AMB Laboratories DIY Audio

[Pinnocchio]

I had this issue as I remember... Play with one of the other pots slowly and it should start getting some traction... I would personally unplug the power then measure each trimpot of the working amp and adjust the pots so they read just about the same resistance on the second amp. Fire up and do the final adjustments.

BTW, leave the amp running for at least 15 min since with the heat, the settings will drift a little and you'll need to adjust again.

Do

Where are you measuring trim pot resistance?

Directly under the board would be ideal but looking at the schematics, you can probably measure VR3 on R26 and R23 (check schematics for proper side of resistance). VR1 from D2 cathode and D1 anode. VR4 can be measured from D1 anode and Q1 Source pin I believe

Do

[amb]

Update: I get through all steps ok now up to offset. With 40mv on both sides, I get like 400mv offset. When I dial that down, the quiescent current goes down with it and I can't raise it up again. I tried the process twice with the same result.

40mV on both sides of what?

[oneplustwo]

Sorry...across r47 and r45.

[amb]

OK.

Were you able to achieve 4V across R7, R8, R9 and R10 (by adjusting VR1)?
And achieve the same voltage (~130mV) across R11 and R13 (by adjusting VR4)? Btw you should also have ~130mV across R12 and R14.

Also, check the voltage drop across your two 22 ohm temporary resistors to see how much voltage is being dropped on them.

[oneplustwo]

OK.

Were you able to achieve 4V across R7, R8, R9 and R10 (by adjusting VR1)?
And achieve the same voltage (~130mV) across R11 and R13 (by adjusting VR4)? Btw you should also have ~130mV across R12 and R14.

Also, check the voltage drop across your two 22 ohm temporary resistors to see how much voltage is being dropped on them.

Yes, got all the previous steps fine. 4V across r7-r10. About 120mV across r11-r14 each.

8.8-8.9V across both of the two 22 ohm resistors.

370mV dc offset.

[amb]

You said that adjusting VR4 to null out the DC offset would change the output quiescent current? Is this on both the hot and cold side? By how much? Are you able to null out the offset at all? At that point what are the voltages across the 22 ohm temporary resistors?

Also, does this happen on only one of your two boards?

[oneplustwo]

Just the r45 side. Actually, since you asked, I checked and the r47 side would go up (went as high as 100mV before i moticed and started turning it down) as offset went to zero. So opposite of the r45 side.

I can null offset but On r45, I could get bias up to 4.5mV at one point in the pots travel (but it was a local peak... Not at max CW.). R47 can be dialed up as expected.

Voltage across the 22 ohm resistors is .5 volts on both.

The other channel is fine BTW.

[oneplustwo]

BTW, the re-wiring of the sigma trafo worked fine.

[amb]

Only 0.5V across the 22 ohm resistors means that there is hardly any idle current. It should be around 9V when the bias is turned up. Are you sure there is 40mV across the output (Caddock) resistors? And they are 0.2 ohms as specified?

I say check all your solder joints and parts orientations carefully. Since you have a good board, it should be easy to compare.

[oneplustwo]

Only 0.5V across the 22 ohm resistors means that there is hardly any idle current. It should be around 9V when the bias is turned up. Are you sure there is 40mV across the output (Caddock) resistors? And they are 0.2 ohms as specified?

I say check all your solder joints and parts orientations carefully. Since you have a good board, it should be easy to compare.

Yes, I get 9V across the 22 ohm resistors when the bias on both sides is set to 40mV across R45 and R47. Apologies for the confusion, I was measuring after I had attempted to get offset to go to zero, at which point the voltage across them is quite low.

I removed the board and checked orientations and reflowed all the joints in any case. Reinstalled and went through the setup process again. Exact same result.

Not sure if this is a clue or not, but it takes a few minutes of warm up before I first get R7-R10 to measure 4.0V. In the beginning, with the pot maxed, I only get 3.6V across R7. But it comes up to 4.0V in about 2-3 minutes.

Also, as I mentioned, when I try to get bias to go down, R45 goes to zero while R47 goes up. (I verified they are 0.2 ohms.)

Any other data I can provide that would help?

Thanks for all the support!