Amiga 4000: Audio issues

I've just acquired an A4000 from a friend and I'm planning on giving it a 'once-over' and bring it up-to-spec a little before he reclaims it. For the most part, it's in good working order, but there's an unusual quirk with the audio. Both sockets on the back are outputting the same channel (not worked out which one, yet) so -- anyone know what causes this and more-importantly, have any suggestions to fix it? Is this is well-known 'A4000 SMD capacitor / Audio' problem, or is this something completely different?

The 4000 will shove out the same sound on both channels if only one audio plug is plugged in.

Have you got another set of leads to try - just in case?

Dave G :cool:

Hiyas Andy,

the A4000 Audio port is notorious for faults

its likely you have a sound doplar caused buy bad filtering of from the caps (i.e. bad caps).

heres a schematic it looks a little busy but its quite easy to follow, also have a look at this thread on EAB might help too.

Quote:

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Originally Posted by davideo

The 4000 will shove out the same sound on both channels if only one audio plug is plugged in.

Have you got another set of leads to try - just in case?

Dave G :cool:

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Good suggestion, but alas, it's not the leads. I've tried three or four different sets and know that these leads/speakers work fine on other Amigas.

The odd thing is that both outputs on the A4000 are the same audio channel. I've tried plugging only one in, tried swapping them round, tried using a 2->1 cable, all to no avail. The other audio channel just isn't there.

Sounds (pun intended) like it's definately a caps problem then :thumbsdown:

Dave G :cool:

Quote:

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Originally Posted by Zetr0

Hiyas Andy,

the A4000 Audio port is notorious for faults

its likely you have a sound doplar caused buy bad filtering of from the caps (i.e. bad caps).

heres a schematic it looks a little busy but its quite easy to follow, also have a look at this thread on EAB might help too.

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I suspected it'd come down to shonky SMD caps... Looks like more soldering is going to be required.

Hi. Have you try to connect the CD out to the cd audio connector? If yes, the 2 chanels is again in trouble? Both in one? If no your problem is before and you must check U402 LF347 type. If yes your problem is after, possibly U403 LM833 type. In all cases you MUST change the capacitors.
Sorry for that, but I believe n any case cause capacitors is not your only problem.:(

Just don't use tantalum capacitors for the audio section (nor for power filtering, you'll be warned!) and all will be fine.

Ordinary electrolytic capacitors is all you need for the audio section, even the 85°C ones will be enough. Remember they need to be 35V or higher.

So, my friend and I have had a chance to take a look at this. It would appear the U402 OpAmp is looking quite minging as one or two of the capacitors around it have defecated their mess on to the board.

So, there are definitely a couple of SMD caps that could do with some love. Do you guys reckon it's worth replacing the OpAmp as well?

Also, how is it best to clean up the electrolytic detritus that's befouled the precious Amiga components? Lemon juice again, or something different?

@Andy

I would change the OPAmp as well - reason being that you will have to remove it to get at the electrolyte that is underneath it - slowly corroding the board -

Remove the U402 - and since you are at it the U403 as well - clean up with a little acetic acid and clean completely... allow to dry then install new caps and components.

its the only way to be sure.