A5000 RAM control line repairs

Whilst soak testing the A5000 after re-flowing the solder around some of the audio components, IC59, a 74AC138 surface mount chip failed and overheated as can be seen in the photo below by the discoloured solder pads and legs on the IC.

The RAM control line area of the PCB after overheating and failure

IC59 controls the RAM select lines and so, due to its failure, the machine would not boot. A POST error of &20141 was being reported by the flashing floppy disk drive light which indicates that it cannot read RAM and the CMOS has become corrupt.

The relevant section of the A5000 schematic can be seen in the image below. As an aside, R299 and R295 are incorrectly labeled and are switched on the PCB.

The RAM control line schematic for an Acorn A5000

After examining the schematic, I replaced the IC and the capacitor as it looked like it had possibly leaked and checked for continuity for all of the control lines. One control line was damaged that linked pin 15 of IC59 to R287 so a short piece of wire was soldered into place. The repair can be seen in the photo below.

The replaced RAM control line de-multiplexer and repaired track using Kynar

After carrying out the repair, the A5000 was restored to full operation and boots and runs reliably again.

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Acorn A5000

POST error calculator

Acorn RISC computer fault codes are reported via the flashing of the floppy drive light in a sequence of 8 blocks of 4 flashes. Each block of four flashes (e.g. long short short long) represents a set of fault and status bits which can be translated into a list of error codes. 

Select the boxes that represent the long flashes from your computer to derive the fault code and its translation.

Fault code:


RISC OS links

RISC OS is the operating system that runs on all Archimedes and RISC PC's from Arthur in the beginning, to RISC OS 2 and onwards to RISC OS 4, 5 and 6 today.