A310: could blown C37 cap break monitor sync and WIMP GUI?

Arc/RPCs, peripherals, RISCOS operating system & ARM kit eg GP2x, BeagleBoard
aspiers
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A310: could blown C37 cap break monitor sync and WIMP GUI?

Postby aspiers » Tue Feb 20, 2018 2:45 pm

Hi there, first post here from a long-term Acorn fan who is very heartwarmed to see that there is still such a great community around protecting the legacy of these wonderful machines!

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?

Many thanks!
Adam

RobC
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Re: A310: could blown C37 cap break monitor sync and WIMP GUI?

Postby RobC » Tue Feb 20, 2018 5:53 pm

Welcome to the forum.

The loss of sync/booting to BASIC would seem to indicate a CMOS RAM issue - have you tried resetting it? Try holding down the delete key as you power on the machine and see if that makes any difference.

If you can tell us which sort of monitor you are using, we should be able to advise on the correct configuration settings.

Also, whereabouts are you located? Someone here might be local enough to help look at the machine.

aspiers
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Re: A310: could blown C37 cap break monitor sync and WIMP GUI?

Postby aspiers » Wed Feb 21, 2018 12:41 am

That was it, thank you so much!! I had tried replacing the CMOS batteries at the weekend, but after all these years I'd forgotten about that delete key trick.

At the weekend the CMOS batteries I found inside when I took the cover off were absolutely ancient (Ever Ready Gold Seal, remember those?), so I immediately replaced them with some I had lying around - it didn't help at the time, presumably either because I was missing the delete key trick or maybe the replacements were also dead.

Anyway, it seems to be working great now - the RISC OS 2 graphical desktop is back and the monitor picture is perfect!

Next problem to tackle is that the floppy drive seems to be faulty. If I stick a floppy in and type the famous *. command, the green light comes on, but no whirring sound from the motor, let alone the loud chugging noises you'd expect, and then it claims that the drive is empty. I'm guessing that that's a fairly common problem ... initial searches suggest maybe track damage due to a leaked battery but I didn't see any obvious leakage. Hopefully it's something simple which doesn't require a replacement drive!

Another problem is that the mouse is hopelessly unreliable - the ball material has deteriorated, and maybe some of the internal sensors too.

BTW I'm in east London - any other enthusiasts in this area?

steve3000
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Re: A310: could blown C37 cap break monitor sync and WIMP GUI?

Postby steve3000 » Wed Feb 21, 2018 7:38 am

aspiers wrote:Next problem to tackle is that the floppy drive seems to be faulty. If I stick a floppy in and type the famous *. command, the green light comes on, but no whirring sound from the motor, let alone the loud chugging noises you'd expect, and then it claims that the drive is empty. I'm guessing that that's a fairly common problem ... initial searches suggest maybe track damage due to a leaked battery but I didn't see any obvious leakage. Hopefully it's something simple which doesn't require a replacement drive!

Not track damage on the A310, as there's no rechargeable battery :)

The A310 floppy drive is known to be fairly unreliable. Most likely is some dust/dirt has crept in, through the front. Take a look with a torch - and be prepared to clean, with cotton buds, the area around the microswitches which detect disc presence.

One other problem I found with my A310 floppy is that it sometimes moves slightly in the case, and when this happens it doesn't spin up because the discs don't go all the way in properly. Try a variety of different discs, and ejecting then firmly reinserting them several times...each time checking if the drive recognises them.

bernie_pt
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Re: A310: could blown C37 cap break monitor sync and WIMP GUI?

Postby bernie_pt » Wed Feb 21, 2018 2:03 pm

Maybe I'm wrong, but if you don't have replaced the dead capacitor, could it be that 12V rail is missing? This could explain why the green LED works (+5V) but the spin motor is powered at 12V. If you can, confirm with a multimeter the voltages at the connector.

steve3000
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Re: A310: could blown C37 cap break monitor sync and WIMP GUI?

Postby steve3000 » Wed Feb 21, 2018 9:31 pm

bernie_pt wrote:Maybe I'm wrong, but if you don't have replaced the dead capacitor, could it be that 12V rail is missing? This could explain why the green LED works (+5V) but the spin motor is powered at 12V. If you can, confirm with a multimeter the voltages at the connector.

Good thought, but C37 only provides 12v to the main pcb podule expansion connector. Floppy drive gets 12v straight from the psu, so would not be affected if C37 missing... also, I'm pretty sure the internal 3.5" drive doesn't use 12v in any case?

aspiers
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Re: A310: could blown C37 cap break monitor sync and WIMP GUI?

Postby aspiers » Thu Feb 22, 2018 12:26 am

Thanks folks, I'll try the cotton buds approach and see if that yields any progress!

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Re: A310: could blown C37 cap break monitor sync and WIMP GUI?

Postby vanekp » Thu Feb 22, 2018 10:08 am

...... the power supply (which still appears to be working fine).


And you have checked all the voltages with a meter to see they are okay and within range?

aspiers
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Re: A310: could blown C37 cap break monitor sync and WIMP GUI?

Postby aspiers » Thu Feb 22, 2018 11:34 am

Not yet but I'll do that. Had a peer into the drive with a torch and it certainly is quite dusty, so that will need addressing.