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Old 19 March 2020, 17:55   #1
turrican9
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Amiga 500 Plus with audio distortion on both left and right channel (Solved)

So I recently bought a supposedly working A500 Plus motherboard with no chips.

I bought an Agnus 8375 NTSC from icomp (switch it to pal when using ACA 500)

I used one of my A500 Rev.6A boards as donor for the rest of the chips except the kickstart where I choose to put in a 2.04 I had laying around.

So the Rev.8A A500 Plus motherboard had some corrosion damage from the battery which I have removed and cleaned up the area. No broken traces as far as I have probed.

So the board seems to work fine except for one thing, audio is distorted on both left and right channel.

what I have done: Paula and the other chips is confirmed working again by putting them back in a Rev.6A board. I even tried another working Paula from another Rev.6A motherboard.

I have changed all the caps on the motherboard. Same symptoms before and after recap. I have done alot of probing to compare the 6A boards audio circuitry to the Rev.8A So far all the probing looks fine on the 8A. But still I get distorted Audio on both channels. Power supplies are fine. I get 12 volt and everything to the audio circuitry as I am supposed to. I know missing 12 volt can give problems with audio. I have recapped Amiga power supplies and also have a modern supply. They all work perfect with all my other Amiga 500s and other Amigas I have.

What could be the issue here? Is the LF347N chip like an audio op amp or something? Could it be something wrong with it? I know it's connected in the audio circuitry. I noticed a slightly different chip in the 6A board with what looks to be the same pinout.

Am I missing something here?

Edit:

So a did some googling and the LF347N chip is indeed the opamp. The 6A board I have just has the other variant (TL084). I have measured the vref voltages (or whatever they are called) on the opamp pins. And both the +12v and -12v also measured on the opamp pins. On the 8A board I measure vref to about 2.41 and +12v to about 11.5v and -12v to about 11.5v. On the 6A board which has the TL084 I measured vref pins to about 2.52v and + and - 12v to about 12.5v. Using the same power supply on both. Looks normal to me?

Edit 2:

Did some more testing at it seem the problem is only there in games that does not dim the power LED. Now, I'm not sure if the audio filters are enabled or disabled when the power LED dims?

Last Edit: Problem solved. Turned out it was a bad op-amp chip. changed it and audio is fine now

Last edited by turrican9; 21 March 2020 at 01:41.
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Old 19 March 2020, 18:19   #2
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You mentioned the 12V line is OK, but what about the -12V line?
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Old 19 March 2020, 18:24   #3
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You mentioned the 12V line is OK, but what about the -12V line?
I updated my post. Refresh the page. I seem to have minus 12v aswell
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Old 19 March 2020, 18:48   #4
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When you say "distorted sound" what do you mean?

Have you done the audio test, and other health checks, in Amiga Test Kit or DiagROM?
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Old 19 March 2020, 18:52   #5
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Quote:
Originally Posted by solarmon View Post
When you say "distorted sound" what do you mean?

Have you done the audio test, and other health checks, in Amiga Test Kit or DiagROM?
Games have scratching and popping sounds. They work fine on all other Amigas I have. So did not see any need to try anything else. There is clearly something wrong. I have not tried Amiga test kit yet. I have it though. I do not have a diagrom.
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Old 19 March 2020, 20:23   #6
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Did some more testing at it seems the problem is only there in games that does not dim the power LED. Now, I'm not sure if the audio filters are enabled or disabled when the power LED dims?

So dimmed power LED is audio filters enabled? Or are they enabled when it's not dimmed? Anyway, the problem seems to be only in games that does not dim the power LED. In games that dim the power LED the audio seems fine. Personally I think the audio filters enable when the power LED dims. So it's only when audio filters are not enabled where I get the scratching and popping sounds

Edit: According to the audio circuitry diagram it says LED Off means bypass audiofilters. found here https://www.amigawiki.org/lib/exe/fe...anual_2014.pdf

If this is true then I must have a problem with my audio filter circuitry since I only have the distorted sound when LED is not dimmed. So that then explains why it works perfect when the LED dims since the filters are bypassed according to the documentation.

Last edited by turrican9; 19 March 2020 at 21:07.
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Old 19 March 2020, 21:24   #7
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The filters are on when the power led is bright, and off when dimmed.
Since the audio filter circuitry is immediately to the right of the battery on the A500+ motherboard it seems likely that you're looking at battery damage.
I don't know enough about opamps to be able to understand how the filter bypass works, but from the schematics you should be able to identify each component and check for corrosion.
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Old 19 March 2020, 22:35   #8
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Quote:
Originally Posted by robinsonb5 View Post
The filters are on when the power led is bright, and off when dimmed.
Since the audio filter circuitry is immediately to the right of the battery on the A500+ motherboard it seems likely that you're looking at battery damage.
I don't know enough about opamps to be able to understand how the filter bypass works, but from the schematics you should be able to identify each component and check for corrosion.
Thanks for confirming. I have been probing like crazy and cannot find anything wrong yet in the audio circuitry. Allthough some of the pads are a bit dark they seem to have continuity. I am using a working Rev.6A to compare results with. Because the audio circuitry itself seems identical to the Rev.8A.

So I wonder what could be causing this distorted sound only when the filters are on. I would assume the opamp are fine when the sound is fine with filters disabled? Or does it use the opamp with filters disabled? I don't know...

Last edited by turrican9; 19 March 2020 at 22:40.
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Old 20 March 2020, 00:13   #9
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Quote:
Originally Posted by turrican9 View Post
So I wonder what could be causing this distorted sound only when the filters are on. I would assume the opamp are fine when the sound is fine with filters disabled? Or does it use the opamp with filters disabled? I don't know...

OK I think I'm getting a handle on how the filter bypass works now.


If both channels behave identically, then I'd be looking at the switching circuit in particular. Check Q301 and surrounding components, and trace the signal path back to U38 (1488 IC - that's clever - they used a spare way on the RS232 transmitter chip to bring the CIA chip's TTL signal level to +/-12v!)
Check that you've got +12v at R307 and -12v at R308.
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Old 20 March 2020, 01:42   #10
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So I meassured some unstable voltages on the opamp. They seemed to vary from about 2.42v to 2.67v. +12v and -12v was stable though. On my Rev.6A boards the opamp voltages was stable at around 2.42v - 2.43v.

So I had this Rev.6A donor board which I took most of the chips from for this Rev.8A board. I took the chance and unsoldered the opamp on the Rev.6A then I cut the pins on the Rev.8A opamp (pain in the ass to unsolder without a proper desoldering station) so i could desolder the pins easily. I then took the opamp from the Rev.6A board and soldered it in my Rev.8A board. Sadly had no socket so I just soldered the chip in.

And guess what? It's working perfect now. With and without filters enabled

I'm glad it was the opamp since I cut the legs on the Rev.8A opamp. so I didn't waste a good chip. Anyway, they are dirt cheap from China. Main thing is I found the problem and everything is working perfectly now

Last edited by turrican9; 20 March 2020 at 03:00.
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Old 20 March 2020, 08:41   #11
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I would also recommend swapping the other op-amp. No point only changing one, might make some things sound slightly strange.
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Old 20 March 2020, 09:31   #12
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Quote:
Originally Posted by Hewitson View Post
I would also recommend swapping the other op-amp. No point only changing one, might make some things sound slightly strange.
The other? What do you mean? Are there two on one board? I don't see it. Or did you mean, install a new op-amp in the donor board?

I simply replaced the damaged op-amp with a fully compatible op-amp and it's working now.
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Old 20 March 2020, 10:25   #13
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Quote:
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And guess what? It's working perfect now. With and without filters enabled
That's great news - well done!

Quote:
Originally Posted by hewitson
I would also recommend swapping the other op-amp. No point only changing one, might make some things sound slightly strange.
The chip in question is a quad op-amp so swapping that out changes all four opamps in the audio filter circuitry - are there any more elsewhere?
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Old 20 March 2020, 13:24   #14
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No, that's them all. All four op-amps are contained within that single package. They can't be changed individually, though that would be good advice if they were separate chips.
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Old 21 March 2020, 05:04   #15
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Apologies. Overlooked the fact that it was a quad op amp.
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Old 21 March 2020, 12:05   #16
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Quote:
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Apologies. Overlooked the fact that it was a quad op amp.
No problem. I got confused there for a little moment
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Old 21 March 2020, 12:35   #17
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I changed the Agnus socket yesterday. The seller had drilled two holes through the groundplane under Agnus socket to poke the chip out. Of course, this did not show in the ebay pictures. In the ebay pictures the Agnus was not removed yet but all other chips were removed. LOL

Anyway, the drilling had caused a few cracks in the bottom of the Agnus socket. One of the cracks went to the edge of the bottom part of the socket and I didn't feel comfortable keeping it like that. Would probably have been fine but I had spare PLCC Agnus sockets so I just soldered in a new one.

Also soldered in the crc232 battery replacement. And installed a modern 1MB trapdoor upgrade to get the full 2MB chip.

Only thing I'm not super happy with is that I have an NTSC version of the Agnus 8375 (bought as new/old stock). It was under half the price of the PAL 8375s listed on ebay. It does not matter much anyway as this A500 Plus will be used with the ACA 500 + ACA 1221. Can force PAL on every boot to classic workbench with this solution anyway. But I would have prefered a PAL version. If the prices was not as crazy I would have bought one.

Everything is working perfect now. Build is almost complete. Will swap the regular Denise that came from my donor Rev.6A board with an Indivision ECS V2. Just to get the full Super ECS. LOL

Last edited by turrican9; 21 March 2020 at 12:47.
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