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Old 18 July 2018, 19:19   #1
Sim085
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New sound issues with A1200

I just moved this A1200 because was doing some cleaning and when connected back again I noticed the sound was not good.

When I play a game the sound is distorted. It looks like something is not playing or playing at the wrong time. I'll try to take a recording in case this helps.

I can confirm that the 1084S speakers are good as I connected two different A500 models and sound was as expected. I can also confirm that the power supply -12v is good, tested this from the inside connector of the A1200. It is also not a a software issue as I have tried different games and the sound of all is wrong. I can also confirm that sound comes from both the RCA connectors as when I unplug these one at a time I can hear a change in sound.

Does anyone know what tests I can do to identify what the issue is? Like which pins and what voltage they should have?

Last edited by Sim085; 18 July 2018 at 21:43.
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Old 18 July 2018, 22:29   #2
idrougge
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So I suppose your A1200 has its original capacitors? They're probably leaking, possibly as far as corroding the connection to the audio op-amp.
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Old 18 July 2018, 22:55   #3
Sim085
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No I had these replaced. I was told there wasn't leakage apart IDE but was safe.


Quote:
Originally Posted by idrougge View Post
So I suppose your A1200 has its original capacitors? They're probably leaking, possibly as far as corroding the connection to the audio op-amp.
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Old 19 July 2018, 00:12   #4
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I'd say there still was leakage. It sounds exactly like corroded audio circuit.
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Old 19 July 2018, 06:04   #5
Sim085
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If a circuit is corroded then should I see a voltage drop on one of the capacitors?



Quote:
Originally Posted by idrougge View Post
I'd say there still was leakage. It sounds exactly like corroded audio circuit.
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Old 19 July 2018, 13:28   #6
Sim085
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I have taken some voltage readings; I am not sure if all are related to sound or if I missed any. Readings are as follows:


The following are replaced capacitors and their voltage:

C303: -12V
C304: -12V
C324: 2.48V
C334: 11.08V

All these readings are when not playing anything.

Can anyone confirm if the above readings are correct or not?

Edit: I have compared the C334 reading with a past reading I did around two months and back then the voltage on this was 2.5V

Quote:
Originally Posted by Sim085 View Post
If a circuit is corroded then should I see a voltage drop on one of the capacitors?

Last edited by Sim085; 19 July 2018 at 14:50.
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Old 19 July 2018, 20:17   #7
Sim085
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So I have looked for other posts which had similar problems and in one of these there was this link: http://amiga.serveftp.net/audio_repair.html

Here is says the first thing to do is to test the LF347 and the page provides expected voltage on each pin of this when switched there is no sound.

As per the attached screenshot mine look to be ok apart for pins 6 and pins 7. The linked page says these should have +2.3V but mine have +11.04V.

I remeber there was a site with the A1200 motherboard where you could select a pint and it would highlight where it ends. Does anyone have a link to this as I cannot seem to find it.

The linked page says:

"Now check the voltage on the second stage outputs (pins 1 and 7). If these are bad, voltages on U403 will be effected.
A problem here is possibly in the audio filter section, located between audio stages 1 and 2. With the computer off, measure approx 20k ohms between pins 3-14 and between 5-8."

However don't know what this means exactly.
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Last edited by Sim085; 19 July 2018 at 20:31.
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Old 19 July 2018, 22:35   #8
pipper
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Worth watching: [ Show youtube player ]
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Old 20 July 2018, 08:37   #9
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Thanks pipper; yes I have gone through this video. From what I understand (I could be wrong) he is checking the ohms between the inputs and outputs of the LF347, i.e. - 14/13 and 8/9 which is in line with what the linked article says. However he looks to be concentrating on the top pins only (8 to 14), maybe his problem is different.

In my case I am getting an incorrect reading on pins 6 (input) and 7 (output). In the linked article it says that when output is wrong in pins 1 or 7 then I must check ohms on pins 3/14 and 5/8.

I have taken all the readings:

14/13 - 0.68
8/9 - 0.68

3/14 - 0.13
5/8 - 0.13

(all readings with 20 ohms on multimeter).

Unfortunately I don't know what the ohms reading should be for 3/14 and 5/8 and where to go from here.

Quote:
Originally Posted by pipper View Post
Worth watching: [ Show youtube player ]

Last edited by Sim085; 20 July 2018 at 08:46.
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Old 20 July 2018, 14:18   #10
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Can anyone pinpoint me to U403/LM833/LM358
The only chip I can find with eight legs (four on each side) on my version 2B motherboard is labelled D215.
(which strangely enough on my version 2B motherboard is populated but not so on the 2B mother in this screenshot: http://amiga.resource.cx/photos/phot...res=hi&lang=en)

Last edited by Sim085; 20 July 2018 at 14:24.
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Old 20 July 2018, 14:32   #11
ajk
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You will find U403 on your A4000 motherboard, not A1200

The chip that handles that stuff on the A1200 is U15 (a 14-pin chip near the keyboard connector, towards the center of the board) which you have probably already been testing.

The A4000 has an extra stage there for what I presume is mixing in the CD audio, if connected.
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Old 20 July 2018, 15:22   #12
Sim085
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Thanks ajk, I should learn to read the titles before scrolling down to the content; the article I linked to is indeed for the A4000.

Edit: Ok I found this page with schematic for the A1200 so will try to follow that one.
https://wait6502.blogspot.com/2017/0...ted-audio.html

Quote:
Originally Posted by ajk View Post
You will find U403 on your A4000 motherboard, not A1200

The chip that handles that stuff on the A1200 is U15 (a 14-pin chip near the keyboard connector, towards the center of the board) which you have probably already been testing.

The A4000 has an extra stage there for what I presume is mixing in the CD audio, if connected.

Last edited by Sim085; 20 July 2018 at 16:06.
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Old 20 July 2018, 16:51   #13
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Ok found the site I was looking for, reading the schematic and looking for stuff on the motherboard was proving too hard.
http://www.amigapcb.org/

(A) Given that:
- C334 reports +11v
- Pin 7 (output LF347) reports +11v
- Pin 6 (input LF347) reports +11v
Then I can assume the issue is not a bad C334 capacitor and relies before the LF347.

(B) Given that:
- Pin 9 (LF347) is +2.3v
Then I can assume Paula is ok.

Are my (A) and (B) assumptions correct?

Last edited by Sim085; 20 July 2018 at 17:05.
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Old 20 July 2018, 18:55   #14
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You could also check the 2 rows of smt caps and resistors behind the keyboard connector, near the audio outputs. I bought a motherboard from ebay and later another one with a known audio fault from Amibay (spares or repair). Both of them had one of the resistors cracked which was the reason for the Audio being so crackly. They weren't the same resistor in each case but they now work as they should although the resistors I used were a physically larger version as that is what I had at the time.

Mind you I don't know of anyone else who has had this problem, I may have been just unlucky/lucky with the fault
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Old 20 July 2018, 21:11   #15
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Thanks Allen1, from what I can see in the schematic (if I understand it correctly), E331 comes after C334 and the output of C334 is already at 11v which seem to indicate the problem is before this.

I did some more readings:

C332 - 8.52V
C331 - 0V

Both the above are with no sound playing. With sound playing there is some voltage on C331 which sometimes goes negative. I can see voltage changing on the various caps when sound is playing but not the C334 which seem to be stuck at around 10V.
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Old 21 July 2018, 20:45   #16
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So continuing working on this; I have found this very helpful video:
[ Show youtube player ]

At 12min it provides a diagram to test sound. I have only tested with audio filter ON and I can see my readings don't match on Q321 and Q331 where expected is -11v but I get 8v. All other readings look to be ok as I only get minor voltage difference.
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