Dragon-32-Repair v2

Ram Repair Board/System for Dragon 32 (and others)

The Dragon 32 comes in (at least) five different flavours. With a variety of DRAM configurations - 16 x 4116, 8 x 4532 or 8 x 4164H/L (half good 4164s).

The layout varies by board design and revision too. Making a single design for a repair board to use 8 x 4164 is not trivial - at best a single design would cover just one of three groups of design.

My answer is to split the problem into two parts - the actual board that will hold 8 4164 ICs and a mechanism for attaching the repair board to the host while providing all of the required signals.

Previously...

My early attempts at this have been a single PCB that plugs into the host through the original memory footprints. I can cover two of the design groups in one go with the caveat that one of them will require a single flying lead to pick up a stray signal but I have a new plan.

And now

The split design consists of a small, minimal board to mount the 8 DRAM chips, literally just big enough for those ICs. The other half is a flexible PCB that is soldered to the host board in place of the original DRAM chips. The flexible PCB is just 0.11mm thick so this sits flat on the host without any clearance issues to the original surrounding components. One end of the flexible PCB is an extended "neck" that folds around to connect to the rigid memory board.

The two boards are not permanently connected, using a regular FFC/FPC connector that holds the flexible PCB strongly once locked in place.

The beauty of this is that different flexible sections can be made to suit the specific host - and it doesn't have to be a Dragon 32 either. Any classic computer that uses single bit DRAM (up to 64k) could work.

Upgrade Shortcut

In the case of the Dragon 32 that I start with, this repair mechanism could also work for the basis of a memory upgrade. By itself the memory board does not give the host access to the full 64K but a simple tweak to the host board can unlock the extra capacity.

On the Dragon 32, the host uses one of the PIA IO pins as a test for capacity. Typically it is pulled high - indicating that the lower 32K is composed of two banks of 16K. If this is, instead, pulled low the host will treat it as a single bank of 64k. Additional work may be required depending on the host board design but in the simplest case, the task of lifting the one leg of one resistor and then connecting it to GND instead of 5V is simple enough for pretty much any DIYer.

Cost

The cheapest option for repair is to remove the original DRAM ics, perform some trace surgery and place the 4164 DRAM ics on the host board. This is not trivial and requires care and attention to the task. Get it wrong and you'll have repair wires running all over to fix the mistakes.

The two part approach is not the cheapest but it is much less expensive than it appears. The single biggest cost is the 4164 DRAM itself. You can easily be looking at £40 to £50 for a set of 8 ics. The two boards would typically cost (depending on how and where you get them made) another £8. Quite reasonable at a price per pcb but you generally need to have at least 5 made in a batch - another £40 effectively. Then there is the cost of the FPC connector but those come in at around (and below) the £1 mark.