ad797 super hot at U9

[aphid310]

the ad797 opamp i've installed at U9 is getting super hot. i'm also having a hell of a time biasing this preamp. i have another working j99 and i've checked all my caps, transistors, diodes, etc against it. can't seem to find the problem. the regulators aren't switched or anything. i did have a problem installing sw2 and had to desolder it at one point (hell with all those connections) and a couple solder beds pulled away but i thought i got them all in there correctly. i know its not the opamps themselves because i've swapped them with the same results. i've also checked for solder bridges.

help!

chris

[tpryan]

If you suspect a problem, the first thing to do is lift one end of L1 or L2 until you get it figured out. That will prevent excessive DC current in U2 and probably eliminate the high temperatures. After that, try biasing U2 without U1 installed. Let us know what you find.

[aphid310]

by lift do you mean remove them completely? if that fixes it, does it point to any particular problem? i appreciate your help!

[aphid310]

okay, disconnected both L1 and L2, then poped out U1. that seemed to let me bias U2 and took the temperature down. should i do it again and connnect L1 and L2 one at a time?

i did notice that once i installed U1 that U2 would get jumpy again. it would start off high in voltage and would drop down slowly. couldn't get a good read.

[tpryan]

Reconnect L1 and L2, install the op-amps, and set the jumpers at J6 and J8 away from the ADJ position. If the voltage at the output of U2 drops to within a few millivolts of 0, you're done.

[aphid310]

okay, i did all that. but it still is getting hot. i went ahead and put it in the chassis to see what it sounded it like. the higher i turn up sw2 the noised ir gets. at 10 or 11 it turns into almost complete noise.

[tpryan]

If the voltage at the output of U2 drops to within a few millivolts of 0, you're done.

What voltage are you measuring at the output of U2? What you should see is the DC voltage homing in on 0v after every click of the switch. If that's what you see, your problem isn't bias compensation or the servo. If you're getting a lot of noise, the first thing to check is the jumper at J1 and J2. Is it the same as your working J99?

[aphid310]

yeah, the jumper setting are the same. i guess noise isn't as accurate to say as distortion would be. i'm wondering if there is a faulty part somewhere. like the regulators or a cap. i have no idea how to test that. i'll come back and give you the voltages on the output. i know they usually start pretty hight like 300 milivolts and steadily decrease but i will double check.

[aphid310]

okay, i finally got some time to gather the voltages. This is with the jumpers NOT set to adjust and Conn1 NOT shorted. Measurements where made by putting the negative lead from the DMM in a screw hole on the chasis and putting the positive lead on pin 6 or U2.

SW2 Click Positions:

1. 2.8mv but increasing slowly
2. 3.7mv steady
3. 3.6mv steady
4. 3.8mv steady
5. 1.7mv but increasing slowly
6. 2.0V it jumped from millivolts to regular volts and started to decrease rapidly
7. 1.8V and decreasing fairly rapidly
8. 1.7V and decreasing fairly rapidly
9. 1.5V and decreasing fairly rapidly
10. 2V and drecreasing fairly rapdily
11. 2V and decreasing fairly rapidly
12. 2.3V and decreasing fairly rapidly

[aphid310]

bump

[tpryan]

OK, but does the voltage eventually wind up at 0V? It might take the better part of a minute.

[aphid310]

some seem to want to but others increase or stay fairly steady.

would a problem with SW2 do any of this?

The only other thing i can think of is there is a problem with that other IC chip or the regulator on the U2 side. I think this because i've thrown the AD979's into my other J99 and they still work.

[aphid310]

any thoughts?

[tpryan]

Send it in and we'll have a look at it.