Trouble with Last Step...

[PLAbass]

Hello!

I've just finished my first A12 and I'm having a bit of trouble with the last step. I'm pretty green with electronics so bare with me. Everything checks out up to the point of installing the opamp, measureing pins 4 and 6 of U1 and lowering the voltage. When I apply power the readings jump around quite a bit but usually hover somewhere around 16-17.5 volts. This is with the ground lead on 6 and the hot on 4. Reversing the order gives me a negative reading. Adjusting R19 does not seem to have an effect. Any ideas?

[tpryan]

Is the op-amp getting hot? Pull out the op-amp and measure the voltage between pin 4 (ground) and pin 1. Let us know what you find.

[PLAbass]

I didn't notice the amp getting particularly hot. Measuring pin 4 to pin 1 doesn't seem to give me a reading...

[tpryan]

You have 16VDC at the output and the op-amp isn't getting hot? Either something is wildly wrong, you're not making the measurement correctly, or both.

[PLAbass]

I could go with either one of those...

The opamp is definitely not getting any hotter. Perhaps I'm taking the measurement wrong. I can get it to stay pretty steady at around 17.48 volts... When taking the initial measurements between U1 3 and 4 I got 18v with no problem. I've inserted the opamp, flipped the board over, applied power and am taking the readings from both the solder joint and the tip of the mill-max recepticle. Is this correct so far?

[PLAbass]

I will check over my work again today with fresh eyes but are there any ideas as to where I might look for a problem? I've realized that I didn't install the 0 ohm resistors at L1 and L2. Would this contribute to my malfunction? Does it mean anything that I'm reading/adjusting the voltages correctly at U1 before the opamp installation but after poses the problem? I believe everything is right but where might I start looking for backwards parts and such?

[tpryan]

With your meter set to read voltages in the 20v range and with the black probe connected to pin 4 of U1, what do you get at U1, pin 3, pin 5, and pin 6? Let us know what you find. Don't install the jumpers at L1 and L2, that may be why you don't have a smoking op-amp.

[PLAbass]

I assume this measurement is to be taken with the opamp in? I will test tonight and report back! Thanks for the assistance!

[PLAbass]

Alright here's the readout (with the negative on pin 4):

pin 3 = 18.09
pin 5 = -18.09
pin 6 = 0

and again, with the negative on 4,

pin 6 = a jumpy 17.48

Does this give any clues?

[tpryan]

The first measurements are with the op-amp removed, and the second measurement at pin 6 is with the op-amp in place?

[PLAbass]

All of the measurements were taken with the opamp installed. I apologize, the measurements do not come up immediately. The voltage at 6 fluctuates quite a bit. If the leads are held in position for a few seconds the actual reading is -17.48.

pin 3 = 18.09
pin 5 = -18.09
pin 6 = -17.48

[PLAbass]

Alright so a couple weeks later and I've finally found the time to assemble the second kit. This A12 works just fine, everything checks out and I'm able to adjust the voltage down to 0 without a problem. I've tried switching the opamps in the disabled kit but it has the same effect so at least I can rule out a bad opamp (though I'm sure this wasn't really a possibility, just makes me feel better). Everything looks fine on the disfunctional one and it looks identical to my functional kit. Not really understanding why I'm getting these reading the way they are... Should I try to install L1 and L2?

[tpryan]

Can you post the voltages you measure at U1, pin 1 and pin 2?

[PLAbass]

With the ground still on 4, at pin 2 I get -6.66v but pin 1 isn't giving me a solid reading. This is with the opamp in place. Between 1 and 2 I get about -50mv (with the ground on 2).

[tpryan]

With the gain at maximum, power off, and the op-amp removed, what resistance do you measure between U1, pin 1 and J7, pin 4 (AGND on the power connector)?