Two special Amiga 4000s: Repairing Jools

In my last post, I created a list of things I needed to repair on the Amiga 4000 named Jools to get it running. Let’s see if it helps.

RAM repair

There were a few traces I needed to patch between U891 and the via holes. It doesn’t look perfect right now, but it should be functional.

I popped in one of the SIMMs, powered it on, and…

Excellent, we now have Fast RAM, and it passes tests. But, there is a problem.

These chips are 512KB in size, and there are 8 of them on the board. Which means that this is a 4MB SIMM. This was concerning.

I put the 4 x 4MB SIMMs from my own Amiga 4000 into it.

Only 4MB is shown. Pulling one of the SIMMs drops it down to 3MB. So all SIMMs are being seen as 1MB. I puzzled over this for a minute and then remembered, there is a jumper that switches between 1MB and 4MB SIMMs. I’ve never seen this set to the smaller position before, but in this case, it was.

Swapping this over and the original SIMM was working as a 4MB SIMM! Unfortunately, the other two SIMMs that were in the machine were too badly damaged to be detected. That is a problem for another day.

RTC repair

I replaced U177 and patched all the known broken traces. Then I put the yellow resistor pack back in and tested again. I also decided to socket the RTC chip, it is surprisingly fragile and can often fail. After this, I found I had missed that the connection on pin 11 was broken.

Running the RTC test in Amiga Test Kit showed that the chip was working.

Of course, there was a problem. The time was increasing surprisingly quickly. Probably 2x faster than it should. There is a small trimmer capacitor for the crystal, it was corroded, so I figured it needed tweaking. I tried turning it and it immediately cracked. I guess it was more corroded than I thought. So, I removed it.

Afterwards, I replaced it with a spare I had on a dead Amiga 500+ motherboard. This will be something to tune at a later date.

Recap and ultrasonic

Now the known faults have been fixed, I recapped the board and CPU board. I then put them into my ultrasonic cleaner, rinsed and dried them.

Starting to look a lot better!

Final testing?

I powered it up, ran some tests… Grr… The Fast RAM is not being detected. It turns out when the board is flexed that data line 6 is not working.

It must have a hairline crack in the trace, I’ve seen this on another A4000 before, and I should have checked for it earlier.

This is the trace.

So, I need to patch this.

In addition, no matter how I tweaked the RTC trimmer capacitor, it was running too fast. I suspect the crystal may be bad too.

Next time

I need to make the (hopefully) final RAM repair before the new SIMM sockets are to be fitted. I also want to figure out why the RTC is running far too fast. Then it will be time to test the power supply and rebuild the machine.

Let’s hope Jops will be a little bit easier!