AMB Laboratories DIY Audio

[amb]

Check your trimpots to make sure they are the correct values at each location, and the values of resistors everywhere (measure them, and compare against the good board).

When you adjust the DC offset with VR4 (a little at a time), also check what happens to the voltages across R11, R13 (and R12, R14). One should increase and the other should decrease, but how much are they varying by?

If this doesn't help to pinpoint a problem, then check all your voltages against the β24 (class AB) operating points PDF and see if there is anything grossly out of whack.

[oneplustwo]

Checked the trimpots and they are correct. If I measure the resistors and compared against the good board, that wouldn't necessarily mean that a discrepancy is in that specific resistor since it will be measured in the circuit overall, correct? But that could give an indication of what to trouble shoot further?

I'll check what happens with R11-R14 as VR4 is adjusted tonight and report back. Will also try to check voltages (I do this with the bias up for 40 mV and ignore the fact that offset is not zero, correct?). There's a lot of them! Does it makes sense to proceed from outside in or left to right or something else?

[Pinnocchio]

Did you short the input to ground while doing the adjustment of the second board?

Ciao!
Do

[oneplustwo]

Did you short the input to ground while doing the adjustment of the second board?

Ciao!
Do

Yup. I used both jumpers per the instructions at J1. Good question though... I hadn't at first but caught it before getting too far.

[amb]

Checked the trimpots and they are correct. If I measure the resistors and compared against the good board, that wouldn't necessarily mean that a discrepancy is in that specific resistor since it will be measured in the circuit overall, correct? But that could give an indication of what to trouble shoot further?

Yes, that would be useful info to help determine what's going on.

I'll check what happens with R11-R14 as VR4 is adjusted tonight and report back. Will also try to check voltages (I do this with the bias up for 40 mV and ignore the fact that offset is not zero, correct?). There's a lot of them! Does it makes sense to proceed from outside in or left to right or something else?

\
Yes, there are a lot of them, but I recommend you take the time and get as many readings as possible. Be careful with the DMM probes -- a accidental short can cause a disaster. Ignore the offset for the moment and just take a full set of measurements, and then try to adjust the offset down to zero and make the problem happen. Re-take some measurements to see where there are big changes.

[oneplustwo]

Is it normal for the amp to take quite some time to stabilize? When I first power on, voltage through r45-r48 start at about 75mV and decreases from there. On the other hand, vunreg starts out around 25V and goes up from there but maxes out around 36V. The unloaded side is about 42.5V from the get go.

[oneplustwo]

Ok... Here you go! Hope it's readable! I tried to circle the potential problem areas.





Also the actual links.

http://farm6.staticflickr.com/5115/7066 ... 00eb_o.jpg
http://farm6.staticflickr.com/5349/6920 ... eae4_o.jpg
http://farm8.staticflickr.com/7205/6920 ... 4e16_o.jpg

[amb]

Can you please add the polarity of the voltages? Many voltage readings are not useful unless I know they are positive or negative relative to ground.

[oneplustwo]

Ok. I think most all of them were correct according to the schematic. Which ones are the critical ones? Is it safe to assume it's the smaller mV readings?

[amb]

Yes, if you look at the schematic from top to bottom, I am interested in the polarity of all the voltages near the center in that direction, across the whole circuit from left to right.