B115 fried speaker

[theheroofourtime]

I replaced the filter caps on a B115 I picked up recently. Everything was fine, it sounded good, but there was some buzzing. The buzz seemed to be coming from the interlock switch. I replaced that and when I turned it back on, with all volume pots at zero, My speaker crackled and smoked. Its completely fried. Did I connect the interlock switch wrong? The switch itself functions. Is the power transformer bad? Any help would be appreciated. This is a mid 70s B115 SS bass amp.

[theheroofourtime]

So it is outputting 54 VDC to the speaker which explains why it fried. Anyone have a good idea as to what is causing the DC output? I don’t see any obviously blown components. I have replaced the filter caps already.

[Hangman]

a good lesson to learn with vintage solid state amps. Always test for DC before plugging the speaker in, Always. it will save you a lot of speakers.

Was this amp working before the filter caps were replaced?

[theheroofourtime]

a good lesson to learn with vintage solid state amps. Always test for DC before plugging the speaker in, Always. it will save you a lot of speakers.

Was this amp working before the filter caps were replaced?

Thank you for the reply. I got it with one of the filter caps missing, so I'm not sure how operational it was. I have tried a different set of caps though and get the same result. Lesson learned. Luckily Amazon sent me a new speaker, no questions asked. Obviously I am not connecting it until I resolve this issue.

[jjmt]

OK. Here's the thing. I don't know squat about solid state amplifiers, but I have something of a hobby in building and repairing tube amps. So, even though I feel quite clueless, I'm intrigued by just how on earth you could be getting 54 volts DC at the speaker. So I looked at this schematic.

If you go to the upper right hand side you'll see that there is 56.5 volts coming into the circuit. There are three paths down to the line that goes right to the speaker jack. On those three path is a diode (middle line, no idea why it is there) and two Q305's. The two Q305 take the 56.5 volts on the top, and then connect downward through what looks to be a 0.33 ohm resistor. You're supposed to see plus of minus 0.05 volts there. You get 54. So, either that diode has shorted (best case) or one of the Q305s has shorted (worst case, as I think those are the power transistors?). Anyway, you can test them. I googled up a thread here that might be useful, involving a multi-meter in diode mode.

Good luck!

[theheroofourtime]

OK. Here's the thing. I don't know squat about solid state amplifiers, but I have something of a hobby in building and repairing tube amps. So, even though I feel quite clueless, I'm intrigued by just how on earth you could be getting 54 volts DC at the speaker. So I looked at this schematic.

If you go to the upper right hand side you'll see that there is 56.5 volts coming into the circuit. There are three paths down to the line that goes right to the speaker jack. On those three path is a diode (middle line, no idea why it is there) and two Q305's. The two Q305 take the 56.5 volts on the top, and then connect downward through what looks to be a 0.33 ohm resistor. You're supposed to see plus of minus 0.05 volts there. You get 54. So, either that diode has shorted (best case) or one of the Q305s has shorted (worst case, as I think those are the power transistors?). Anyway, you can test them. I googled up a thread here that might be useful, involving a multi-meter in diode mode.

Good luck!

Thanks! I was actually suspecting the power transistors and was going to replace them. I'll check out that diode as well. Thanks for the link. Should help a lot.