Amiga 500 Zydec Superboard 1.5MB Refurbishment


Introduction:

A few years ago I was lucky to snag an Amiga A500 Rev 6a on Gumtree. As the floppy drive was not working the seller bundled in a load of original games, an external drive, and a couple of Zipstiks – all for the price of £30.

I was confident I could get the drive working again and came back home thinking I’d hit the jackpot. Back home I opened the trapdoor to see if anything was in there. To my surprise there was a Zydec Superboard 1.5MB expansion lurking in there – extra Jackpot! This expansion is also known as AMRAM1M5 Issue 2.

This type of trapdoor memory expansion is known as a Slow RAM expansion. Despite the name this memory expansion is not any slower than the onboard memory – this name is given because it is neither ChipRAM or FastRAM.

Amiga Memory Types:
ChipRAM starts at the base of the Amiga memory map (0x000000) and is used by the custom chips for video & sound. The custom chips have priority over the CPU so the CPU has to wait to read / write to ChipRAM when the custom chips are using it.
FastRAM starts after the ChipRAM area in the memory map (0x200000). The Custom chips have no access to this memory area so the CPU can read / write to FastRAM without delay.
SlowRAM starts higher up in the memory map (0xC00000). This RAM shares the ChipRAM bus. Although the custom chips cannot access this memory the CPU still has to wait for them to release the bus before it can access it. For the CPU this has the same speed of ChipRAM but without the ability of the custom chips accessing it.

Battery Leakage:
Anyone who is into vintage computers will be fimiliar with the Ni-Cad memory backup / clock battery. Over time these batteries leak out a strong alkaline fluid that destroys PCB traces and corrodes component legs. Many Amigas have been badly damaged by leaking batteries.

The Superboard has a battery that keeps the onboard clock ticking over.
This battery had leaked and had corroded a few IC legs and lifted the soldermask off a few traces.


As well as the battery leakage this board was quite grubby from the solder flux used in the assembly process. The plan was to clean up the whole assembly and find an alternative to the leaking battery.

A CR2032 coil cell would be an ideal replacement as the onboard Clock uses just a few microamps to keep it ticking over. The M6242B clock IC requires a minimum supply voltage of 2.0v so a 3V CR2032 cell has enough voltage to run it including the 0.5V voltage drop due to the the in-circuit diode.

The original battery is a rechargable Ni-Cad so if a CR2032 was used then the circuit would need to be modified to disable the recharge feature.


PCB Clean-up

Gary PCB
The Superboard comes with a Gary adaptor. This needs fitted to the motherboard Gary socket to allow all the memory on the expansion to be available. Normally these trapdoor expansions are 512KB so extra circuitry is need to allow the full 1.5MB on the board to be seen by the Amiga.

The Gary adaptor plugs into the motherboard Gary socket, then the Gary chip is placed into the socket on the adaptor.
The adaptor has a couple of Glue-Logic ICs and a 100nF decoupler cap. The twin-flex lead goes to the main Superboard PCB



The Gary PCB was cleaned, a better socket put in place, the PCB break-outs sanded flat, and the fraying twin-flex cable tidied-up.



Memory & Clock PCB
Once the leaking battery was removed the whole PCB was washed.
The corroded IC legs were cleaned and re-tinned, and the exposed traces were given an overcoat.
The two old electrolytic capacitors were replaced by new Panasonic items.
The recharge circuit was then modified to allow the CR2032 to run the clock, but not allow the cell to be recharged.



Modified Battery Circuit
The Post-It note schematic below shows the changes that were made to the recharge circuit:


After all the refurbishment was done the Superboard performed flawlessly in the A500 🙂

// Document End

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