PM

The fade-out problem appears to be fixed! Thank you to everyone for all your suggestions, and for encouraging me to troubleshoot before blindly shotgunning in a bunch of new parts.

I just ran the set for over 2 hours, worked great. I now have full B+ at 400v on test lug L22 (in Philco manual) as expected, and no longer need to short across the horiz ringing coil.

When I was removing the Hor Out tube to get resistance and voltage readings for each pin, I noticed the VOS board at that corner was flexing a lot. On closer inspection, the grounding tie / clasp next to the HO tube had actually come loose! I checked all those clasps earlier today and verified this one in particular was connected to ground. I soldered all of them a long time ago after putting the VOS board back. But I didn't use a lot of solder, and I guess this one was either a cold solder joint, or got cracked loose when moving the TV around or changing tubes. So that ground connection was good for a cold start, but after that corner of the board got heated up, it must have moved just enough to make it fail. I added a little new solder, so now that side of the board doesn't pull up when taking out the HO tube.

My next problem is the sound output level is rather low -- the set used to have plenty of volume. But I'm going to bask in the victory glow a little before I tackle that.