2x A4000 motherboards not working - Green, Grey Screen and other issues

[jbenam]

Hi mates,
I'm trying to repair two A4000 motherboards for a friend (he just needs one working, so I can cannibalize the other for components) and I'm a bit stuck. More than stuck, I'm lacking enough time to do all the tests of the sort, as I'm going back to UK next week. He also gave me two A3640s, I've repaired one but the other's still dead. Not a priority, as at least I've got a working one.

Anyway, here's what happens:

First motherboard:
Grey screen most of the time. Power LED goes brighter. Caps Lock can be turned on 7 times then stops working (buffer full). CIAs have been socketed, so I can swap them around, sometimes I get a green screen if I keep both mouse buttons pressed, other times I don't. Buster was busted (hehe!) and gave me a yellow screen, so I socketed it and placed the Super Buster 7 from the other motherboard. With the new Buster I get to the light Gray screen. Removing the RAM gives me Green screen as expected. Re-done the solder points on the Chip RAM slot and reflowed the CPU connector. ROMs have been tested in another A4000 and are working. Battery was removed ages ago, so there's not much damage. The capacitor near the U975, U976 latches has leaked, but damage doesn't seem to be extensive.

Second motherboard:
Gray screen. Doesn't detect at all if there's RAM or not, so I suspect it might not be reading the CPU or the ROMs. ROMs have been tested and are working though... This one wasn't as lucky as the first one was in terms of battery damage. There's green everywhere around the RTC area. Is there any component in that area that can prevent the A4000 from booting if fault?

Equipment:
Hot-air station and soldering station. I don't have an oscilloscope (yet) so I'll have to make do with a multimeter. I have sockets of every size, so if needed, I can swap chips around with the other dead motherboard.

I would love to at least get one working before I have to go... Help me guys, please

Thanks

[ancalimon]

After I plugged four 32mb rams (couldn't find 4mb simms and ordered 4 but they haven't arrived yet) on my A4000T, I started to get green screen. I tried rams one by one and one turned out to be causing the green screen. Tried another simm and it's now working with all four slots populated.

Green screen means chipmem error so I have no idea why a simm module was causing this.

[jbenam]

Originally Posted by ancalimon

Green screen means chipmem error so I have no idea why a simm module was causing this.

Oh yes, I forgot to say that the RAM sticks I'm using for Chip RAM are tested as working on my personal A4000. I've not added any fast RAM at the moment as I don't want to render troubleshooting any more complicated than already is

[FOL]

Second board, remove the latching chips, check under them.
Also check the surrounding area, so corrosion through the copper holes (double side holes).

[jbenam]

Originally Posted by FOL

Second board, remove the latching chips, check under them.
Also check the surrounding area, so corrosion through the copper holes (double side holes).

Thanks for the tip - found a crapload of battery acid under the latching chips.

I've tested a few vias, and they seem to work. I'm pretty sure there's AT LEAST a broken one in there though... I'll give a look at the schematics later.

What do you guys usually do with acid-encrusted solder? It's a pain to remove I'm just using a lot of flux with my trusty desoldering braid along with the soldering iron at maximum temperature - it's helping a little bit (some doesn't want to go away no matter how much I try though) but it's superslow and I don't want to risk damaging the pads...

[alenppc]

In my case, I fixed one board by soldering a gazillion jumper wires all over to fix traces eaten away by acid. It's a painstaking work, just finding all traces and figuring out which ones are broken...

Also I find that 4000 CPU slots tend to be prone to cold solder broken joints so try reflowing the CPU slot to see if that fixes the issue. Also, scratched or broken traces next to the CPU slot can cause green screen as well.

[jbenam]

Originally Posted by alenppc

In my case, I fixed one board by soldering a gazillion jumper wires all over to fix traces eaten away by acid. It's a painstaking work, just finding all traces and figuring out which ones are broken...

Also I find that 4000 CPU slots tend to be prone to cold solder broken joints so try reflowing the CPU slot to see if that fixes the issue. Also, scratched or broken traces next to the CPU slot can cause green screen as well.

That's the problem right now, time The first motherboard looks quite good actually in terms of acid. Pretty much zero on it.

By the way, no more green screen now - I get a white screen then it goes back to grey. Maybe the _IORST line is pulled down the entire time by something? Damn, I do not have an oscilloscope to check that

Anyway, I did reflow the CPU slot and I'm pretty sure the CPU is getting recognized, as I get a green screen correctly without RAM (without a CPU card it won't even get to the RAM check).

EDIT: Checked _IORST, looks fine - 4.17V, my own A4000 has 4.19V. Checked SCLK for broken tracks and that one seems fine too... Urgh.
EDIT2: Green screen is back and now it happens on every boot