2 Oct
2013
2 Oct
'13
12:52 p.m.
So I have recently been given half a dozen Acorns - one Model A, 4
Model B+ with 64K and one 128K B+.
I'm going to donate the Model A, at least, to TNMOC at Bletchley, but
I'd like to keep at least one for myself.
The thing is, whereas I wouldn't just turn on a 1980s business
computer with multiple moving parts, the donations chap at TNMOC has
put The Fear into me by sternly warning me Not To Turn Any Of Them On,
because They Will Blow Up.
Tht's going a bit far. It is a very bad idea to jsut power on any old
computer, but they don't _always_ blow up.
So, how does one go about testing a mucky old Beeb that's not been on
in 25y or something & has been stored in a dusty attic?
Give it a good clean, I guess, but can one hook one to a PC PSU and
test its internal PSU in isolation?
Yes.
The BBC PSU is a separate unit. It links to the mainboard with 7 wires
IIRC. The 3 black ones are all ground (0V). The 3 red ones are all +5V.
And the other one (purple?) is -5V. Actually, IIRC the Beeb will run (but
sound doesn't work) with only a +5V supply.
There are at least 2 versions of the Beeb PSU The origianl one (in a
black case IIRC) was a linear supply. A mains transformer and 3 separae
5V regulators. This oen was well-known for takign the varnish off tables,
it ran so hot. Most were replaced by the later one, so this supply is
now very rare. If you have one, hang on to it (but don't run it!). IIRC
the 3 red wires are acutally separate 5V outputs, one form each regulator.
The laster PSU in an off-silver case was an Astec switch-mode unit. With
that one., there is on 5V output, conencted to the 3 red wires.
I _think_ very early revsion mainboards have 3 separate 5V connecitons,
one for each PSU output, each feeding soem of the chips. On later boards,
all 3 5V tags are connected together. It is still a good idea to connect
to all 3 tags when testing, the current drawn is ratehr too much for just
one connection.
So what I would do :
With the machine unplugged from the mains, take off the cover and
keyboard. Then disconnect all 7 PSU wires (the -5V one is difficult to
spot, it's in the corner near the PSU). Tkae the PSU out (3 screws o nthe
bottom. Removeing the mainboard from a Beeb is not as easy as it might
be, IIRC you have to desodler the wires to the video output BNC socket.
Inpsect the PSU for burnt components, bulging or physically
leakingcapoacitors, etc. Check the fuse is good and the right rating.
Put the PSU back in place but do not connect the wires ot the mainboard.
Connect a 6V 6W bulb (or thereabouts) between one fo the red wires nad
one of the black wires (assuming this is the later SMPSU, I can explains
what to do with the linear one if you really have that one). Power up,
the bulb should light. Check you have 5V across it.
If you can't get the bulb, then a 4.7 Ohm 10W resistor would also be a
suitable load. That might be easier to find.
If so, then power down, reconnect the wires -- correcrtly -- to the
mainboard, connect up the keyboard, and try the machine out.
-tony