Installed OPA2604 into U4 - oscillating now?

[Fdesalvo]

I posted this in the wrong area prior.

After installing a socketed OPA2604 into U4 the following occurred:

-On some patches, Master and EQ LEDs would light up. On some, the master led would turn off after lowering the master via the front panel.
-No changes in noise level when adjusting OD1 via the front panel
-Low, pulsating bass tone/oscillations on some patches

After installing the socket, I used a meter to confirm continuity between the pins and traces and also between the opamp legs and traces. I have a 4 pack of new BB/TI OPA2604AP's and each one responded the same way.

Any idea of similar experiences?

[vansinn]

You forgot to mention which ADA model ;)

Well, too tired for analysis now, so just a quick shot off the hip..
Some fast / high slew-rate opamps may require a compensation capasitor to avoid freaking out; don't remember if this is the case with this one. but do some searching on the opamp model and compensation.

Does the processor behave the same way if you reinstall the original opamp?

[Fdesalvo]

Ha!  My apologies, it's an MP-1.  I understand I can add a small cap in parallel with the IC's fb resistor, but that's not worth the trouble.  I'm going to put in a lower noise version of the same opamp back into the socket.  I was perplexed because the OPA2604 is part of the noise mod and suspected something was wrong in this particular MP-1.

Thanks for taking a swing!

[MarshallJMP]

No need to add caps.These opamps have a miller capacity inside to avoid oscillations. I have put in lot's of these in MP-1's and never had these problems.

Got some more info about the patches you use where this happends (settings and stuff)?

[Fdesalvo]

Hey there.  I just put the 5532 back in and same issues. Noises persist even with the header removed from the tube board and u4 removed.

Preset 6 for example.  Loud mororboating. Master led illuminated. OD1 no impact. Od2 no impact. Master at 9.5. Adjusting down to zero changes pitch of noise and silences it at the bottom, but master led stays lit.   

[Fdesalvo]

Anyone else have any ideas?  Desperado!

[MarshallJMP]

Check your work, my guess something has happend.

[Fdesalvo]

I'm used to tracing signal flow through potentiometers, so I'm at a loss with where the master sits in the circuit.  I'm sure the issue is pre-master, so can you tell me where the master's circuitry is on the circuit board?

[Fdesalvo]

OK for posterity's sake, looks like the master is controlled by a TC9176P chip @ U13.  I'm just not sure how I damaged the board by installing the socket.  All traces are in tact.