797 overheating (solved--two bad 797s, just my luck)

[tpryan]

What voltage do you measure at U1, pin 1 with the IC removed?

[paavo]

2.4mV, falling to 0 over 45 seconds or so.

[paavo]

That's with J6 jumpered to Normal.

I'm trying my best to follow up on the servo's failure to null out the voltage at U1, pin 6, but can't seem to bring the big picture together. U3 is outputting around -11.74V with an IC in U1 and J6 jumpered to Normal.

[paavo]

Wow, didn't expect this, but just for kicks, I left my probes in U1, pins 1 and 4. After settling at 0, which is when I ran over to the computer and reported resuls, it dropped back down even further and is now drifting between -5.3 and -5.4 mV.

[paavo]

That's with J6 jumpered to Normal.

I'm trying my best to follow up on the servo's failure to null out the voltage at U1, pin 6, but can't seem to bring the big picture together. U3 is outputting around -11.74V with an IC in U1 and J6 jumpered to Normal.

That's -11.74V at U3, pin 1, which I'm guessing isn't quite as crucial as pin 7 (pin 7--since there's nothing at U2?--starts off reading 48mV or so, then rises at a rate that's unchanged when I remove my DMM's leads and hold them in space...ah, that mildly conductive NYC air....)

[tpryan]

Do you get the same 16.6V output with either 797 at U8?
You may have two bad 797s, but the odds of them being defective out of the chute are remarkably high.

[paavo]

One registers 16.6V, the other 16.1V.

[tpryan]

Try a new pair of op-amps. For testing you can use 5534s, which I'm sure you can get locally.

[paavo]

Try a new pair of op-amps. For testing you can use 5534s, which I'm sure you can get locally.

I was about to ask about this; thanks much!

And thanks in general for helping out here.

[paavo]

The internet has not been kind to the bricks-and-mortar electronics industry. Can't find a 5534, but from what I'm seeing, the LM741 is configured similarly, and I might be able to get my hands on one today. Any danger of using that one for testing purposes?

[tpryan]

Useful for testing but guaranteed to sound terrible.

[paavo]

I've tried twenty places trying to find a suitable stand-in, and nothing. If anyone's got ideas as to where I might find a cheap replacement for an AD797 in NYC, I'd be obliged.

[paavo]

Useful for testing but guaranteed to sound terrible.

Well, it's looking like a moot point, but I'm willing to spend a buck (and subway fare) to isolate the problem here. If it sounds awful but handles voltage correctly, that'd tell me one thing; if it sounds awful and repeats the problem I'm having with the AD797s, that'd tell me another, right?

[paavo]

n/m. Just ordered some more 797s overnight from DigiKey, so...I dunno. Completely defeated at this point.

[paavo]

Two new AD797s overnighted from DigiKey, and everything's working. Voltages and heat, at least.