PPI A404

Perry Babin

Paid Member
2003-11-20 11:01 am
Black probe on one of the non-bridging speaker terminals for all of these...

DC voltage on pin 3 of the driver board?

DC voltage on pin 4 of the driver board?

DC voltage on speaker terminal with 22v?

Polarity is important for all of those.

If you touch a speaker or load across the channel, does the amp draw current or does the load drag the voltage down to 0v?

Check the 100 ohm resistors on the driver board.

Resistance A?

Resistance B?
You have a first version A404, not a A404.2. So you have a discrete channel design. By your description I say ohm out the outputs for the damaged channel first. The drivers might be shot also. To have rail voltage on the speaker terminals you surely have shorted transistors, its just a matter of ohming out the bad ones and in 99% of the cases its the outputs and possibly the little MPSA 06 and 56 drivers located near by... Any DC offset after that is likely due to damaged input transistor pairs.

The reason your getting 40 amperes current draw is because the shorted outputs are directly dumping the power supply rails into your speaker load. This will cause the excessive current draw your experiencing. You should not connect anything to a channel that exhibits any voltage output on its speaker terminals above a few milli-volts DC. I always test this with a meter first then connect speakers. you might end up buying test speakers on a regular basis doing what your doing.

It always helps if you post a picture of the amp your working on. Both Perry and others including myself can use that picture to guide you with if its high enough clarity. I just finished 14 of these myself for a collector so my memory is fresh with them currently.
Check your outputs they are the highest probability of failed devices, and also the to-92 drivers connected to them the MPSA-56 and 06 devices they tend to get real beat up on output shorts....hope this helps some...
Will post a pic up as soon as my camera gets done charging..

I replaced Q6,Q7,Q8,Q9--outputs

I replaced Q19,Q20,Q21,Q22

Also replaced Q17,Q18,Q29,Q30,

I have the black probe on left rear negative terminal and red probe on right rear + terminal..

If i measure directly across the output termianls for the right rear channel or use amps ground termianl as reference in every way i test i get 22 volts..
I only have a A204 here to reference to so i will wait for the pic. But it sounds like you completely replaced the channel so far. There are 2 each MPSA18's < input pair > and the rest are MPSA06 and 56 up to the outputs.

Possibilities include the following:
1: possible solder bridge
2: possible shorted cap or diode
3: possible shorted mute fet
4: possible bad new device installed or TO-92 transistor installation error
5: possible bad trace or solder via

In the mean time I would carefully compare the nearest good channel that mirror images the bad channel to make sure everything is correct between the good channel and the bad channel visually.