Shorted output transformer? 1962 Princeton problems

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Looks like your amp has been "sympathetically" serviced meaning some important components have been updated while leaving as much as possible original and intact. I love seeing those blue molded caps still in Brown and BF amps. Has the capacitor can been replaced? If not, it could be causing some of your problems but as other have said your voltages look OK... It looks like the solder joints have been touched up but hard to tell. Since it is a such a nice and valuable amp, you might want to consider sending it back to your tech to check things out again so you don't toast anything important like original transformers along the way. It does sound like it could be a cold solder joint on a heater though if not an issue with the filter caps in the can.
 

AlbertaGriff

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Looks like your amp has been "sympathetically" serviced meaning some important components have been updated while leaving as much as possible original and intact. I love seeing those blue molded caps still in Brown and BF amps. Has the capacitor can been replaced? If not, it could be causing some of your problems but as other have said your voltages look OK... It looks like the solder joints have been touched up but hard to tell. Since it is a such a nice and valuable amp, you might want to consider sending it back to your tech to check things out again so you don't toast anything important like original transformers along the way. It does sound like it could be a cold solder joint on a heater though if not an issue with the filter caps in the can.

Cap can has been replaced, yep. Last year.

It may end up back with the tech... last time he had it for 6 months...
 

D'tar

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Your voltages suggest your heaters are working. They also suggest you are biased in cutoff so no sound is reasonable. With -300ish vdc on the 6v6 control grid your bias circuit is suspect. Check the grounds and the entire path to the grids. Ill go look at a layout quick as I have other suspicions as well.
 

peteb

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If you want to be very careful, use a half size fuse and do all initial testing using a light bulb limiter. DC testing should not risk the OT.

the DC checks out enough that you are past the need for a lightbulb limiter.
 

D'tar

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I beleive there is enough evidence to proceed with your meter to verify a few things before the power gets turned on again.

Caps drained amp unplugged. Measure the resistance from the circuit side of the bias diode to chassis. Should be roughly 22k
 

AlbertaGriff

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I beleive there is enough evidence to proceed with your meter to verify a few things before the power gets turned on again.

Caps drained amp unplugged. Measure the resistance from the circuit side of the bias diode to chassis. Should be roughly 22k

I'll have to start again tomorrow afternoon as I just got to work for a night shift.

So nobody seems concerned that I haven't been able to read heater voltage anywhere?
 

Lowerleftcoast

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I think @D'tar is on the right track. The negative bias is down past -300VDC. Something has interrupted the path to ground.

Below... the red arrow points at a suspect. If the wiper in that pot is not making contact with the resistive element the path to ground may be interrupted. Try working the pot back and forth. The green arrows point at possible places where the solder is not making good electrical contact. You can connect a jumper wire from the wire (purple arrow) to ground to bypass the (green arrow) solder joints.

Inkedprincy.jpg
 

AlbertaGriff

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Thanks I will try that and get the measurement D'Tar asked for. I also wonder if one of the solder joints on the diode is cold.

The little bias pot was what the tech added. He's pretty good, but I'd never seen one like this before.
 

Lowerleftcoast

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The voltage is making it past the diode. Either there is not a path to ground or the bias pot is a high value, like 300k, and the wiper has failed. The bias pot is of interest.

EDIT: The style of the pot is fairly typical. Pots are mechanical devices they fail. Get dirty... Possiblly it was damaged during soldering? Who knows?
 

AlbertaGriff

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The voltage is making it past the diode. Either there is not a path to ground or the bias pot is a high value, like 300k, and the wiper has failed. The bias pot is of interest.

EDIT: The style of the pot is fairly typical. Pots are mechanical devices they fail. Get dirty... Possiblly it was damaged during soldering? Who knows?

Well with that info, my current guess would be the wire beneath the bias board is no longer connected, giving no grounding. I will look closely there tomorrow afternoon. Thank you so much for the help.
 

Lowerleftcoast

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Well with that info, my current guess would be the wire beneath the bias board is no longer connected, giving no grounding.
You can meter from the circuit to ground checking for continuity and/or resistance.

Among other places, check from the diode (the side nearest to the pots) to ground. It may have a high resistance, 300k or so, or it may show *OL* (open loop= no connection). The schematic doesn't show another path to ground so maybe a good reading can be made?
 

AlbertaGriff

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You can meter from the circuit to ground checking for continuity and/or resistance.

Among other places, check from the diode (the side nearest to the pots) to ground. It may have a high resistance, 300k or so, or it may show *OL* (open loop= no connection). The schematic doesn't show another path to ground so maybe a good reading can be made?

I meant the grounding wire you pointed out with the green arrows.
 

AlbertaGriff

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I beleive there is enough evidence to proceed with your meter to verify a few things before the power gets turned on again.

Caps drained amp unplugged. Measure the resistance from the circuit side of the bias diode to chassis. Should be roughly 22k

Circuit side (-) of the diode reads 635k ohm to chassis. The + side is 104k.
 

King Fan

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I think we’re getting close. Hey, D'tar cheats — he looks at pics *and* numbers. :) Griff, I like your idea about the underboard connection on the bias board. By all means have a look. Also for grins how about checking resistance at these remaining two test points?

AAAE9703-C72F-489F-BB6E-127C220F3BD1.jpeg
 
Last edited:

AlbertaGriff

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I think we’re getting close. Hey, D'tar cheats — he looks at pics *and* numbers. :) Griff, I like your idea about the underboard connection on the bias board. By all means have a look. Also for grins how about checking resistance at these remaining two test points?

View attachment 994943

Looking for V DC correct?
The -ve side of the diode is -356, as is the -ve side of the bias cap. The +ve side of the bias cap is -355.
 




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