If you own an Amiga, especially an A4000, read this!

[Castellen]

Had a look at that article, and it is incorrect about using a non-polarised capacitor.

It's perfectly normal to use the polarised capacitor Commodore fitted.  In fact you could use either, but since the non polarised ones are usually bigger, more expensive and there were exsisting polarised capacitors in the design, it made sense to do what C= did.
The negative side of the capacitor is biased to ground, so providing the bias on the op-amp's input doesn't get messed up, the circuit is fine.

The facts are that it would appear to be a very large bad batch of electrolytics, or more likely just a bad quality capacitor.  Apparently various manufacturers have had electrolytic chemical problems over the years.
I've repaired a lot of A4000T, A4000D, CD32 and A1200 with this problem.  In almost all cases it's only the 22µF capacitors, and not just the ones in the audio output section.
From memory there are around 5 of these 22µF capacitors in the A4000D, which I have all seen with leaking problems.
Some of the other SMD electrolytics can have the fault too, but from experience, the 22µF ones are definately the worst.

The problem should not occur on the A3000 as these are all through hole components.  Only the surface mount capacitors in the A4000D/T, A1200, A600 and CD32 would appear to suffer from this leaking problem.  Leaded capacitors usually just tend to "dry out" rather than leak.


The biggest cause for concern in almost all Amiga models is the RTC battery failing.
These batteries are generally rated for a life of around 6 years, but some of the ones out there now will be double that age.
There's a very high chance of the battery leaking corrosive material onto the motherboard which can cause a number of faults.

I'm actually in the process of writing a guide on this, my work so far is online at:

http://amiga.serveftp.net

under the battery replacement section if anyone wants a look.

Lots more technical info to be added as I get time to write it :-P

Anthony.

[Castellen]

OK, just read the other section of that website regarding the A3640 repair.
Most of the information is pretty good, other than the removal procedure of the capacitors.
I would definately NOT suggest "twisting" them off the PCB!!  That's a very good way to rip off solder pads.

The correct way is to use a hot air SMD toldering tool, but at home, two small soldering irons can be used, one in each hand.
Another way is to heat one side, lift it slightly, heat the other, lift that slightly, continuing until it's removed.  Not really ideal either as it's just as eay to lift pads doing it that way.


As for the advice of never "poking around in the computer" while powered up...
Geez, I'd never get anything repaired!
There's possibly a slim chance of killing yourself with 5V if you had an unusually low body resistance and you layed in a bathtub full of water, then tightly grabbed a large contact area with the 5V supply on it.  As far as I know, the lowest voltage causing death was about 32V or something.
Ohm's law = current = V/resistance.  Takes at least 20mA though your chest in the right place to cause problems, meaning you need a body resistance as low as 250 ohms!!
Normally it's 100k ohms or so.

Of course power supplies are a different story.  The voltages inside them have a very high chance of doing you damage!

That's today's lesson in electrical safety :-)  

[Castellen]

Good point, and especially around car batteries.
Only 12V, but several hundred amps through your metal watch strap and your wrist is going to get extremely hot extremely quickly!

The biggest failure I usually see in day to day repairs is the failure of high power devices (transistors, etc) and sometimes dry joints on the legs of through hole components subject to a lot of thermal stress.
Leaking/dry electrolytics is still a big problem, mainly with older equipment.  Also it's common to get "resistive" tantalum and ceramic capacitors from time to time.  These typically cause bias upsets in audio circuits.

[Castellen]

The A4000T is electrically similar, but the output amplifier and those 22µF coupling capacitors in question are fitted to the audio/video module which unplugs from the motherboard.

I have repaired these before due to leaking capacitors causing audio problems, so you may well have the same problem.  There are some 22µF capacitors on the motherboard as well, so would be worth replacing those at the same time.

A4000T schematics (and of the module) are here:

http://amiga.serveftp.net

Look in the schematics section.

[Castellen]

Thanks Dalamar :-)

@darkcoder:
Yes, you could replace the audio/video module.  But a new replacement module would NOT be cheap!

I saw one for sale on either A.org classified or Amibench a few months ago for about £5.
Or you could chuck up a wanted add.

Though the module itself is not very complicated, so repair is quite simple if you can SMD solder.

Or send it to me in NZ for repair if you're desperate, would still be way cheaper than a new one though.

Anthony.