Amiga 600: Retro Restoration

Every week I check the local auction websites to see if there are any interesting vintage computer related things to bid on. This time around, there was a lot, including an Amiga 600. So, let’s take a look.

The Auction

The local auction posts up photos on a Friday night of everything they have. This time around they had a few interesting things. I submitted some absentee bids on the following things:

You can see the TV on the left of the first photo, that is a Sony KDL-15G2000. It is a fantastic 4:3 1024×768 LCD TV for use with vintage computers. I recommend checking out Kari’s videos, as she uses this screen quite a bit.

On the right of the second photo is an Amiga 600 with software and accessories. Then on the last photo is an Acorn Electron and Vic20, both with cassette machines and software.

Out of these, I won the TV and the Amiga. I forgot the invoice with all my details on was in the box, so I quickly brushed that out on my phone.

The Amiga

The Amiga 600 was quite yellowed on top, but appeared to be in good condition. Inside the box was the PSU, tank mouse, external floppy drive, two small boxes of disks and boxes for a few pieces of software.

I took out the Amiga to see what was inside, and how bad any potential capacitor damage would be.

Actually, under the hood, it isn’t too bad at all. The warranty seal was still in-tact, so I didn’t expect it to have been recapped already. But the capacitors haven’t leaked to the extent of causing obvious damage yet.

The motherboard is a revision 1.3, which is one I haven’t encountered before. Revisions 1.5 and 2.0 appear to be more common.

Testing

With that, I decided to test it. I popped in an Amiga Test Kit disk and ran several of the tests on it. Almost everything passed, but there was a problem.

The problem wasn’t the bad block on that disk, that is a known fault on that particular disk. The real issue was that the keyboard was completely dead. No key worked, and no reboot. Which was a little concerning.

Debugging

I grabbed a keyboard from another Amiga 600 and tried that. Same problem, so at least I know that the keyboard itself isn’t the cause.

Amiga Test Kit can be controlled by the mouse, and there is a software reboot option in there. I hooked the original keyboard back up and tried this. Suddenly, the keyboard was working and I could test it.

It was happening this way every time. Cold boot and the keyboard wouldn’t work, but a soft reboot caused the keyboard to spring to life.

The keyboard controller on the Amiga 600 is a chip on the motherboard, and that needs to be reset on power-up, so this indicates to me that the power-on reset isn’t working properly. The reset circuit on an Amiga 600 is … not well-designed.

The 74F27 is way overkill, and commodore left the rest of the inputs to that IC floating, which is not something you do with CMOS chips. There are other ways this could have been improved. But I suspect it was a workaround to a problem they encountered.

The most common cause for the reset circuit to fail is the larger capacitor, C612. This typically leaks and causes a lot of damage. So, I removed it and replaced it. There was no obvious leakage or damage caused. But it still didn’t solve the problem.

I probed around and saw that at the trigger for the 555, it was getting stuck at GND. Which was weird. I checked the other side of the transistor Q622, and that was at 1v. Which is also weird, the inputs to the NOR gate were 0v, so it should be around 5v. Nothing logically about this made sense.

I tried replacing the transistor and testing the NOR gate chip, just in case that was bad, but it checked out fine.

It then occurred to me, what if the trigger input for the 555 had somehow become an output? I pulled the 555 and replaced it with a known-good one. And it worked! I was now seeing the roughly 0.5 second pulse on bootup I would expect, and the keyboard controller worked every time!

Recap

After I had sorted the reset problem, I got to work recapping the whole board. Some of the solder joints for the capacitors were already looking a bit tarnished, which is an indication of early leakage. But there was no permanent damage anywhere. This is the end result.

More Testing

The final thing I wanted to test was the external floppy drive. I hooked this up, popped Amiga Test Kit into that drive and ran the test.

Spot on! I appear to have got quite lucky with this lot.

I reassembled the Amiga, blowing out all the dust bunnies as I went, I could then give it a real test. I pulled out a demo disk for Lotus III and tried that out.

It probably would have helped if I had hooked up a controller 😂.

Conclusion

As always, I wanted to write this blog post so that next time I have a machine fail like this, I remember what to look for.

I’m likely going to keep the external floppy drive, but sell the Amiga 600 on, once I’ve cleaned it of course! I don’t need another one in my collection, but I did want the restoration project.

As for the TV, that will be coming with me to the Norwich Games Festival later this month, where I’ll be bringing a bunch of Amigas and a BBC Master 128 with the Valiant Turtle.