PPI A600 offset

[miniman82]

I have around 1.7v DC in the left channel, and am unable to null it out by adjusting the offset pot VR5 on the board. Is this generation amp prone to driver card issues like the later ones were?

 

[Perry Babin]

What is the DC voltage on the power supply terminals of the op-amps?

Is the offset pot for the other channel need to be near the end of travel for the offset to be set?

Does it have the standard driver board or is one of the black driver boards?

 

[miniman82]

The other channel seems to respond correctly, I’m able to null it to 0mv and the control is just slightly to one side of center. The OPAMPs all check in at +14.3/-15v, so that seems OK as well. The driver cards are the original style, looks like white ceramic material.

 

[Perry Babin]

Will the offset channel drive a 2 or 4 ohm load to clipping (symetrically)?

Compare the voltage on the pins of the 2 driver boards. Which ones are different? Post the voltages for those that are different.

Check the 100 ohm resistors:
http://www.bcae1.com/temp/ppidriverboardresistoroverlay01.swf

 

[miniman82]

Only pin I see different is pin 12, see attached. Pin 12 on the bad channel is sitting at -2.5v while the good channel is at close to zero. I zoomed in with a loop, looks like the trace goes to the collector of one of the two larger SMD transistors on the card.

 

[Perry Babin]

Did you check the 100 ohm resistors behind the driver transistors?

http://www.bcae1.com/temp/ppidriverboardresistoroverlay01.swf

 

[miniman82]

I can't view the animations anymore, flash doesn't exist. What does the animation show?

 

[Perry Babin]

Flash exists except for those who do what they're told without question.

See attached.

 

[miniman82]

I get 98 ohms for both drivers on both cards.

 

[Perry Babin]

Compare the resistance from pin 12 to pin 15, to the negative rail and to the emitter of the large NPN driver transistor on the driver board, for the two boards.

 

[miniman82]

Bad channel:

12 to 15= 12.77k


Good channel:

12 to 15= 12.64k


Negative rail to emitter just causes the meter to count up, it’s charging a cap somewhere.

 

[Perry Babin]

With 1.7v offset, I'd expect it to show up at the feedback terminal of the driver board but you have it at 0v. Was it 0.000v?

Is the DC offset pot on the other channel near centered?

Are the ±15v regulators very nearly the same magnitude from the secondary ground?

 

[miniman82]

Which pin is feedback, I’ll check.

 

[Perry Babin]

Diagram from another amp that uses the ceramic boards.

 

[miniman82]

Rechecked, I have -.150v or so at pin 4. Meter lead probably wasn’t making a decent connection last time or something. I assume the -2.5v present at pin 12 is where the offset is coming from?

 

[Perry Babin]

With the black probe on one of the non-bridging speaker terminals, is the speaker terminal offset positive or negative?

With the black probe on the same point, what's the exact DC voltage on terminals 3 and 4 of the driver board?

 

[miniman82]

-1.7v on the left channel

Terminal 3: -.003v
Terminal 4: -.150v

 

[Perry Babin]

Use that same point for the black probe.

It doesn't look like you have much offset on the input.

What's the range of offset on the two channels? Read on pin 3 of the driver boards.

 

[miniman82]

So both cards are apparently seeing the same thing, I get a range of around -6mv to +15mv for them both when adjusting the offset pots. Not sure if this is significant, but I also noticed the card on the good channel gets warm but the one for the bad channel stays stone cold.

 

[Perry Babin]

Why is there -3mv on the input if you can get +15mv?

Does the bias pot change the idle current for the defective channel?