A1200 - Distorted Sound

Lamaman71 · 08 April 2022, 21:19

Hi Folks, recapping my A1200 I thought I might resolve the (slightly) distorted sound issue - but it didnt.

It comes from both channels but all channels work ok. Sound is not terrible - but is slightly distorted. I have cleaned the phono sockets and double checked the new caps.

Any suggestions for where to look next?

Daedalus · 08 April 2022, 21:44

A couple of things to look at: First, the power supply. If either the +12V or -12V rails aren't working properly you'll get distortion (but the machine will still work otherwise). Second, if it's an early revision, it might have incorrect resistor values in the audio circuitry resulting in too high an output level and clipping when playing high levels. Third: if the capacitors had leaked, there might be damage to the board or other components in the area.

If you have access to an oscilloscope, using AmigaTestKit's audio test function is helpful for tracking down the source of a problem. It's not unheard of for the op amp to die, or for a resistor or ceramic capacitor to fail.

Lamaman71 · 09 April 2022, 02:56

Thanks Daedalus. I reflowed the op amp and thought Id fixed it - but the damn problem seems to have reappeared...When I recapped I didnt see any sign of leakage but I will double check.

Do you have any idea which resistors or caps I should test? There are quite a few!

Daedalus · 09 April 2022, 21:24

In general, pretty much any in the audio output area can cause issues with the sound. If it is an early revision board (rev 1A typically), there are a couple of feedback resistors at the op amp that aren't correct. They result in excess volume, and clipping when multiple channels are played. It's easy to see this with an oscilloscope - the sine wave from AmigaTestKit will be fine when one channel on each side is running, but when both on each side are running, the waveform is clipped.

Damion · 12 April 2022, 06:05

I had the same problem with one of my A1200 boards (nearly drove me batty). The board looked fine, but the problem was a corroded via in between C383 and C384, near the op-amp. (I also replaced the op-amp with one of these). Your issue might not be identical of course, but if you poke around in that general area I bet you'll find it.

08 April 2022, 21:19	#1
Lamaman71 Registered User Join Date: Sep 2021 Location: Katy, USA Posts: 154	A1200 - Distorted Sound Hi Folks, recapping my A1200 I thought I might resolve the (slightly) distorted sound issue - but it didnt. It comes from both channels but all channels work ok. Sound is not terrible - but is slightly distorted. I have cleaned the phono sockets and double checked the new caps. Any suggestions for where to look next?

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08 April 2022, 21:44	#2
Daedalus Registered User Join Date: Jun 2009 Location: Dublin, then Glasgow Posts: 6,334	A couple of things to look at: First, the power supply. If either the +12V or -12V rails aren't working properly you'll get distortion (but the machine will still work otherwise). Second, if it's an early revision, it might have incorrect resistor values in the audio circuitry resulting in too high an output level and clipping when playing high levels. Third: if the capacitors had leaked, there might be damage to the board or other components in the area. If you have access to an oscilloscope, using AmigaTestKit's audio test function is helpful for tracking down the source of a problem. It's not unheard of for the op amp to die, or for a resistor or ceramic capacitor to fail.

09 April 2022, 02:56	#3
Lamaman71 Registered User Join Date: Sep 2021 Location: Katy, USA Posts: 154	Thanks Daedalus. I reflowed the op amp and thought Id fixed it - but the damn problem seems to have reappeared...When I recapped I didnt see any sign of leakage but I will double check. Do you have any idea which resistors or caps I should test? There are quite a few!

09 April 2022, 21:24	#4
Daedalus Registered User Join Date: Jun 2009 Location: Dublin, then Glasgow Posts: 6,334	In general, pretty much any in the audio output area can cause issues with the sound. If it is an early revision board (rev 1A typically), there are a couple of feedback resistors at the op amp that aren't correct. They result in excess volume, and clipping when multiple channels are played. It's easy to see this with an oscilloscope - the sine wave from AmigaTestKit will be fine when one channel on each side is running, but when both on each side are running, the waveform is clipped.

12 April 2022, 06:05	#5
Damion Registered User Join Date: Mar 2008 Location: US Posts: 315	I had the same problem with one of my A1200 boards (nearly drove me batty). The board looked fine, but the problem was a corroded via in between C383 and C384, near the op-amp. (I also replaced the op-amp with one of these). Your issue might not be identical of course, but if you poke around in that general area I bet you'll find it.

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