Ampeg B100R - nothing but hum

[wagdog]

Greetings all,

A friend brought over his Ampeg B100R yesterday. It produces nothing but a low hum and the power on indicator light (blue) no longer comes on.

I know a bit about tube amps, but don't really get into solid-state. I know enough about the theory and operation of them, but that's about it - I typically don't mess with them.

Anyway, I disconnected the speaker load and with the power off tested the power transistors for open shorts as best I could. I believe they are fine, as are the IC's in the amp.

Plugging it in, I powered it on (with no speaker load) and starting measuring voltages starting at the rectifier. The +40 side is fine, however there is no voltage at the - side of C34/C36 where there should be -40v. The same for C40 (obviously if there is no -40 there won't be a -16).

That's where I stopped as I didn't want to pull the board out and disconnect parts to test in isolation yet.

Visually, the rectifier looks fine as does C34 however I suspect these parts may be where the trouble lies. Before pulling things apart, I was hoping to run it by you folks and see if you think that would be where to start looking, or somewhere else.

Thanks in advance,

Wag

[soundguruman]

Check Q 5,6,7,8,9 for shorts. Bad output transistors.
D 20: open diodes.
Also, I bypass the optocoupler. (as worthless as it is...)
Caused by bad speaker or speaker cable. Be sure to check that so it won't happen again.

[Jazz P Bass]

Absolutely 'no -40Vdc' sure sounds like the rectifier.
Or an open solder joint.

[soundguruman]

Absolutely 'no -40Vdc' sure sounds like the rectifier.
Or an open solder joint.

something is loading the -40 down to 0 and blowing the rectifier
Like a bad output transistor?

The way it works is,
the speaker is tied to the output transistors
and you have a shorted speaker cable, etc....
That blows the transistor.
and when the transistor blows, the -40 is dragged down to 0
and that blows the rectifier, or blows the circuit track, for that matter.
so whether or not a load is connected, it's still got an open circuit after it blows.
so I suspect the transistors are blown open and so is the rectifier.
and if it melted the circuit tracks, that's par for the course.

[Enzo]

If the output were shorted enough to load -40 down to zero, we'd have the plus side dragged down too, and the fuses would blow. The output transistors have no path to ground other than through the load - which the OP had removed. SOMETHING would be getting real hot.

If you don't have good power supply, nothing else will work either. STart there. I suspect that both negative sides of the bridge did not fail, so my wager is on either a bad solder joint on the negative corner of the bridge or a busted connection between there and the minus end of those first filter caps.

SO you are used to tube amps, OK, most tube amps have a positive B+ supply and a negative bias supply. Two supplies opposite polarities. Now if your bias supply was missing (you lost the negative supply)what would you do? WHy is that any different here, just because there are transistors out there instead of tubes?

[wagdog]

Thanks for the info folks, much appreciated. I will take the board out and check it out ASAP.

@Enzo: I hear ya - I just wanted to be sure there wasn't something else I should be looking at. I really don't know much about transistors as far as how they might fail; for instance would it look normal, but pass its current to ground if it were shorted. Just wanted to check - thanks for the help.

[wagdog]

Thanks again for the help. Got back into the amp this evening and fixed it. Here's a shot under the board:



The two bottom pins of the rectifier had a poor solder joint. Surprised the amp lasted as long as it did, this one is pretty well used.

Again, thanks all for the assistance.