Amiga 500+ Restoration & Upgrades

I’ve been slowly but surely branching out into Amigas from Commodore’s 8-bit range of computers, but one of the first computers I bought was a Commodore Amiga 500+.

This turned out to be a somewhat special unit, with serial number “000657”.

The A500+ is a 16-bit computer from the 1990s, and is a more capable version of the A500 on which it is based- it features the same 7.16MHz 68000 CPU and full-sized keyboard, but has ECS graphics (as opposed to OCS graphics), 1MB Chip RAM, and a battery-backed RTC.

The machine was in excellent condition and even came with several accessories and its original box, however it was sold as “untested”. It even had intact warranty seals, which meant that it must be pretty much completely original.

Unfortunately, in the A500+ (and other machines of the era with a battery-backed RTC, such as the Acorn Archimedes 3000) this is not a good thing. These have Varta 3.6V alkali rechargeable barrel batteries soldered to the mainboard, which leak corrosive battery alkali during storage – this stuff is notorious for going to town on the PCB, damaging ICs, sockets, traces, ports, and connectors, and killing machines.

As such, I didn’t even attempt to power up the computer in its current condition – I removed the warranty seals and opened the computer for inspection. After removing the keyboard, RF shield, and internal 3.5″ FDD, I found that the damage was (luckily) limited to just the battery area itself, not even the surrounding components.

A500+ battery leakage.

This was the most isolated battery damage that I’d ever seen on an A500+, as all the others had coverage across most of the board (even to the ports on the back of the computer) and were pretty much irreparable. So, I set about cleaning it up.

Past this point I’d recommend wearing a set of nitrile gloves and cleaning your hands and workspace regularly, as battery alkali is pretty nasty stuff.

The first step was to remove the source of the corrosive material: the battery itself. I used a small pair of clippers to cut its three legs close to the PCB, taking care not to cause damage to the board – then, I pulled it off and put it straight in the bin.

The second step was to neutralise the remaining corrosive material. I removed the board from the case, removed all of the socketed ICs, and bathed it in white vinegar for a while to neutralise the alkali, scrubbing the most affected area with an ESD-safe brush – then I washed the board off with 99.9% IPA to remove the leftover alkali salts and vinegar, again scrubbing the most affected area. I carefully disposed of the vinegar and IPA solutions, then cleaned all of the IC sockets with contact cleaner and reinstalled the socketed ICs.

Finally, I desoldered the battery legs using my desoldering station (a Duratool D00672) and a large amount of leaded solder and rosin flux, necessary because the corroded material on the battery joints makes them difficult to remove. I scraped back the darkened solder mask on the battery joints, and tinned the bare copper with solder.

After this, the battery area looked pretty good (no corroded ICs or components, no damaged traces or vias), and I was quietly confident that the system would work OK.

Before testing the machine, I wanted to restore the RTC functionality by installing a new battery – both NiCad or NiMH batteries would be a suitable replacement as these don’t leak like alkali batteries do, however I decided to use an adaptor board with a CR2032 coin cell holder as coin cells are readily available and are easy to replace. This fits in the original battery footprint, and is fitted with a diode to prevent the coin cell from being charged.

After checking that the PSU was working OK, I did a quick power-on test with an RGB SCART cable connected and the computer seemed to boot up fine. However, I couldn’t test it any further as the internal 3.5″ FDD wouldn’t load disks.

Commodore Amiga A500+ boots for the first time.

The original internal 3.5″ FDD was a slimline Panasonic unit – initially it would make the correct “clicking” noise when waiting for a disk but wouldn’t detect when one was inserted, and shortly afterwards it just died completely.

Original Panasonic 3.5″ FDD.

I disassembled, cleaned, and serviced the internal FDD, but still wouldn’t attempt to detect disks – this ruled out a mechanical issue and pointed towards a probable logic issue, so I replaced the electrolytic capacitors on the drive board, but to no effect.

At this point I decided to just swap the entire drive unit out for an earlier (and more reliable) Chinon unit from a spare A500, which seemed to work fine.

Commodore Amiga A500+ loads disks for the first time.

Now that the computer seemed to be working reliably, I wanted to perform some preventative maintenance to help keep it that way – specifically, by replacing all the electrolytic capacitors on the mainboard.

These capacitors typically comprise aluminium windings insulated by a liquid electrolyte, which can dry out over time and negatively affect performance (even failing dead short), or physically leak and cause corrosion to the PCB and surrounding components.

Unlike the later A600 and A1200, the A500 and A500+ were manufactured using through-hole electrolytic capacitors – being an established technology for the time, these capacitors are generally reliable and rarely cause problems. However, in this case the previous battery leakage may have compromised their performance, so I replaced them out of caution.

I usually remove all of the capacitors at once using my desoldering station (a Duratool D00672), then install the new ones one-by-one whilst taking particular care to ensure that the value, voltage rating, and orientation are correct – electrolytic capacitors are polarised, so must be installed the correct way around, else they’ll blow up during use. I then clean up all the remaining flux residue or heat marks using 99.9% IPA.

You can’t always trust the markings on the PCB silkscreen, as sometimes mistakes were made in the design from the factory (take the Commodore CD32, for example), so care must be taken to match the orientation of the new capacitor with the original.

I used a commercially available capacitor pack from AmigaStore.

After testing everything again to make sure that the new capacitors were installed correctly, it was time for some performance upgrades.

I fitted a KickStart 3.1 ROM, a CPU relocator, an internal IDE interface and IDE-CF adaptor, and an internal WorkBench 3.1 CF card, allowing internal storage without the need for a HDD; I also fitted a 1MB Chip RAM expansion which fits into the PCB’s expansion slot.

The A500 and A500+ do not have an onboard IDE interface like the later A600 and A1200, so an adaptor is required to emulate one – this requires a KickStart ROM version of at least 2.05 for IDE support. The adaptor fits under the CPU, and needs to be wired into three signals on the mainboard (/OVR, /INT2, and /INT6).

After all this work was performed, I did some finishing up: I thoroughly cleaned the mainboard with compressed air and an ESD-safe brush; I thoroughly cleaned the case inside and out using Cillit Bang and a microfibre cloth; I also disassembled and thoroughly cleaned the keyboard.

The computer still seemed to boot OK following all of my modifications. However – and I know this well – just because a computer boots, doesn’t mean it’s working properly. Thorough testing is necessary to verify operation, so I did as much testing as I could.

  • All keys registered correctly.
  • Internal IDE interface worked OK.
  • Internal 3.5″ FDD read and wrote from/to disks OK.
  • External drive port worked OK.
  • RGB video output worked OK.
  • Composite video output worked OK.
  • Dedicated stereo audio output worked OK.
  • All joystick and mouse inputs worked OK.
  • RAM expansion worked OK.
  • RTC worked OK.
  • Power and drive LEDs worked OK.

Another restoration complete – happy days!

Published by themightymadman

A conscientious, intelligent and committed graduate engineer, with excellent interpersonal skills, an eye for detail and a keen interest in hardware design, mathematics, and software development.

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