Ampeg BA-115 Blowing Output ICs

[stereomonostereo]

Hey guys,

I had an Ampeg BA-115 (100 watt) blow both of it's output ICs and take out the mains fuse. I replaced the two ICs, IRFP-140R and IRFP-9140R, with an IRFP-240 and IRFP-9240. I powered the unit up and it worked, until I put a signal through, at which point it blew the fuse again. The FET gate resistor R49 reads 38 ohms instead of 47. I'm getting -25 at D of Q5 and -36 at S. Is my problem in the voltage mismatch?BA115_Schematic[1].pdf

[nickb]

Hey guys,

I had an Ampeg BA-115 (100 watt) blow both of it's output ICs and take out the mains fuse. I replaced the two ICs, IRFP-140R and IRFP-9140R, with an IRFP-240 and IRFP-9240. I powered the unit up and it worked, until I put a signal through, at which point it blew the fuse again. The FET gate resistor R49 reads 38 ohms instead of 47. I'm getting -25 at D of Q5 and -36 at S. Is my problem in the voltage mismatch?[ATTACH]29483[/ATTACH]

-25V at drain if Q5 is wrong as it should be close to zero.
Don't plug in a speaker and use light bulb limiter until you get this fixed.
The gate resistor is most likely a meter reading / tolerance issue I would not worry about it.
Test Q1, Q4 and associated resistors.

[Enzo]

Not sure whwy you used different parts for the outputs.

It would be extremely unusual for a 47 ohm resistor to change to 38 ohms. More likely there are parallel current paths that cause it to read low. If you suspect it, disconnect one end of it and remeasure it.

The source end of Q5 is at V-. Schematic says -40, but -36 is close enough. You have -25 on the D, which is the amp output, because either Q5 is being turned on or it is failing.


Once you replaced the output transistors, you also need to check those zeners, D6-9, and Q1, Q4 are suspects.

Important to set bias per the note on the schematic.

[stereomonostereo]

I used the different chips because they were what I had on hand. Should I stick with the 140 and 9140?

Q1 and Q4 test okay, as do the Zener diodes.

My mistake, I meant -25 on G of Q5.

[Enzo]

Q1 controls Q5, so what is on the base of Q1? What is on the gate of Q2?

[stereomonostereo]

I've got -4.1 on the base of Q1, and 36.2 on the gate of Q2

[Enzo]

Sounds like your bias is not working. Did you try to adjust the bias when it "worked" the first time? Does it adjust at all?

The power amp requires +16 and -16, are both present and up to voltage?

[stereomonostereo]

I'm only getting +11v instead of +16v. I've traced back to R68, where I'm getting +34 on one side and +11 on the other. The resistor reads good. Looks like I've got a short somewhere in the chain

[stereomonostereo]

When I pull U3, my +16v appears. I replaced it, and I'm back down to +13v. I'm getting +3v on pin 7. It seems to be right in that area. I could be missing something simple, as I'm more of a tube amp guy than solid state.

[stereomonostereo]

Looks like U3 was at fault. Put the output mosfets in, adjusted bias to 5mV, and everything seems stable. Thanks for the help guys!