Author Topic: Low and distorted audio output on my Amiga 4000 (Read 1006 times)

servitus · « **on:** April 14, 2014, 11:27:10 PM »

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 ?

danbeaver · « **Reply #1 on:** April 14, 2014, 11:32:25 PM »

Quote from: servitus;762546

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 ?

Yup, contact Anthony Hoffman via ahoffman(at)clear.net.nz and schedule a diagnostics session & repair!

spirantho · « **Reply #2 on:** April 14, 2014, 11:40:34 PM »

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.

servitus · « **Reply #3 on:** April 14, 2014, 11:47:26 PM »

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 ?

Quote

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 ...

Castellen · « **Reply #4 on:** April 14, 2014, 11:59:03 PM »

Quote from: spirantho;762548

I believe from memory there's a 7905 regulator for the -5V.

The audio stage runs at 24V, from +12V and -12V only. The 7905 regulator is only provided for Zorro expansions that might happen to use -5V, nothing to do with the audio stage.

Looking at your measurements, the problem is that the -12V supply is missing from both op-amps. From memory, the track for this physically runs under and around the cluster of three 47uF capacitors, so you'll probably find the track under there has corroded open circuit somewhere. Will be easy enough to find without the capacitors fitted.

When done, you should measure about 1 Ohm between U402 pin 11 (and U403 pin 4) to the -12V pin on the board power connector, which is the middle pin on the side that's not ground. I can't remember the pin number and I'm too lazy to look at the schematic.

spirantho · « **Reply #5 on:** April 15, 2014, 12:22:19 AM »

You're right about it being +/- 12V, but I think I did have a problem on this bit of the circuit. I may be misremembering it but I think a ceramic capacitor had gone short which was shorting the legs of the 7905 and messing the entire -12v line up.
It was something like that anyway.
I've seen a number of failed ceramic caps in AGA Amigas.... Check for discoloration, that can be a giveaway.

First, though, check the connections as Castellen said, I'm only going on a vague memory! It may help though.

Lozspd4 · « **Reply #6 on:** April 15, 2014, 11:37:58 AM »

Hi check your -12v Signal comes in from the back of the mother through a via just above the first amp between amp and thats u404(amp) and c443 there is a via which supplies the amp the -12v Signal if the via is damaged due the the c443 leaking it will be damage it and will not suppy the amp with the -12v siganl. or the -12v line under u404 is damage. I Had the same Problem and this is what caused it in my case.

Regards Laurence

Author Topic: Low and distorted audio output on my Amiga 4000 (Read 1006 times)

servitus

Low and distorted audio output on my Amiga 4000

danbeaver

Re: Low and distorted audio output on my Amiga 4000

spirantho

Re: Low and distorted audio output on my Amiga 4000

servitus

Re: Low and distorted audio output on my Amiga 4000

Castellen

Re: Low and distorted audio output on my Amiga 4000

spirantho

Re: Low and distorted audio output on my Amiga 4000

Lozspd4

Re: Low and distorted audio output on my Amiga 4000