Ampeg BA115...problem

[kenny_viper2]

I have an Ampeg BA115 and It keeps blowing fuses. I know there are a few threads on this but I figured I would start a new one.

I have ordered new driver and output chips since that seems to be a common problem. I plug in the amp, and turn it on and the amp starts to slowly smoke. I shut it down and open up the amp to discover where the speaker chord connector connects to the circut board, the connector on the speaker chord is a little brown, hence the source of the smoke.

What should I do to start to troubleshoot this, I can have access to a multimeter and basic tools. Thanks

[Enzo]

Well, for starters, disconnect your speaker until you are sure the amp is not putting out DC voltage to it. In fact, take the wires off the speaker, and connect it to some other amp and see if it even still works. It might already be half melted.

[kenny_viper2]

Well, for starters, disconnect your speaker until you are sure the amp is not putting out DC voltage to it. In fact, take the wires off the speaker, and connect it to some other amp and see if it even still works. It might already be half melted.

It doesn't blow the fuse anymore, I replaced Q1 and Q4, but now R34 is smoking when I turn the amp on. The speaker is fine, and is disconnected.

I plugged in headphones and a bass and it was way distorted for a moment, but it was working but then nothing(then the smoking started). R34 is charred pretty good. Any suggestions? Order R34?

[Enzo]

Did you check for DC voltage on the speaker wires?

Q1,Q4 are drivers, how about output tranasistors Q2, Q5? Either of them shorted. We don't usually see drivers short on their own. And while there, check the four diodes next to them, D 6,7,8,9. They are all zeners, but just check them like they were regular diodes. If they act OK that way, they are probably fine. Looking for any that are shorted mostly.

If R34 gets hot, that means excess current is flowing through it. That immediately makes Q4 suspect. I don;t care if you already replaced it, test it. The excess R34 current comes from somewhere. It is either through Q4 or from Q2. If R41, R35 and further on down are not burning up, that points us towards Q2. Check the health of the output gate resistors, R33 and R49. If either of Q2 or Q5is shorted, replace both. If one is bad, the other may test good, but it has seen stress looking at the shorted one. This is not the place to be cheap for a two dollar part.

Since you replaced the drivers, check all four of those 220 ohm resistors in series with them. R34, R41, R35, R31. An open one will screw you.

And also since those were cooked, check anything working with them, in particular the bias circuit, whioch is Q# and surroundings. Check Q3, but for me, they cost under 25 cents, so I would just replace it. Make sure the bias control AP1 is OK, does it adjust down to about zero ohms and up to something like 500? I imagine the upper resistance will read a bit low due to parallel circuittry, but you an tell if there is a smooth rise from the zero end to whatever the highest reading is.

R50, R42 feed the bias circuit from the +/-16v rails, and there are caps C31, C20 to keep it smooth. We want no open resistors or shorted caps here. And that leaves R43, R44 around Q3, check them too.

C17 is probably OK, but check R28 for open or burnt condition. 3.3 ohm 1 watt. Those parts are your zobel stability network - very important.


I wil be surprised if Q2 or Q5 is not shorted, but certainly other problems could be at fault.

Don't power it up until you have checked ALL the above parts cold.