PPI A600 offset

[miniman82]

I don’t understand your question, with probe on pin3 as requested and offset pot fully CCW I get -6mv and fully CW I get 15mv. Is that not normal?

 

[miniman82]

Bias pot does nothing on the bad channel, works normally on the good channel.

 

[Perry Babin]

You have offset on the output of the amp but you don't have the offset adjustment maxed out. If it goes to +15mv, why is it at -3mv.

The actual voltage may also change, depending on where you have the black probe. The non-bridging terminals is where you need to have it.

 

[miniman82]

I’m not following what you’re saying.

You asked me to put the black probe on the non-bridging terminal, which I understand to be the common bussbar between the two channels. This makes sense to me, since it’s the common point of the secondary of the power supply.

Next you said check the range of the offset pots while reading pin 3 on the driver cards, and I noted a range of -6 to +15mv with the positive probe there.

So I don’t understand what you mean when you say how could it go to 15 if it’s -3, both cards do the same thing when reading pin 3. Did you mean read the output terminal instead???

I’m lost.

 

[Perry Babin]

If you have -3mv on the input pin and the offset can drive it more, why don't you set it as far as it can go to get rid of the offset at the speaker output?

What are the positive and negative regulated supply voltages on the op-amps? Use the non-bridging terminal for the reference.

 

[miniman82]

Um, because it doesn’t get rid of the offset no matter how far I turn it?

On the bad channel the pot will only vary it from -1.7 to -1.5v at the output terminal, good channel works like I’d expect it to passing through 0 and varying positively or negatively like it should.

 

[miniman82]

I think the problem must be between the driver card and the output stage somewhere, it’s like the positive half of the output stage isn’t responding or something.

 

[Perry Babin]

What's the resistance across the 2-pin header for the bad channel?

On the driver board, confirm that all of the traces are intact between the transistor terminals on each end of their trace. The traces sometimes open.

 

[miniman82]

Both channels have 1.147k ohms at the header.

 

[miniman82]

Traces look good to me through a jewelers loop, only thing I can think is something with the bias circuit would you happen to have a diagram of that section?

 

[Perry Babin]

I would do a terminal-terminal ohm-check of the traces.

I'm assuming that you've already checked the transistors on the driver board.

Diagrams of the driver board have been posted on the forum before. See if you can find them with a search.

 

[miniman82]

Yes, checking the individual devices with diode check mode was one of the first things I did since I know these driver cards are suspect.

 

[Perry Babin]

Have you tried swapping the two driver boards to see if the problem follows the board?

 

[miniman82]

Hoping I don't have to do that, this board looks like it will get damaged really easily.

Safe to assume you think it's the driver card?