18 Aug
2006
18 Aug
'06
2:10 p.m.
Hi,
Please note that I have never seen inside an M2, so this is mostly
speculation. Feel free to ignore all my ramblings...
A friend of mine needs to repair the power supply of a TRS-80 Model II.
I've found a schematic but without part numbers and values! He needs to
I assume this is a schematic in the technical reference or service manual...
Be careful, In the case of the M3 (a machine which I _have_ worked on),
the PSU was bought-in from ASTEC. ASTEC did not supply (or allow to be
publisehd?) their schematics, so it appears somebody at Radio Shack
reverse-enegineered the board to provide a schematic for the manual. And
they didn't do a very good job of it. Suffice it to say the transformer
shown on the diagram bears little relationship to reality. And I seem to
rememebr some components are mising from the control circuit.
Te M2 PSU schematic may be fine, of course, but do check it carefully.
know the type of transistor Q1, there are no markings
on the case of the
transistor. I've checked the power supply of my Model II, and there are no
markings, too. The Technical Reference Manual has all schematics for the
Model II, but the schematic for the power supply is useless without
component values. Might a BU208 for Q1 do the job?
Hmmm... Chopper transistors are always hard to substitute because they
work under such unusual conditions. It may well be that the reason the
original chopper isn't marked is that it's not really a standard part,
but was selected to meet some criteria that you haven't a hope of
discovering.
That said, I've found the 2SD1942 to be a good 'universal' chopper in
most small single-stage SMPSUs.
-tony