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.

 

[pucho812]

whenever faced with a challenge of such nature start with the obvious.

1. I always start with the PSU and make sure it is sound, in working order.
2. do you have another channel to compare it to? if so, you can swap 990's around and see if it comes back. Ideally you can work out which 990's need attention and which do not. It may not be all 4. That is assuming it is the 990's Isolate, divide and conquer.
3. If you are positive it is a 990, do you have the correct schematic for the 990. You can run it on a opamp test jig as sold by five fish labs and others but in the end all that will tell you is if the opamp is faulty.
4. a good DMM will allow you to check diodes and even check the transistors for the basics( all all junctions working?). You can also get a transistor checker from peak electronics (dca75). The peak electronics transistor checker is very nice and easy operation. It's allowed me to match transistors and do full diagnostic on them. it's not cheap as it will run you the cost of a couple of 990's but it is worth it if you deal in a lot of discrete circuits like I have had to.