Ampeg BA-115

[Tvrepair]

I repaired unit but when you plug to a speaker it blows fets in output section with test speaker in bulb in series it works fine help

[Jazz P Bass]

If the outputs 'blow' when a speaker is connected, then it was not fully repaired.
Without a load connected always look for dc volts on the output.
If a high value is observed, the unit will blow the outputs.

[Tvrepair]

No fixed had .3 volts on output will work fine on protected speaker

[glebert]

Are the FETs from a known good source, ie no counterfeits?

[nhbassguitar]

I'm confused about the test setup. What do you mean by "protected speaker?"
Do the outputs blow at idle (volume at 0), or only at some volume setting and above?
Was this amp abused before it came into the shop? Asked to deliver too much volume in a band setting? If so, the voice coil of the driver that was in service might be partially shorted.
Voltages look ok according to the schematic?
Bias adjusted properly per note on schematic?

[nevetslab]

Looking at the schematic, we're dealing with single N-Ch and P-Ch MosFETs IRFP 140 & IRFP 9140 devices. There is a power resistor R61 on the negative side of the Source terminal thru which Bias current is measured to set it to 5mV (which would be 5mA Id @ idle). While in testing/servicing mode, did you check to see if the Clipping Eliminator circuit works (the circuit around U3A which has a VacTEC driven from the amp output to alter the feedback in U3A. You'd also need to verify the driver xstrs Q1 & Q4 have equal emitter current. In this amp's output stage circuit, there's no simple way to measure the Id of both the N-Ch and P-Ch MosFET's as there are in multiple MosFET circuits having individual Source resistors.

Bass Amplifier PCB Schematics BA115a (103XXC_1).pdf

[g1]

With a bulb in series with the speaker, you are limiting the output current. It is that current that is blowing the Fets.
So it is not 'working right' with the bulb, it is just that you are protecting it from damage due to excess current in the output devices.

[Jon Snell]

It looks to me that the DC on the speaker out line is present for too long, causing damage to the output FETS. Never use Fleabay FETS, they are not what you think!.
We presume the set up procedure for quiescent current and the DC off set is as required and you monitored the DC on the Drains of the output FETS Q2 & Q5 and the Emitters of Q1 & Q4. U3 b sets the DC off set.
This is a very simple amplifier but people get confused between current and voltage. This uses Norton current mirrors to drive the output stage.
What was the original fault and what was replaced?