RCA 8-PT-7034 issues

[bigaudioal]

Ok found some issues with the RCA 8-PT-7034. In addition to the new C171 5uf lytic popping during initial power up after recapping, I found a dead primary on the VOT. Now also found a dead focus control pot. All very much on the same circuit.

Below is a link to the schematic. PDF pages 31-32 or printed pages 23-24. 2nd page of schematic (printed page no. 24 at bottom). Find focus control to left of damper tube (6AX4). Trace it back towards damper tube. Then straight up to VOT (top right of page, T152). Just under the VOT is C171. That's the lytic that popped. So focus control toast, primary of VOT toast, and C171 blew.

Anyone have any ideas about where I should go from here? Would hate to install new VOT, focus control and lytic and have them damaged by an issue I do not know how to diagnose which affects this circuit. Thanks for any advice anyone can provide.

http://www.earlytelevision.org/pdf/rca_8PT7010_SVC.pdf

[miniman82]

How's the fly ring out, good? It's an autoformer style fly so presumably if there were turns shorted in exactly the right place, you could wind up with high voltage pulses in all sorts of strange places. Ohm out each section, see if the values make sense. Terminals 1/2/3/damper cathode I would expect to be a relatively low value and pretty close in number to each other (perhaps 8-20 or so ohms), and obviously the HV overwind going to the 1V2 will have a higher number to the damper cathode than the others.

Another possibility is that since this is a low B+ chassis, there could be an issue with boost voltage. Anything connected to terminal #1 on the fly has B+boost voltage, so if boost is abnormally high things could start melting in a hurry. Out of curiosity, what voltage rating did C171 have? I see 440v at the plate of the vert output tube, so that cap better have been rated at least 500 working volts/550 surge to be on the safe side.

Incidentally if the VOT blew it's primary and the cause wasn't a ton of boost voltage frying it, the only other reason that would make sense is C153 being shorted to ground.

*EDIT* missed C156, so C153 being shorted wouldn't do much of anything.

[bigaudioal]

Originally Posted by miniman82

How's the fly ring out, good? It's an autoformer style fly so presumably if there were turns shorted in exactly the right place, you could wind up with high voltage pulses in all sorts of strange places. Ohm out each section, see if the values make sense. Terminals 1/2/3/damper cathode I would expect to be a relatively low value and pretty close in number to each other (perhaps 8-20 or so ohms), and obviously the HV overwind going to the 1V2 will have a higher number to the damper cathode than the others.

Another possibility is that since this is a low B+ chassis, there could be an issue with boost voltage. Anything connected to terminal #1 on the fly has B+boost voltage, so if boost is abnormally high things could start melting in a hurry. Out of curiosity, what voltage rating did C171 have? I see 440v at the plate of the vert output tube, so that cap better have been rated at least 500 working volts/550 surge to be on the safe side.

Incidentally if the VOT blew it's primary and the cause wasn't a ton of boost voltage frying it, the only other reason that would make sense is C153 being shorted to ground.

I did ohm the sections of he fly and they all seemed good. C171 was rated at 450. That's what I installed. Schematics also call for a 450 volt cap. Isn't C153 connected to ground?

Thanks for these tips too!!!

[miniman82]

Had to edit my post, I was in haste and missed another cap in that line which makes my previous statement invalid.

HOWEVER COMMA

If for some reason the vert out tube weren't biased correctly and drawing too much current, like if it weren't getting a drive waveform from the osc section for example, it's possible it's just drawing a ton of current and burned the VOT that way instead. Only way to know for sure is power it up with a DMM attached to the VOT red wire: if it flies up way high, you've got boost issues that need to be sorted. If it sags below what you'd expect, you may have a short somewhere.

What's it doing right now, with vertical obviously not working? What's boost measure at?

[bigaudioal]

Originally Posted by miniman82

Had to edit my post, I was in haste and missed another cap in that line which makes my previous statement invalid.

HOWEVER COMMA

If for some reason the vert out tube weren't biased correctly and drawing too much current, like if it weren't getting a drive waveform from the osc section for example, it's possible it's just drawing a ton of current and burned the VOT that way instead. Only way to know for sure is power it up with a DMM attached to the VOT red wire: if it flies up way high, you've got boost issues that need to be sorted. If it sags below what you'd expect, you may have a short somewhere.

What's it doing right now, with vertical obviously not working? What's boost measure at?

Got ya. Thanks. I do not have another 4.7uf 450v cap at this point. Blown one is still in set. So need to order some parts and then thrown that in there and fire it up with meter at red wire of VOT to see what it is reading. What should it be reading at that point?

[Eric H]

Since you're using the factory manual this probably doesn't apply but, there are two different versions of this chassis with different vertical output tubes and circuits.

See Phils Old Radios about halfway down this page: https://antiqueradio.org/rca16.htm

His set didn't blow capacitors, it just had no vertical.

[bigaudioal]

Originally Posted by Eric H

Since you're using the factory manual this probably doesn't apply but, there are two different versions of this chassis with different vertical output tubes and circuits.

See Phils Old Radios about halfway down this page: https://antiqueradio.org/rca16.htm

His set didn't blow capacitors, it just had no vertical.

Thanks for posting that link. I will check it out.

[bigaudioal]

Update on this set. After replacing the VOT, the new electrolytic that popped on initial power up, and a bad focus control, the set powered up and has a great picture. Photos make the picture looked a bit washed out, but not like that in person. Working great for a little 8 inch portable set.