Back in June 1988 was one of the best days of my life, when I received a brand new A310M for my 13th birthday. I've had it ever since but fell into disuse over the years. This weekend I attempted boot it for the first time in a few years. No problem with the monitor, but turning on the computer resulted in a fairly spectacular simultaneous bang and not only release of what on first sight was presumed to be magic smoke, but also some actual short-lived spark or even flame, out of the vents next to the power supply. Neverthless, astonishingly the machine soldiered on, booted up fine, and indeed the WIMP GUI appeared to be fully functional! (I upgraded from Arthur to RISC OS 2 I think at some point in the first few years.) Alas, no such luck after subsequent power cycles - even though apparently the power supply is still fine, the WIMP GUI failed to materialise, instead showing the text-only greeting:
Code: Select all
RISC OS 1024K
ARM BBC BASIC V version 1.04 (C) Acorn 1988
Starting with 20732 bytes free.
Worse than that, the monitor no longer syncs probably with the video signal, so the text is rapidly moving both vertically and horizontally (which can be minimised somewhat by tweaking the monitor's sync knobs but is still unusably bad).
Opening the case revealed the infamous C37 16V 100microfarad decoupling capacitor to be completely blown, with its severed head quite some distance away. However I can't imagine how an explosion of that would magically travel through the power supply and make it out of the side vents, which makes me wonder if the explosion was a separate incident with the power supply (which still appears to be working fine).
I've trawled these forums and the rest of the internet for info about this C37, however the problems I'm seeing don't at all match the normal symptoms of a totally dead machine. Is it conceivable that a dead decoupling cap could cause a) loss of monitor sync and/or b) a windowing GUI to fail to boot? I was delighted to discover that the service manual is still available online and packed with helpful info:
http://chrisacorns.computinghistory.org ... 300_SM.pdf
and it even seems to suggest that a decoupling capacitor could be responsible for interfering with video:
VIDEO NOISE (ACORN FCO E011 REFERS)
Breakthrough of system noise occurs onto the screen. This shows as rippling on screen and is particularly noticeable with colour monitors. This seems to be particularly prevalent when running 'Arcwriter'.If this occurs, solder a 1N4148 diode in parallel with R67 (adjacent to VIDC IC17) with the cathode (dark stripe) at the end nearest Q13. Remove the decoupling capacitor 'A' nearest to IC17, between it and Q12 either by using desoldering equipment or by cutting the capacitor out, leaving wires long enough to attach a new component. Replace this capacitor with a 22μF 6.3 V or higher axial lead electrolytic capacitor, eg Farnell part number 030 34229. Observe polarity - positive end furthest away from Q12.
However I'm not sure which decoupling capacitor 'A' refers to.
More worryingly for this soldering newbie, just before that it says:
Note: There are problems inherent with removing components from four-layer boards. Components should only be desoldered from the board using vacuum desoldering equipment. An acceptable alternative would be to cut the wires to the components concerned, leaving enough of the wire on the board to allow the new resistor wires to be soldered to them. The new wires should be cropped close to the resistor, but allowing the new solder joint to be effected. On removing R39, which is no longer required, crop the wires back to the board
I don't have vacuum desoldering equipment. Would that approach also work for capacitors as well as resistors?