Reviving an Amiga 600: From Dead Video to a Clean Boot

I managed to score an Amiga 600 motherboard which was faulty for £41. This weekend I attempted to fix it. Here is my progress.

The motherboard

This is what the motherboard looked like when it arrived.

Things to note immediately:

• There is no ROM. Unfortunately, I have no 2.05 ROM in stock, but I have other ROMs I can test with.
• The keyboard connector cap is missing.
• There is quite a bit of corrosion and dirt, including a lot of capacitor leakage.

In general, nowhere near the worst I’ve seen. I was hoping for a fun challenge.

Testing

I tested for shorts between the voltage rails, there is always a risk of fire with corrosion in the A600. Everything seemed fine. I popped in a DiagROM and tried powering it up… The result was a black screen. But it had sync, which means the clocks are likely good.

Reset circuit

This hasn’t been recapped yet, so I immediately looked at the reset circuit. This uses a 555 timer and one of the capacitors for it is an electrolytic, which had leaked.

The reset circuit can be a little complicated, but this is the part I’m focusing on:

This may look sparse in this diagram, but all these components are very close together on the PCB. C612 is the electrolytic. Long story short, replacing this got the machine to boot DiagROM! But the boot time was longer than expected. There was still an issue.

I tested C611 and it was showing that it was starting to fail, pulling the Trig signal lower than it should be. I replaced this and I also replaced the 555 because it was heavily corroded, just in case this was also misbehaving.

After these changes, the reset circuit was very predictable.

More testing

All the standard DiagROM tests passed, so it was time to test a proper Kickstart ROM. I had a Kickstart 3.1 ROM available which I popped in and connected a Gotek drive to boot Amiga Test Kit.

Every test passed! But there was a caveat, the composite and TV out video was dead. This is pretty normal on Amiga 600, 1200 and CD32 machines (and on the CD32 it affects the S-Video too). I shall explain why, and the fix I have designed for it.

Composite fix

There is a Sony CXA1145 chip on these versions of the Amiga. This is a video encoder chip that takes the analogue RGB signals, and produces a composite video signal (and a few other things). It also acts as an audio buffer so that the audio can be sent to the TV out without introducing noise to the normal audio circuit.

You’ll notice on the motherboard that there are some components that are square metal cans, a few of them are welded together. These act as the filter circuit for the CXA1145. Unfortunately, even a slight amount of capacitor corrosion gets into these and destroys them. There are currently no replacements for these (although they are easy to design, and it has been on my TODO list for years).

Fortunately, Sony made a couple of more enhanced versions, the one I use in my fixes is the CXA1645. This is almost pin compatible, but not quite. But, it has the filter circuit built-in to the chip! The biggest downside is it drops the audio filter.

There are websites that show you how to modify the motherboard to accept a CXA1645 (or CXA2075, another relative), but I designed my own way that is easily reversible.

But first, we need to remove the CXA1145. I used the JBC hot air and tripod system to easily and safely remove the chip.

Note that I had already done the recap at this point and cleaned this area a bit, just to make sure this was not causing the dead video. It was not.

My method then requires soldering a male connector where the chip was so that my socketed module can slot into place.

This is a double-sided PCB (components on both sides) that rewires the CXA1645 to the pinout of a CXA1145, and adds an audio buffer that is missing on the CXA1645.

It would be very easy to desolder the male header and put a different chip (or the original) in instead.

Finishing up

I replaced the audio connectors whilst I was doing the recap, I had to remove them to get to two of the capacitors, and found them quite corroded inside. I also put a new cap on the keyboard connector. I was going to 3D print one, but I ended up just taking one from one of my scrap A600 motherboards.

I gave the motherboard an ultrasonic clean and performed a final round of tests. I then put a 2.04 ROM into it. The board wasn’t quite the challenge I was expecting. It turned out to be a relatively easy fix. But, it is now ready to keep a new owner happy for years to come.