Dead A4000 Desktop

[bjjones37]

Hi guys,

Been awhile...  When my Amiga 4000 desktop died I sort of dropped off the radar.  I never ditched it though and I am hoping to get it working again.  I have been looking for the past week for another motherboard without much success.  The battery had leaked before I got it off of ebay a few years back and only lasted me a couple of months.  Right now if I turn it on it initializes to a solid green screen and then flops back and forth between green and black. I tried testing some of the traces that looked damaged with an ohmmeter just in case the battery acid had eaten through.  They tested OK.  I have to admit I do not have much experience repairing Amiga motherboards.  I have successfully repaired it's baby brother, the AtariST. I was really excited when I first got it and was looking forward to learning AmigaOS.  It was a real downer when it crashed.  Any ideas?  

Any vendors out there still selling A4000 motherboards?

Thanks,
bjjones37

[bjjones37]

Don't mean to start the Amiga Wars all over again, but I would like some recommendations.  If I have, say, $300-$400 to drop on another Amiga, what can y'all recommend?  I know...I know, it depends on what I want to do with it. First and foremost, I want to explore the AmigaOS.  Of course gaming is sort of a given.  Perhaps some light programming.  And somehow network it to my Windows box - NIC, serial, parallel, Flash card, whatever.  I am strictly a hobbyist so a little of everything without too much of any one thing.  I do not require a lot of speed but I do insist on a hard drive.  Oh, one more thing,  I do not want to buy an Amiga that is going to die a month after buying it.  My  Atari 800XL and Commodore 64 still work fine for goodness sake.

I have WinUAE, but it just is not the same to me as using an actual Amiga.

Any thoughts?

[bjjones37]

Don't mean to start the Amiga Wars all over again, but I would like some recommendations.  If I have, say, $300-$400 to drop on another Amiga, what can y'all recommend?  I know...I know, it depends on what I want to do with it. First and foremost, I want to explore the AmigaOS.  Of course gaming is sort of a given.  Perhaps some light programming.  And somehow network it to my Windows box - NIC, serial, parallel, Flash card, whatever.  I am strictly a hobbyist so a little of everything without too much of any one thing.  I do not require a lot of speed but I do insist on a hard drive.  Oh, one more thing,  I do not want to buy an Amiga that is going to die a month after buying it.  My  Atari 800XL and Commodore 64 still work fine for goodness sake.

I have WinUAE, but it just is not the same to me as using an actual Amiga.

Any thoughts?

[bjjones37]

I took some photos, not sure where to upload them to. And do I need to reduce them in size as they are 2+ MB each?

Close examination has shown a cracked ceramic capacitor, C700, tied to the GND1-4 inputs of the Buster chip. +5v is on the other side.  Not sure what purpose it servers.  Might this be causing at least a part of my problem?  

The cap is a 1208 0.22uF but I can only find 1206s.  I am hoping that will not be an issue.

[bjjones37]

I think I might have identified the problem.  I have located five more caps with hairline cracks in them.  They are C500, C455B, C211A, C211B, and C130.  C130 is associated with U130 which contains logic gates used to generate BR_W - or Board Read/Write.  A problem with this signal would certainly prevent access to chip RAM. I am still waiting for the .1uF caps to arrive but they should be here soon.

[bjjones37]

I have not been able to locate a Tech manual for the A4000.  I have to generate my own logic diagrams.  Here is one for the Chip RAM logic.

[bjjones37]

You have the right machine,but buying one with battery damage is always a risk. Its not that its unreliable,it is just that batteries are a serious problem. Sadly its a known problem but people still dont remove the batteries or replace them with the same type batteries.
Capacitors are also a problem on many of these machines.
Its a good sign that your machine shows some life. it may be repairable.
can you put up some pictures of the battery damage-we may be able to advise you on it. You don't mention where you are located.
green screen is usually a sign of chip ram troubles and could be as simple as a tarnished socket,bad chip ram simm or the damage may be traces and or chips.

Here are a few pics.  If you need to look at something else just let me know.  I took more shots. I have replaced 14 capactors so far, six of which were definitely bad.  But still the same symptom.

[bjjones37]

I recently read that the A3640 had three electrolytic caps installed backwards on them.  Mine is Rev 3.0 and it appears that they may be backwards. I see a thin greasy film around a couple of them.  If they are blown could this possibly cause my symptom?