24 Apr
2011
24 Apr
'11
11:10 a.m.
One thing about taking it apart: before you do, record what card is in whic=
h slot. If you have multiples of a given card (for instance, more than one=
Or more generally 'make notes'. I never take anything apart without a
notebook and pen to hand to record cable positions, board positions,
which plug goes where, etc. It saves a lot of time later ;-) Some people
like ot use a digital camera for this but I am not sure if it will show
what needs to be recorded in all cases. So even if I had one, I'd want a
notebook too.
You may want to arbitrarily number the slots (say '1' at the bottom, it
doesn't matter if you are consistent!) and then afix labels to the PCBs
(e.g. tie-on tags with the string passing through the handle rivets) to
indentify which board came from each slot.
[...]
(which is available on BitSavers). You may find that
the cards are actual=
ly not in the "best" order. But if you record the original sequence, you c=
an always get back to a situation where the machine probably once worked. =
Indeed. It is very useufl to have a configuration that you know has
worked. If it doesn't work now, it means a component has certainly
failed. And that's something you can trace. Rather than having a setup
that maybe shouldn't work even if all the parts are good.
=20
When you check voltages, don't just look at levels. If you have or can bor=
row an oscilloscope (a good tool to own for this hobby), look at the power =
Actaulyl, it's not an instruemtn I use all that often, but anyway...
lines for AC component. A small ripple can be
accepted, but the filter cap=
acitors in these machines are well beyond their lifetimes. If you are the =
In some cases if the smothing capacitors still have some capacitance
(albeit much less than they should do), you won't seen much, if any.
ripple on no-load, but you will when you connect the dummy load (or the
rest of the machine).
In machines with swtiching regulators (and I have an idea that the 8/a is
one of these), a dreid up or open-circuit output capacitor in the
regualtor cicuit can put high voltage spikes (around 15V) on the output.
These will hopefully trip the crowbar circuit, but it's something to
watch for.
cautious sort, disconnect the filter caps and perform
a leak-down test, rep=
lacing parts that fail. If you're really cautious, just replace them - tha=
t's what we do with power caps if they're over twenty years old. There's a=
date code on the part. =20
If I did that, I'd have replaced every capactior in every machine I own,
including the machine I am currently typing this on. I haven't. In fact
many of my machines, including ones that are 40 years old, are still
runnign with all their original capacitors.
Yes, capacitors fo fail. I've replaced a number over the years. And they
can fial from just old age if the electrolyte dries up. But in m
experience it's not the problem that some people claim it is.
-tony