PT explanation please

[Randall]

I am working on a chinese pignose G40-V, a real piece o'crap. Blows fuse, variac shows dead short. I systematically lift everything from the ss ps board until nothing but the PT secondary is connected. Still dead short. Nothing on the simple board looks shorted, can find no path to ground with ohmmeter. Board ground is also now lifted. Disconnect one side of secondary and read 355 vac. Hook it back up, dead short.

I'm thinking PT, but I don't understand why? Anyone?

[Jazz P Bass]

The Red & Red Vac winding goes to a bridge rectifier or the .047uf bias cap.
That's it.
What more do you need to lift?
And have you lifted them?

Is the filament winding still connected to the board?
If it is, lift that.

With the high voltage winding & filament winding disconnected from the board, measure both windings.
If there is an internal short inside the transformer, you may see an abnormally high filament voltage.

Schematic link: http://www.freeinfosociety.com/media/pdf/3290.pdf

[Randall]

The filaments don't go to the board, straight to sockets, no tubes, I get 7.3 vac. I didn't lift anything on the board. The diodes test good in circuit, the caps are charging, no shorts. I did not lift C1 .047 bias cap, but I did measure across it, no short. I measured both windings d/c from board. 335v and 7.3 v.

[Jazz P Bass]

I am confused.
What is the issue?

First post: "I systematically lift everything from the ss ps board until nothing but the PT secondary is connected"

Second post: "I didn't lift anything on the board"

First post: "Still dead short."

Second post: "The diodes test good in circuit, the caps are charging, no shorts."

[eschertron]

The filaments don't go to the board, straight to sockets, no tubes, I get 7.3 vac. I didn't lift anything on the board. The diodes test good in circuit, the caps are charging, no shorts. I did not lift C1 .047 bias cap, but I did measure across it, no short. I measured both windings d/c from board. 335v and 7.3 v.

More detail is better.
"the caps are charging, no shorts". Charging how? does the PS work or not?
"The diodes test good in circuit". How did you test them? I've had rectifier diodes that test OK on my multimeter, but DEFINITELY break down under high voltage. IMHO the only thing a multimeter diode tester is good for is verifying the polarity of the component... and selecting the right value LED to use in my clipping circuit
As you 'divide and conquer' the power supply, be sure to specify which components are still in circuit with the "no shorts" and "dead shorts". Otherwise heads will spin.

[Randall]

Sorry if I was not clear. I lifted the wires that feed the rest of the amp one by one from the PS until the PS is not connected to anything. I put a MM diode tester on the bridge rectifiers in circuit, they all read as I would expect. The caps were measured in circuit with an ohm meter where I watched them charge all in like fashion. Nothing looks suspect under the circumstances. By dead short, I meant while powering up on a variac with ammeter, it pegs the ammeter right away, indicating a short somewhere.

I understand that components should ideally be measured out of circuit, but also know that there is something to be learned from measuring in circuit at times when we are looking for a shorted component. My actual question was, can a PT measure a secondary voltage while unloaded, but actually still be faulty when under load? And if so, how?

[eschertron]

My actual question was, can a PT measure a secondary voltage while unloaded, but actually still be faulty when under load? And if so, how?

That's a question that I'd like answered too. I hope you don't mind if I hang out for the duration.

I'm curious if you can make the PT misbehave on some other load: a resistor, or perhaps a rectifier/filter breadboarded just for this test.
Conversely, since I think the pignose's bridge rectifier may be bad take another PT (surely there's something laying around?) and, with proper fail-safes in place, apply juice to the pignose's rectifier and filters. Divide and conquer while you're waiting for the forum gurus to check in.

And since you are using a variac, and you don't blow a fuse when applying power to the PSU, I have to ask if you took voltage readings with and without the PT connected to the rectifier/filter caps. I know you reported 355vac on the unloaded secondary; what do you get when it's connected?

[g1]

Have you tried removing the power tubes?
The variac can not show a "dead short". The current meter can show excessive current draw, but that is not necessarily a dead short.
After trying it without the power tubes, if it still has major current draw, set the voltage low on the variac and take some voltage readings in the power supply.

[Enzo]

Understand the limitations of an ohm meter. It only uses a volt or so to test things. A cap that is fine at a volt or so may leak like a screen door at say 100v, and you are asking it to handle 350v. Not only that, if a board arcs somewhere, that can ONLY happen at high voltage, a meter will not expose an arc.


I don't know if you actually stated it this way, but with the secondary wired removed from the board - hanging in mid air - the amp no longer draws heavy? It only draws when that secondary is connected? If so, then SOMETHING is shunting that supply.

Try disconnecting the center tap of the output transformer, looks like a white wire on the drawing. Still draw heavy?

[Randall]

I don't know if you actually stated it this way, but with the secondary wired removed from the board - hanging in mid air - the amp no longer draws heavy? It only draws when that secondary is connected? If so, then SOMETHING is shunting that supply.

Try disconnecting the center tap of the output transformer, looks like a white wire on the drawing. Still draw heavy?

Again, adjusting my terminology, the transformer draws heavy current on the variac with no tubes, and nothing connected (including PT CT) except the two red secondary wires. Nothing else connected. Not even chassis ground. It does not draw with one of the secondaries lifted.

Thus, feeling no support for my question of whether a PT can shunt or short or whatever you wish, by it's own fault, I will regain access to the amp and dig into the PS board. Thanks.

[g1]

The kind of PT failure you are asking about would be quite rare. I won't say impossible but highly improbable.
Like Enzo said, try disconnecting the OT center tap (white) and see what happens.

[Tom Phillips]

... the transformer draws heavy current on the variac with no tubes, and nothing connected (including PT CT) except the two red secondary wires. Nothing else connected. Not even chassis ground. It does not draw with one of the secondaries lifted....

Randal,
I'm perplexed about why this discussion seems to be going in circles but it seems clear to me that there is something on the board that is sorted (or just presenting a very heavy load) across the whole secondary.

With the two red secondary wires connected you have the problem. When you disconnect one of those wires the heavy current draw stops. That pretty much shows that the problem is not inside the transformer. The fact that the heavy current draw continued after you lifted the center tap and ground connections indicated that the heavy current is not flowing from an internal shorted winding to the transformer core. My guess is that one or more of the diodes in the rectifier bridge are shorted.

The confusion, I think, stems from statements such as "...nothing connected (including PT CT) except the two red secondary wires. Nothing else connected..." Stick with either "nothing" or "something" rather than "Nothing...except" All the troubleshooting clues are in your report. The search for something inside the transformer is just a hypothetical side issue now. I'm "almost certainly" sure.

Cheers,
Tom

[Enzo]

Then the order of likelihood is: shorted rectifier, problem on the OT circuit, shorted caps. And remember, your hand meter cannot detect caps that act shorted at more than a couple volts. My personal guess is an arc inside your OT.

[Randall]

I finally regained access to this amp, and dug into the PS board again. After lifting a few things, it left only the rectifier diodes, all of which tested as good out of circuit. I replaced all four, and that was indeed the solution. It makes sense looking at it now from this side of it, but I was not seeing thru that fog. So thank you guys once again for your wisdom and help, I surely do appreciate it!

[Tom Phillips]

...it left only the rectifier diodes, all of which tested as good out of circuit. I replaced all four, and that was indeed the solution...

This may be a case where the diode(s) were failing at a voltage higher than the test voltage of your meter. Glad to hear that you found the solution.