Yet Another A500+ Motherboard - Part 1

This is getting to be habit... Another A500+ motherboard has arrived. This one just gives a black screen despite the best efforts of the previous owner.

Next patient please!

On first inspection the repair wires look to be correcting the classic signs of battery damage. They were pretty well done but a bit thick for my liking and, as I have some uber thin kynar wire knocking around, I decided to remove them with the intention of replacing them.

Previous Repairs

Then I inspected the board again. To my surprise, it looked very, very clean. There's not much damage from the battery. I poked around with a screwdriver to expose where the battery had leaked and remove the damaged conformal coating.  Where the copper was exposed I ran a smidgeon of solder across the tracks. But there wasn't really much damage at all.

One of the least damaged I've seen...

Continuity testing revealed that the only issue seemed to be from the cluster of resistors/diodes near the AUDIO FILTER section on the board. On the opposite side of the board one of the pads has fallen off probably through exposure to the battery leak and then a hot soldering iron. No biggie to sort this out but it shouldn't cause a black screen. Hmmmm.

Missing pad - no problemo!

I tried testing around Gary who is another prime candidate for causing a black screen, especially if his socket is corroded by the battery. No problems.

Strange.

Black screens are listed as being a CPU fault but, in reality can be caused by any number of things. Dodgy logic chips, broken tracks at Gary's socket (as mentioned), busted ROM etc.

Anyway, just to be sure I swapped over the CPU with a spare but this made no difference. Then I swapped Gary with a spare but this still made no difference. I don't have a spare 8375 Agnus or a 2.04 ROM but I do have a couple of 1.3 ROMs. Just out of interest - I wasn't even sure the 1.3 ROM would work on an A500Plus - I swapped out the ROM for one of these.

And it booted.

Well, this is awkward..

This is a surprise to me as I would normally expect ROM faults to give a red screen. In any case, the board boots to the familiar KS1.3 white screen. This is good news but is a bit of a pain as the 1.3 ROM with only boot from an internal floppy drive i.e. DF0. The big advantage with 2.04 is that it will happily boot a bare board from an external floppy drive. To do any more testing I have to dismantle my own A500+ and swipe the drive from it for a while which is a bit of a pain...

Before I start dismantling I decided to carry on looking over the board in case of any other issues. There were a couple of minor things. The pads at jumper 9 and jumper 7A are gone. This is partly my fault. Slightly over exuberant cleaning, battery acid and the heat of a soldering iron were too much for them. They were there when I started cleaning and, well, you can see the result. They were in poor condition but had been that way since before the previous owner. Their non-existence is not really an issue for a standard Rev 8A board though.

JP7A - Gone, but not forgotten.

J9 - Missing, presumed gone.

The mouse and joystick ports have pins that are damaged. I'm not sure if I did this. Oops. Again, it's not a big deal as I have a big bag of these connectors that I accidentally over-ordered a couple of years ago...

It wasn't me...honest..(probably was actually)

In part 2 I will dismantle my A500+ and attempt to test whether there are any other problems on this board. I might also replace the mouse and joystick connectors. 😀