Recapping an Amiga 1200 and evicting a Varta: a weekend log

Back in 1993 I bought my A1200 from a little shop on London Road in Sheffield with money I had been told was for a new boiler. The boiler waited. The Amiga did not. Thirty-odd years later it had started doing two things that every A1200 owner learns to dread: the sound had gone thin and quiet on one channel, and when I opened it up there was a faint fishy smell. If you know, you know.

This is my log of putting that right, plus a side quest involving my old A500's trapdoor memory expansion. I'm writing it for anyone with an A600 or A1200 in a cupboard, because the problem is the same on all of them and the earlier you deal with it, the less damage there is.

The problem: surface mount electrolytics

The A600 and A1200 use small surface mount electrolytic capacitors, and after thirty years they leak. The electrolyte is corrosive. It sits on the board quietly eating pads and the thin traces around them, and the audio section is where people usually notice first. Mine had the classic green-brown crust under three of them, and one pad lifted when I touched it, which tells you how long it had been going on.

Let me be clear about what this job is and isn't. The A1200 runs from an external power brick, so there's nothing on the main board that can hurt you. The brick is another matter. Old power supplies can hold a nasty charge long after they're unplugged. If you're a beginner, leave the brick closed, buy a modern replacement supply, and spend your nerve on the board.

Tools

• A decent temperature controlled iron with a fine tip
• Hot air station (a cheap one is fine, I used 330°C with a wide nozzle)
• Plenty of flux, flux remover, and isopropyl alcohol (99%, not the stuff from the chemist with water in it)
• Kapton tape to protect the plastic connectors and nearby chips from the hot air
• A capacitor list for your board revision, and a photograph of every area before you touch it
• Patience, and tea

The process

1. Photograph and label. Every cap position, its value, and which side the stripe is on. The silkscreen is not always clear and you will not remember on Sunday what you saw on Saturday.
2. Remove the old caps. Some people use the "twist" method, gently twisting the can until the legs shear. It's quick but it can rip a weakened pad straight off. I used hot air and tweezers, lifting each one as soon as the solder went.
3. Clean properly. This is the step people rush. Isopropyl and cotton buds until there's no residue at all, then look at every pad and every via under a magnifier. The corrosion travels further than you think.
4. Repair. My lifted pad needed a short bodge wire to the next via. Check continuity of anything near a leak site with a meter before you fit the new part.
5. Fit new capacitors, polarity checked twice. I went with new electrolytics of the right value and voltage. Some people fit tantalums. I'd rather not: they can fail short, and a short on a 30 year old board is a bad afternoon.
6. Clean again, test, then close it up.

Result: both audio channels back at full volume, crisp as I remember. The first thing I played was the title music from Turrican II, at a volume that made the pigeon on the shed roof leave.

The side quest: the Varta

While I had everything out, I opened the A501 trapdoor expansion from the old A500. It has a little rechargeable Varta battery for the real time clock, and these leak too. Mine had started. Blue-green fuzz across two traces.

• Get the battery off the board today. Not next month. It keeps leaking.
• That electrolyte is alkaline, so a little white vinegar on a cotton bud neutralises it. Then isopropyl to clean the vinegar off, and dry it thoroughly.
• Check the traces nearby with a meter. Repair anything open.
• If you want the clock back, fit a coin cell holder with a diode, or simply live without the clock. I've lived without it for 30 years.

If you've never seen the machines this is all about, this is a lovely history of the Amiga and why people like me still care:

Any questions, ask away. And if you have an A600 or A1200 in the loft, please, open it this year.

Cheers, Pete