DC offset OPA 134

[tpryan]

See if you can adjust U4, pin 6 to be the same voltage as U4, pin 3 using R28. Is U3 still getting hot?

[rdatman]

No, there is no way to equalize those voltages-- the potentiometer does nothing to either lower the voltage at pin 6 or raise the voltage at pin 3.

16XX gets hot in less than 30 seconds from cold start up.

[tpryan]

I'd replace both U3 and U4. You can repair the cut trace with a solder bridge or snip of solid wire soldered across the gap. Remember that it's much easier to snip the ICs out and then remove the pins one at a time. You're much less likely to damage the PCB that way.

[rdatman]

Thats a good tip, thanks! I've done it that way before if the solder sucker was not doing a good job.

I will install a socket at the OPA134 location. Any guess as to how this might have happened? I have not had any problems that I could find-- just located the problem upon trying to install a 100k multi turn pot?

[rdatman]

I removed/replaced the two opamps. The problem is gone. Since I don't know which one was burned out, it's going to remain a mystery as to what actually occurred.

Thanks for you help. I did install a socket on U4 and bonded pins 5 and 8 so that the DC offset would work for chips that are wired like the OPA 627.

Is there a reason that that chip wouldn't operate properly, I have read that it is not designed for a circuit that amplifies by 60 DB or more, but that kind of information is vague in nature. I have not done any acoustical tests or oscilloscope measurements yet, but I think the circuit is stable.

I also ordered some 1115 chips to try those as well.