Low and distorted audio output on my Amiga 4000

[servitus]

Hi - I have the usual audio problems due to leaking condensers in the audio circuit. I followed the audio repair guide of Anthony and got the following findings so far:

I downloaded the recommended audio utility and started the test... First of all, the audio output volume is fairly low but not unusual distorted as far as I can judge the 1kHz signal. But when playing a game the sound is very distorted and crackling all the times and is hardly recognizable.

I already changed allmost every part in the audio circuit with new ones: U402, U403, C430, C440, C431, C441, C432, C442, C403, C405, C406 and all electrolyte condensers on the whole motherboard (and some resistors within the audio circuit).

The I successfully measured 750 ohms between 8-9 and 13-14 on U402 (power off). But... I measured only 300 ohms between 5-8 on U402 (instead of 20 kOhm as described in the repair guide), whereas between 3-14 I measured the described 20 kOhms.

Unfortunately I don't own an oscilloscope and it would be a bit expensive to buy one only for this task. But I measured the voltages with my multimeter and got the following results (power on / no audio / both cables connected to my 1084s monitor):

U402
-----
Pin 1: +5.0V to +5.9V
Pin 2: +5.8V to +6.0V
Pin 3: +6.45V (changeless)
Pin 4: +11.8V (changeless)
Pin 5: +4.85V to +5.2V
Pin 6: +4.92v to +5.2V
Pin 7: +4.95V to +5.15V
Pin 8: +4.5V to +4.8V
Pin 9: +4.45V to +4.55V
Pin 10: +2.35V (changeless)
Pin 11: +1.59V (changeless)
Pin 12: +2.35V (changeless)
Pin 13: +6.0V to +6.2V
Pin 14: +6.48V (changeless)

U403
-----
Pin 1: +6.25V (changeless)
Pin 2: +6.05V (changeless)
Pin 3: +0.95V (changeless)
Pin 4: +1.6V (changeless)
Pin 5: +0.95V (changeless)
Pin 6: +4.15V (changeless)
Pin 7: +4.5V (changeless)
Pin 8: +11.85V (changeless)

I also measured VREF between GND and C403 => 2.35V.

Any ideas how to proceed ?

[servitus]

thanks for the useful hint danbeaver... but those repeated bits you've posted are pure waste of server capacity and completely cocks the clarity of the thread...

I know the the voltage on pin 11 of U402 is way too low and wrong poled (+1.59V instead of -12V). Moreover the resistance between 5-8 on U402 is also much too low (300 ohm instead of 20 kOhm).

Possibly someone of you experienced a similar situation and came up with a solution ?

I believe from memory there's a 7905 regulator for the -5V. Check the  voltages and the capacitors (including ceramics) on the voltages around  the op-amp, particularly the -5V on the regulator.       

Great - thanks spirantho for the information, I'll check this and come back with the results ...