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!

Well, getting a blank screen on boot many times is a accelerator contact problem so check the cpu slot and connector on the accelerator well for tarnishing. Use a non silicon contact cleaner on it. I have also taken a sewing needed behind the contacts to bring them out a tiny bit,but be gentle and don't go crazy here!. The power supply can also cause these symptoms-Dirty power can be as bad as low voltages-is the ripple you see within the PSU specs?..
The fact that soldering a capacitor makes things better makes me think the whole boards capacitors needs reworked. I use Panasonic brand caps personally.
I don't like to see under 4.92v on the 5v rail loaded. I have had 4000's that were picky under this voltage.I had a machine that gave me fits with a ethernet board and transciever that caused flakiness when voltage dropped.
As for using thru hole caps to repair broken pads this should work fine as long as you got a good solder job on the via the leads go through.
Double check for a bad connection in the rom sockets while you are there.
A4000's are usually quite reliable other than what users do to them :p.

mech

in this case I feel sure the unreliable component is the user. :) Of course I know how to do it properly now, but it's a bit late. :)

I think I measured down to 4.7V at one point, which seems very low. I reckon the next thing to do is to take off all the 47uFs and re-do them with through-holes from the nearest solder points, rather than try to use SMTs where a track has lifted.....

Thanks for the advice, I think it's a low voltage problem. Would low ESR caps help?

Well.. I changed all the 47uF caps. They're all through-hole, and they're all soldered properly to the solder pads _and_ the hole where possible.

The +5V line varies from 4.87V (the highest I had) down to 4.7V, when running with just the CPU card. Adding the original 4000 Zorro backplane lowered the voltage to 4.64V, enough to boot the Amiga sometimes, but not enough when other hardware is in.

Can anyone help? It's working perfectly except for this wretched voltage problem!