Help with blown JE 990 op amps please!

[skidmorebay]

I have (had) four DIY JE 990's from Hairball in my Igor Sontec EQ. I don't know what happened, but all four of them suddenly failed (PSU issue I think.)
With a +4dbu tone at the input, they were outputting 20V RMS on the output! I also had around 1V DC on pin 3 of the inputs (which seems to have fried an ADC channel in my interface!)

Any idea which components on the 990's might have been fried? I would love to be able to replace a few active components rather than buy four new op amps.

Thanks!

Schematic below:

 

[JohnRoberts]

Modern VOMs have a diode test function. You should be able to probe transistor junctions looking for opens or shorts. Besides the obvious base emitter junction diode, you should be able to measure a base collector diode. Some of these measurements can be compromised by connected circuitry but this is a good start for troubleshooting.

[edit- indeed losing 4 at the same time sounds like a common fault, like PS over voltage. /edit]

JR

 

[ccaudle]

they were outputting 20V RMS

RMS usually implies AC voltage. Do you mean that the gain is unexpectedly very high, and the +4dBu input is now 28dBu at the output, or that there is 20V DC on the output? Very different things. Over voltage from a power supply would not be expected to change a feedback path and increase gain, but could easily cause DC voltage on the output.