More Memories...

As I was perusing the Amiga Facebook pages I came across a post by a chap who had dragged his A500 out of storage and was going to give a young relative a lesson in retro gaming. Sadly, this was not to be as the A500 refused to start.

With the quantity of helpful people on the group several pieces of advice were given and it transpired that the memory expansion was the cause of the non-boot. The OP (that's 'Original Poster' for you non-hip and trendy people out there..ahem) was about to throw it in the bin but I asked if he could send it to me instead as I was looking for another repair project. And he did.

Package Arrive Safely

It's a fairly generic 512Kb RAM expansion as was the trend in the late 80's and early 90's. It has four 256k x 4 bit CMOS dynamic RAM chips and, bizarrely, three of them are socketed. The battery was left on too but this was quickly despatched.

Argh! Curse you Varta!

Varta removed - clean up on aisle 5

I took some vinegar and applied it liberally to remove the battery residue (fizz-tastic) and then washed away the vinegar with some distilled water and allowed it to dry thoroughly. After a bit more clean up I was ready to test.

First test...

I used my A500+ spare motherboard with an external drive and the excellent Kier's test disk. There is an intermittent fault with this motherboard in that it sometimes won't boot and I get a yellow screen. Normally, I have to re-seat Agnus and then it works. Sometimes it works just by breathing near it. That's for another time. At this point, the motherboard is working...

...and so is the RAM expansion. Well that was easy.

Before I got too cocky I decided to run the RAM test and see what transpired. It was not necessarily good.

Aw, crap.

I re-seated the three RAM chips and got the same result. It looks like there is one dodgy chip in there but the question is, which one?

So I labelled them 1, 2 and (duh) 3. Then ran the RAM test again and took a picture of the board and the results. Then I moved the chips around and did it again. Rinse and repeat. Very quickly, I honed in on chip number 3.

Okay so I did 2, 1, 3.

Error on FEDC

Order now 3, 1, 2

Error now on 7654 - it's either chip 2 or 3.

Order now 3, 2, 1

Error still on 7654 - It's looking like it's chip 3 at fault

Order now 1, 2, 3

Error moved back to FEDC - It's chip 3 alright..

Whenever chip 3 moved, so did the RAM errors. I think we have our faulty RAM chip. Although I am not an expert in this, what  happens is the chemicals from the battery leak and attack the metal legs of the chips. In some cases it's just superficial but sometimes the corrosion can creep up the legs and into the actual chip destroying it completely with no chance of repair.

Rather annoyingly these are of a later vintage than the chips I was playing with here and I don't have any others that might be suitable. I guess I need to get on eBay to look for a replacement chip and see if that recovers it. But in the meantime, I will be doing some more reading up on this type of chip, just to be sure that my rudimentary detective work is correct.

Special thanks to Neil for providing the RAM expansion. :)