AB763 shorted diodes

[Randall]

This is a head scratcher for me. A BF Bassman comes in popping fuses. It lights up my dim bulb tester like a dead short. I lifted the red wire from the rectifier to the first caps and standby switch, still shorted. In testing the diodes I find three on one side failed, but what I don't understand is, two are shorted and one is open. If one is open how can there appear to be a short? Or how does the circuit pull too much current with only one side of the secondary connected?

https://vintagefenderamprepair.com/p...matics-layouts

Edit It's an AB165 actually.

[bob p]

if you lift the connection between the rectifier diodes and the first filter node and you're still shorting, then isolate the bias circuit to verify that it's not shorted. then look at the PT. of course this assumes that you've already pulled all of the tubes connected to the 6.3v filament supply.

anything that's loading the PT could be causing the problem.

[Randall]

With the three bad ones out it didn't short. I replaced all 6 of them, and it powers up OK. I just don't get how or why it would pull too much current with one open diode on one side.

[Chuck H]

Perhaps that diode is only open at your tester voltage, but not at HV? If you're up to it you could put it back in the circuit without the shorted diodes and see what happens. Strictly for educational purposes and maybe not worth the time since the amp is working now.

[Randall]

"Perhaps that diode is only open at your tester voltage, but not at HV?"

Hmm.. I still don't get to why that would act like a short, unless that open diode goes shorted under HV. But is that possible?

And the amp is buttoned up now. This is a case where customer knows it needs more work, but doesn't want to pay for it, just wants to pay as little as possible to sell it. So, I don't feel like spending extra time on it.

[Chuck H]

If the customer is happy, you're done. But yes, IMO it could happen that a diode testing open at low voltage may well pass voltage/current at substantially higher voltage. Tolerances are pretty small with silicon devices. I could believe the difference between open and conducting could be a few hundred volts. It might have been good to know, but maybe unnecessary. I have no other explanation.

[bob p]

Chuck makes a good point. Testing a diode with a 9VDC battery powered meter (or maybe even lower voltage if you're using a modern meter than runs on penlights) is NOT the same as testing a diode under HV conditions. Your diode may test one way with low voltage / low current and behave entirely differently at HV / high current. Basically, all that the diode tester is good for is telling you that the diode is BAD. Don't sweat it. You know it's bad, you replaced it, the amp works, you're done.

[Enzo]

Hmm.. I still don't get to why that would act like a short, unless that open diode goes shorted under HV. But is that possible?

Yes, it is exactly possible. I preach this all the time, your meter does not put real world conditions on parts. A cap that checks fine at a volt or two can leak like a screen door at a couple hundred volts. Your diode might check open at a volt or two, but that does not tell you that at anything over say 20 volts, it conducts heavily. Like a 20v zener, more or less.

Take a 1N4001 rectifier. It is a 50v part. But put 400v across it in reverse, and it will break down. That is what 50v inverse rating means. It would test OK on a meter all day, but fail every time in a 400v circuit.

[Mick Bailey]

"There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."

Comes to mind whenever I see something I think impossible, infeasible or that I haven't seen before.

[52 Bill]

I once had a Bassman that would cycle loud and soft like a bad tremolo. Turned out to be a power supply diode that would go open at high voltage dropping the B+ by half until it cooled off and then conduct again.

Tested fine with a meter but broke down at high voltage.

[drewl]

I've been seeing this in older Mesa Rectifier amps.

The 4007's that run dc filament and switching start breaking down but test fine.

Burnt PC board underneath is a clue.