First A12 - Trouble with last step

[bensampson]

I just finished building 3 N72's and now I'm on my first A12 and at the last step I can't figure out what's going on.

I got to the part where I calibrate it to +/- 18V and that works fine, but when I'm on the last step measuring between U1 pins 4 + 6 and my DMM is set to the lowest setting my meter's off the charts. When DMM is set to the second highest setting I get 13.1V. It doesn't seem to change when I turn R19.

Where should I be looking for error?

Thanks

-Ben

[bensampson]

BTW I have 2 leftover resistors that don't seem to be on the parts list - 2 resistors with 1 black stripe

[tpryan]

What happens if you swap op-amps?

[bensampson]

ooh good call...I switched to another op-amp and now it's looking better, on the lowest voltage range on my DMM and with R19 turned fully counterclockwise it's at around 40, and it goes down with every turn. I'll get it down to 0 and test it with a mic and let you know what happens.

What does this mean for the other op-amp, is it busted? If so is it possible to get a new one by tuesday?? Jeez I'm getting nervous, coming real close to this session and I have 3 more A12's to build and get working!!
I could pay extra for overnight shipping if need be.

[tpryan]

All of the op-amps tested fine when they shipped, so it's a bit of a mystery. Getting a new one by Tuesday is possible, but shipping will be expensive. E-mail me for details.

You won't get signal through the module until you install L1 and L2, or jumpers instead (See step 8 ). That's what the two zero ohm resistors are for.

[bensampson]

aaahh missed that part about the 0 OHM resistors. That's probably it.
Once I get this one working I'll swap the op-amps again to make sure I wasn't testing it incorrectly. If it really is busted, does that mean I did it somehow, if they were fine when they left? How would I have done this?

I appreciate the quick replies Tim

[bensampson]

So I tested all 4 op-amps you gave me and they all work fine except for that one. It gives me a voltage reading of 10.4V (on the second to highest DMM setting, 200). All the others are reading near zero on the last DMM setting.

I got signal to pass BTW, happy to have a working A12 now.
I'll email you about the op-amp.

Thanks Tim! Hopefully this will be the last problem you hear from me, 3 A12's to go and I'm just waiting on the op-amps for the J99.

Thanks for being so patient.