Amiga 500 AMRAM1M5 Expansion

ram_full

Introduction:

Recently I got back into the Amiga scene after a 20-odd year break and managed to snag a semi-functioning A500 Rev6a on Gumtree. When I got it home it had an AMRAM1M5 Ram Expansion lurking inside.

The A500 can be expanded by adding two types of memory – Trapdoor Memory (“Pseudo-Fast” or “Slow-RAM”) or CPU-only Memory (FAST RAM). The particular expansion is the Trapdoor variety and comes with a piggyback mod-board that allows the full onboard 1.5MB to sit in the memory map at $COOOOO thru $D7FFFFF. As a bonus this board has a battery-backed realtime clock.

Closer Inspection:

Any retro collector will tell you one of the plagues of old electronic items is the onboard battery. After 20 odd years these have been known to leak and corrode anything in the nearby area. Amiga 500+ owners will be very familiar with this as the onboard clock battery can eat away at the PCB – known affectionately as “Death by Varta”.

This expansion has suffered the same fate but not as bad as it could have been:
ram_leak
The leakage has got under the solder-mask to the right of the battery, and some pins on the IC to the left have went green.

First thing to do is remove that old battery and clean up the area and assess the damage. The wires connecting to the mod-board have become a bit frayed so they will need a tidy-up as well.

The Gary Mod Board

ram_gary

This mod-board needs attached to the main PCB in order to get all the memory to appear in the memory map at the correct location. The GARY IC is removed from the main PCB and placed in this mod-board. Then the mod-board is placed in the GARY socket.

It was a bit mucky and some of the soldering looked rushed. I decided to clean it up and give it a quality machined-pin IC holder. The frayed wires got a tidy-up as well:
ram_gary_clean

The Battery Replacement

The leaking Ni-Cd battery now needed replaced with a new item, or something else that could be removed easily if needed. The ideal solution would be a coin-cell and holder. As luck would have it the onboard clock IC only draws a tiny 3.5uA so a CR2032 battery would be ideal and should last at least 5 years.

I found that a CR2032 holder that I use on the SYEMB05 was suitable – there was enough room for it on the PCB. The original battery-charging circuit needed a little modification so as not to charge the CR2032 but still allow it to run the clock when the main power was off.

ram_clean_mod

And Finally:

ram_clean_full

I cleaned and washed the whole unit as there was a lot of residual solderwave flux on the board especially around the main connector. The rough PCB break-out tabs were sanded to give a smoother finish. The job wouldn’t be complete without a Re-Cap so the 2x 22uF electrolytics were replaced with new Panasonic items. The exposed trace to the right of the battery was covered with green overcoat – this was where the battery leak had lifted the soldermask off the trace. The corroded legs of the LS139 IC were cleaned and re-flowed.

Modification “Post-IT” Schematics:
The top drawing shows the original circuit on the board for the battery charging circuit.
The bottom drawing shows the modified circuit so the CR2032 cell can be used instead.

ram_battery_circuits

// END

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