Ampeg B-410 Blown Fuse Diagnostics Help

[The Dude]

Both a B-E junction and a B-C junction should read like a diode (observe polarity for a PNP or NPN transistor). C-E should read open. This is out of circuit. In circuit, you may be reading across other circuit components. I would check them in circuit. Then, if something seems shorted/open/leaky, etc., pull them and re-test.

[Helmholtz]

In this case I cannot get to that point as the amp is pulling way to much current. I am dialing up on a variac and it starts to draw too much current with only a small amount of AC applied. Should I use a bulb limiter here and then test for DC on the output?

I was hoping that removing the load would stop the overcurrent (as it often does when there's DCV at the output).

Though I have a variac with a current meter, I generally prefer to use the bulb limiter.

[misterc57]

I thought I had isolated the bridge recitifer and it was overcurrent. Pulled each diode out and they each tested good. Put new ones in and still have the issue.

I then went back to see if I could find shorts in the finals and I see lots of shorts between 22 and 100 ohms on each 2N4348. Argh.

So, going to put this one aside for now. I have another identical amp head I want to see if I can fix. It has DC on the output and I am going to open a new thread for that one.

Thank you, MC

[g1]

I thought I had isolated the bridge recitifer and it was overcurrent. Pulled each diode out and they each tested good. Put new ones in and still have the issue.

So you have tried it without load connected, as Helmholtz asked?
If trying to start up on variac or bulb limiter, with load connected, you can sometimes get over-current even with an amp that has no fault.

[misterc57]

So you have tried it without load connected, as Helmholtz asked?
If trying to start up on variac or bulb limiter, with load connected, you can sometimes get over-current even with an amp that has no fault.

Yes. All testing has been with no load.

For SS I have learned here to check for DCV on output with no load as the first step. This is when I discovered the overcurrent.

Thank you