Acorn BBC Archimedes A310 with RISC OS 2

I bought this A310 (base unit only) from ebay in late May 2015. I knew little about the state of this particular machine and the ebay listing stated "Once plugged in and powered on the green light shows at the front - no further testing done - please bear this in mind when purchasing.".

An Acorn BBC Archimedes A310 1MB with RISC OS 2

photo wakeuk2015 (ebay seller)

From the ebay listing photo's I could at least discern that the machine had a backplane due to the presence of the AKA16 MIDI card but beyond that, there were no indications of what condition the machine was in.

Acorn BBC Archimedes A310 rear view with AKA16 MIDI Podule fitted

photo wakeuk2015 (ebay seller)

When the machine arrived, I was pleased to discover that it was in what appeared to be its original factory condition although it was in need of a clean and had some minor hardware issues that had to be addressed. The machine was in such an original state, it still contained the original MEMC chip (named Anna) and the accompanying PAL chip.

The original ARM MEMC1 chip and its associated PAL

The MEMC chip was revised and most Archimedes owners replaced it with the MEMC1a chip which, along with the PAL delivered an approximate 10% performance increase in memory access. The MEMC1a chip provided fixes to the MEMC chip which allowed ARM3 upgrades, support for Acorn SCSI cards and FPU podules (only in A400 series machines) to name just a few of the benefits of the upgrade.

The ARM MEMC1a chip and PAL chip fitted to the A310

I've documented the repairs in the Repair and Change history below.

Expansions in this A310

This particular A310 has just a few expansions installed which are:

Repair/Change history

Although the motherboard was getting power when the machine arrived, it was not booting correctly as I suspected may be the case from the original ebay listing. The cooling fan was also not spinning which was indicative of a power issue with the machine.

Using a multimeter, I determined that the 12V line was only delivering 1.4V. This is indicative of a component shorting to ground which is a common fault. In the Archimedes A310, Acorn specified three 100uF Tantalum capacitors rated at 16V as decoupling capacitors for the power supply on the motherboard which can be seen in the photo below.

The Archimedes A310 power connections

Arguably, the voltage rating for these capacitors is under specified and, as a general rule of thumb the voltage rating should be 2 or 3 times higher than the working voltage in use. Whilst the 16V rated caps are fine on the 5V lines, it's is not quite enough for the 12V line so a 25V or higher rated tantalum capacitor would be a better option.

An original 15V 100uF Tantalum capacitor.

The capacitor that de-couples the 12V line is designated as C37 on the PCB. Regardless of the rating, a quick way of working out whether the C37 is at fault or not is to simply disconnect one leg of the capacitor from the motherboard. If the machine begins to respond correctly then the capacitor is at fault and needs replacing. Doing so on this Archimedes caused the machine to boot into life.

  • I replaced C37, C38 and C39 with 100uF 25V rated tantalum capacitors to ensure that there's no chance of failure of any of the three tantalum decoupling capacitors for some time. Incidentally, the A400/1 series has 25V 100uF capacitors specified for the equivalent decoupling capacitors.
The A310 schematic showing decoupling capacitors
  • The floppy disc drive had a thorough internal clean as although it activated, it did not seek or read a floppy disc. After cleaning with a dry brush and then cleaning the heads with Isopropyl Alcohol using some chamois tipped sticks, the drive reads and writes perfectly.
  • The video output was suspect so, using some VIDC test software, I was able to boot from floppy and automatically test the VIDC chip which spooled the test results to the floppy disc. (Thanks to Steve Harrison for providing the test software). The tests revealed that the VIDC chip was working correctly so the issues with video output is elsewhere. The issue on the RGB video out turned out to be a faulty cable. After changing the cable and switching to a VGA monitor, I was able to re-configure the machine to output a VGA display and boot to the desktop.
  • No video output on the mono connector was resolved when replacing IC4 with the Ultra VIDC Enhancer which was fitted to improve monitor compatibility and replaces IC4 as part of the fitting process.
  • An Econet module was fitted to allow easier data transfer to the machine given the fact that the machine only has 1MB of RAM and a 2-way backplane, upgrading the machine is impractical so installing an Econet module allows connectivity to other Acorn machines removing the immediate need to install a hard drive for data storage.
  • The battery holder was replaced as one of the contacts was so corroded, it snapped off.
  • A MEMC1a upgrade was fitted replacing the original MEMC chip (Anna) and the associated PAL.
  • There was no sound coming from the speaker under normal usage. Replacing Q9 and Q11 (2N3904 transistors) with BC548 functional equivalents fixed the problems.

The A310 op-amps and transistors making up the filtered audio out

  • In December 2015, a Simtec 4MB RAM upgrade was sourced to take the machines RAM capacity from 1MB to 4MB making the machine more practical in use.

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A310 maintenance

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