From: rogersh%p2g@uk.ac.man.cs (rogersh)
Newsgroups: comp.sys.acorn
Subject: A3000 PSU, RAM upgrades, RGB output intensity, and RF interference
Keywords: hardware screwup
Date: 13 Apr 92 10:59:53 GMT
Organization: Another 5 years of glorious Conservative rule...


	There was a major problem with my A3000 PSU which I discovered
last night and fixed. Other A3000s are probably suffering from the
same problem - read on...

	For some time I had been worried about the intensity (brightness)
of the image my A3000 was generating - whenever I flexed the top of the
case or knocked the machine the intensity flickered uncontrollably,
sometimes for minutes on end. It also seemed to be affected by hard disk
operations, but flexing the top of the plastic case was the most reliable
way of reproducing the problem. In the end, tired of banging the top of
the case to fix the problem (sometimes this crashed the machine), I decided
to open it up and find out once and for all what was wrong.

	My first suspicion was that the case lid was putting too much
inward pressure on the metal plate at the back that mounts all the I/O
ports, causing it to bend inwards thus flexing the PCB. Although putting
pressure on this did seem to have some effect on the output intensity
I came to the conclusion this wasn't the problem. Then I checked whether
the RAM upgrade was causing it. No luck there - pushing it in had no
effect at all. I did discover however why banging the lid hard caused
crashes - this knocked the RAM upgrade against the floppy drive, causing
the top row of RAM chip pins to all short out against the metal case.
A piece of insulation tape and a couple of pieces of cut eraser soon
stopped that from reoccuring (and incidentally made the upgrade a lot
more stable - previously it was wobbling backwards/forwards on it's
pins quite easily).

	Anyway back to the intensity problem - next candidate was the
floppy drive. Pushing/pulling on this had no effect either. Then I tried
pushing the PSU cover down. BANG. Machine crash. Ok so reboot and try
again - this time more gently. Result: massive flicker on the RGB output.
However this didn't explain how the cover could be affected by the
lid of the case, since there is a small (but sufficient) gap between the
top of the PSU cover and the underside of the lid. However I noticed that
the flyleads from the PSU to the main PCB did loop up above this level and
were pushed down by the lid when you pushed it on. Flexing these flyleads
gently caused the RGB flicker, and violent pushing/pulling caused crashes.

	So off came the PSU cover. (To do this you remove the warning sticker,
use a long shaft screwdriver to unscrew the PSU mounting screw accessed via
the uncovered hole, pull out the various flyleads connected to the PSU, and
lift it out of the case.) The problem (to cut a long story short) was that
the flyleads were badly connected to the PSU - however the problem arises
in such a way that I think other A3000s may be suffering from the same problem.

	The power supply leads from the PSU to the main PCB are soldered at
the PSU end using crimped pins - the wires are crimped with connectors which
cut through the insulation when they are fitted and provide pins to solder
onto the PSU's PCB. All the power leads (the two to the main PCB and the two
to the floppy drive) are pulled together with a cable tie before leaving the
PSU cover. Unfortunately these crimps were badly fitted on my machine, and had
mangled the insulation on three of the four wires really badly, all but
missing the core of the wire. In fact the +5V line to the main PCB was
connected by a single strand of the wire core which was just touching part of
the crimp and actually sometimes sparked whenever the wire was flexed.
This was the cause of the RGB flicker and the crashes.

	Anyway the solution was to desolder the crimps from the PSU's PCB,
chop them off the flyleads, strip the ends of the flyleads, resolder them
(this time properly without crimps) to the PSU's PCB, and reinstall the PSU.
No more problem - since performing this operation I have had no unreliability
in RGB output intensity, and no anomalous crashes from banging the machine.

	In order to perform the resoldering operation you need to completely
remove the PSU from the machine, pulling the flyleads off the main PCB
so you can work on them. I also cut the PSU cover flaps off while I was
at it so I can now lift the cover on/off the PSU without having to remove the
whole damned thing. It goes without saying that you'll need a solder sucker,
a soldering iron, wire clips, pliers, screwdriver, some solder, and
relevant experience. In fact unless you're confident about this don't
do it - and in any case don't claim that I advised you to do it. If you're
not experienced at this kind of thing and think you have a problem,
then see your dealer and show him this article. If he has a repair facility
then maybe he'll do the necessary for you - and maybe not.

	Hope this helps people out there - If my machine was like this
then it's quite likely other A3000 PSUs (made in Taiwan in bulk) are also
suffering similarly. Certainly I've seen articles on this newsgroup
complaining of exactly the right symptoms. If anyone else subsequently
discovers they do have this problem then please mail me, post an
article, or mail Acorn. If this is a frequent problem then the right people
(Acorn) need to be made aware of it.

	Lastly the PSU cover is just a piece of textile - this is one reason
why the A3000 pumps out so much RF interference. Casing the PSU in metal
wouldn't be too hard, nor would putting metal covers on the underside
of the lid, around the sides of the machine, and under the main PCB - a
few grounding flyleads connecting these together would then ensure near total
RF containment (cf Faraday's Cage)...

	-Huw

[ H.J.Rogers  INTERNET: rogersh%p4@cs.man.ac.uk                    ]
[    ,_,      JANET: rogersh%p4@uk.ac.man.cs                       ]
[  :-(_)-o   "I'll be back..."                                     ]
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