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Old 09 July 2012, 12:15   #1
Spirantho
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Join Date: Jul 2012
Location: Aberystwyth, Wales
Posts: 16
Amiga 4000 boot problems with capacitors

Hi everybody,

First post here, so be gentle.

My Amiga 4000 has been a bit ill for a while. It's in an Eagle tower, and has a Prometheus (Voodoo III, RTL8029), Deneb and a Highway. Hard disk is an U2W SCSI 37GB. 128MB RAM, of course, running OS 3.9 and 4.0. Power supply is a Corsair (I think) CX430 through AmigaKit adaptor. I've wired up an extra +5V line to the motherboard to lower strain on the single-wire plug.

Now, back in the day, it was acting unreliably, so I changed the capacitors. The problem was that I didn't really know what I was doing, and many of the capacitors lost their SMT pads. I managed to get the caps on, but normally linked to the through hole or something so they're connected but not as they were originally. I'm using mostly SMT capacitors with the occasional radial electrolytic where I had to.

The symptom is this: when the machine runs, it tends to run faultlessly. It's now stable for hours, and is a pleasure to use.
But turning it on usually results in.. nothing. I mean nothing. No error screens or anything. Just... nothing. Occasionally, it'll start up, particularly if I push on the CSPPC or the Zorro backplane (Eagle 7-slot one). But not always.

If I remove the Zorro backplane it fires up... but doesn't seem to reboot correctly. If I replace it with the original backplane, it maybe works and then stops working when I load it with expansion cards.

Try it with the original 030 card (it's a revision B board, or 2 or something) and it fires up to the kickstart screen. Unless I put cards in.

So in other words, it works as long as nothing is attached. The CS-PPC is good, the 030 is good, the Zorro backplane is good (and recapped) as far as I know (it works all right when it works), and the original backplane is good.

I noticed when I looked with my scope that the voltage (about 4.9V without cards) had quite some ripple so I resoldered the caps, and it got better.

What I want to know is can I do anything about it? I don't want to write off a resurrectable board. I can now do SMT but most of the pads have gone so that's no good.

I suspect I just need to lower the ripple still. Does anyone have any suggestions to help my poor 4000?

Thanks to anybody who can help!
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