Repairing a Commodore 128 and DIY Chris's RAM tester review

By Michael Doornbos

April 14, 2021 - 3 minutes read - 611 words

I have several items from DIY Chris that we’ll review over the next week including a GAL PLA and flash-able ROM drop in that can hold kernal, character or BASIC ROMs.

First on the list is the RAM tester.

The problem child 128

I have a Commodore 128 which developed a bad RAM issue a few weeks ago. These old computers just sometimes sacrifice a part to the component gods.

Well character rom seems to be working anyway

RAM issues when there are 16 chips can be difficult to diagnose if the machine boots up most of the way. Luckily it showed up on the C64 mode as well which helped narrow down which chip.

The 128’s RAM banks are split into two parts and the C64 mode only uses the lower bank of 8 chips.

They all looked fine on the Oscilloscope which was expected since the machine boots. The easiest course of action is to desolder them, socket and then try them one by one with known good replacements.

A good desoldering pump can make this process MUCH easier

Came off pretty cleanly. I eventually decided to do the upper bank and all of these ceramic capacitors while I was at it

I don’t have a large stash of 4146 RAM chips lying around, so it was off to eBay and some other questionable places to look for some spares. Your mileage may vary. And by vary I mean you’ll be lucky to get 75% of the ones you buy third hand to work.

But how do we know that chips you buy on the internetses are good? Well, trial and error of course!

Bleh.

Luckily there is a cheap(ish) RAM tester for these chips available.

RAM tester

DIY Chris offers this no frills RAM tester. It’s got a ZIF Socket, a jumper to select 4164 or 41256 chips and is powered by USB.

It couldn’t be easier. You pop a chip in, press the button.

It’s either bad:

Or good (which does take a solid minute to complete all of the tests)

It works

Simple, reliable, inexpensive. If you need to test a lot of these, it seems like an easy purchase decision.

The only thing I can think that’s “negative” about the package is that there was no jumper included. It’s a tiny thing and most of us have them lying around, but I was trying to think of SOMETHING that wasn’t all raves and praise ;-)

A working 128

I decided to remove all of the RAM chips since I was doing them anyway and install sockets. These things are just a little tricky sometimes, but patience and okayish desoldering equipment can really help. The bottoms of my chips are plastic, so be extra careful with the heat. I use Arian Black’s reflow station trick to get these chips off of old board and the line between getting them loose and melting them is close.

Replaced all of those ceramic capacitors as well

So thanks to the RAM tester, I managed to find 16 working RAM chips and the trouble maker 128 is back to it’s old self.

Sorry about the blurry picture, my video capture is on the other monitor

What’s next?

I spoke with Chris about making a version of his drop in RAM replacement board like the one he has available for some C64 boards and it’s something he’s exploring. If he does one, I’ll be sure to get it and do a VERY short review on it in my 128. Should be a 10 second replacement since my board is now fully socketed.

Follow Chris on YouTube.