In this article, I shall be looking at simple DIY repairs, and
general fault diagnosis to keep your Amiga in tip-top working
condition. This article isn't for those of you who wish to carry out complex
repairs, but does detail some of the basics for finding the majority
of minor faults. I list these below for some novices.
There are two types of Amiga power supply, and these are easily
differentiated. The first uses a transformer and weighs about the
same as a household brick. The second uses thyristors and weighs
about the same as a can of beer. Apart from the obvious, which involves checking the fuse and the
wiring in the plug, repairs on the lightweight type end here. The
reason: the thyristor variety employs a capacitor, holding some 300
volts, even after switching off, so leave it alone. That one is a
dealer repair. The second type is more flexible, but before you do anything, unplug
it! Underneath, where you would expect to find the four screws,
Commodore chose in its wisdom to cover these with little cream plugs.
Despair not, these can be levered out with a sharp point, then sling
them. Opening up the two parts of the case will allow the innards to
be removed. The first test is continuity, and this involves turning the meter to
resistance setting and placing each of the probes at either end of
the three cables which run from the plug to the circuit board. Check
for continuity. Forty percent of Amiga faults are with power supplies, and there are
usually three causes. Firstly, the cable breaks after lots of use,
where it travels into the power supply, hence the continuity check.
If you find one of the lines broken, de-solder the whole lot and
replace the cable.
Secondly, there is a sneaky 400 mA fuse on the circuit board. Again
check this for continuity with a multimeter. If it's blown, as is
often the case, you'll get one from most electrical or television
shops. Thirdly, and more tricky, are the two diodes which sit on the centre
of the circuit board. These are rectifiers, and are a common source
of problems. To test a diode, remember that it will only allow a
current to pass through it one way. The component is grey in colour,
with a black stripe at one end - this is the cathode. Connect your multimeter, again with resistance, across the two ends.
Anode to cathode should produce zero resistance, while the converse
should give infinite resistance, registering no current. The diode is a 5 volt, 9 Amp type. If you find one of them, or both
are faulty, you can get them again from Maplins or Tandy. Make sure
you solder the cathode at the correct end. Once completed,
re-assemble and test. Never test the unit while it is dismantled. Another word of caution: be careful with the wires from the
transformer to the circuit board as a few years of use makes them
brittle, especially at the point where they solder onto the
transformer tags. Other than the points I've mentioned, there is little else the
average punter can do to effect a repair. If all these fail, sling
it. Do consider buying one of the higher-rated power supplies if you
are replacing. The original version supplied wasn't intended to take
the strain of all the third-party add-on we have all accumulated over
the years. The power supply in the Amiga 2000 is a completely different kettle
of fish, and this can only be accessed by removing the lid of the
2000. Again, however, there is a 400mA fuse on the circuit board,
but you will have to remove the whole assembly, disconnect the drives
and lifting the assembly away in order to gain access to the screws
which hold the perforated metal skin over the circuit board.
Actually diagnosing a fault can be difficult, and not always as
obvious as you might think. Some characteristic malfunctions are
easily traced, if the obvious alternatives are eliminated. A classic example is a problem with the mouse only operating in one
axis. It's useless messing about inside the Amiga unless you are
sure the mouse itself works perfectly, so the mouse needs checking on
a fully-working Amiga to determine where the true fault lies. Once the mouse has been tested okay, then it's more often than not a
problem with one of the odd or even CIA 8520 chips, and these are
usually blown by carelessness, i.e. not turning the Amiga off before
connecting a peripheral. There are a few tips regarding CMOS
devices: electro-static discharge or ESD. This is not an Amiga
related illness just a natural phenomenon related to the abrasion of
non-conductive materials, such as rubber soled shoes and nylon
carpets. Unfortunately, electrostatic discharge (ESD) charges can build up in
the human body to hundreds of thousands of volts, and to any IC/CMOS
device this is fatal. There is a simple remedy, and this involves
discharging the build-up to earth. The easiest way is to grip an exposed metal surface (a cold water
pipe or tap leading away from it). It is only effective if not
painted. Domestic wiring circuits usually have an earth strap to the
cold water pipe system, so this conveniently discharges the static. There are a couple if general tests we can do before looking further
into individual components. The first is the IC 'thumbs down'. This
is essentially a firm location test, ensuring the IC's are happily
planted firmly in their IC sockets. Believe it or not, this
represents about 10 percent of Amiga faults. So, we look around the
motherboard of either the 500 or 2000, and identify all of the
socketed ICs. With a thumb at either end of the IC, we apply firm downward pressure
to see if it has become dislodged. The power supply can be
disconnected, as well as the keyboard, and the Amiga tested. There is no problem trying out the Amiga with the lid off - the
motherboard only has two supplies, one of 5 volts and one of 12
volts. If the thumbs down test didn't work, we can do a general hot-spot
test. This needs a little care, unless you have asbestos-tipped
fingers. Connect the power supply to the Amiga, and leave it on for about 5
minutes. Carefully move over the surface of all the ICs and see if
you can feel any localised point of high temperature. By this I do
not mean general heat. There is always heat generated by ICs, and in
particular the main processor (00, 020, 030 or 060).
What I refer to is a small localised point over the surface of the IC
where there is a considerably 'higher' temperature than elsewhere.
If this is the case, then you can feel reasonably confident in
suspecting something is a bit dodgy. You might be exceedingly lucky, and get away with a through clean of
the legs of the IC(s), and this is done using a cotton ear bud/swab
and some alcohol. You will need to be careful removing any ICs from
their housings, but don't be too timid, just careful. Once cleaned, the IC needs returning to its socket. Be very careful
to ensure the IC is placed the correct way. This is easily checked
by firstly, identifying the semi-circular notch at one end and
matching it with the notch illustrated on the motherboard, and
secondly, looking to see if all of the pins are offered up accurately
to the holes in the socket. It all sounds fiddly and precarious, but
believe me it isn't difficult, once you've cracked it. If neither of the two previous tests have worked and we still have no
go, the ideal solution is to ask a friend you can try the suspected
ICs out of his or her Amiga, thus identifying by substitution. A
word of warning however. If the fault is consequential, in other
words if your 68000 / 68020 / 68030 / 68040 / 68060 is blown because
you have a short in the edge connector, then you will also blow your
friends one as well and popularity may be thin on the ground. When Commodore designed the Amiga 1200, they decided to use cheaper
surface mount technology, which makes replacing particular components
somewhat difficult. So some electronics experience as well as some
knowledge in soldering is@2 DEFINITELY@5 recommended...
Should you feel confident that your diagnosis is correct, and you
have bought a new IC, you must be careful at installation. Firstly,
carry out the static discharge procedure before even handling the new
IC. Secondly, remove the IC from its packaging and holding it
between thumb and middle finger of one hand, lay the legs flat
against a resistant surface, such as a table top. Apply gently
pressure to all of the legs, bending them towards the centre of the
IC. Repeat this procedure with the opposite legs. The purpose of this
exercise is to create a right angle between the body of the IC and
the legs. When they are bought the legs are splayed out and will not
fit into the relevant IC socket. Once completed, plug in and test. You will either be ecstatic, (maybe even electrostatic!) or
disappointed with your attempts. It really gives a buzz when you fix
your Amiga for the first time, and you realise you have saved
yourself a packet. Don't despair if you fail and there are faults which this limited
article can only hint at - then you can always admit defeat and
toddle off to your dealer. At least you've had a go, and if nothing else, you know what your
Amiga looks like from the inside out !