5 Oct
2011
5 Oct
'11
4:53 p.m.
Here, I'll start: Anyone know if the original
TRS-80 Model I power
brick suffers from the same failure modes as those old C64/VIC-20 power
bricks? Any repair options? I've got an old one that seems to be
working OK for now, just wondering if it'll stay that way...
From what I rememeber, the power brick contains a
transformer with
effectively 2 centre-tapped outputs [1]. The tap is conencted to
system
logic grounmd. One set of outputsi s fed as AC to the keyboard unit,
where it is rectified and smoothed to provide the +5V ad -5V rails. The
other set of outputs is rectified in the brick by 2 diodes (biphase
rectifier) and fed to the keyboard unit where it is smoothed and
regulated to provide the +12V line.
[1] I do not know if it's one long transformer winding with 5 conenctions
or 2 separate centre-tapped windings with the taps joined. In other words
if it's :
||(--------------
||(
----)||(--------------
)||(
)||(-------------- Centre tap
)||(
----)||(--------------
||(
||(--------------
Or
||(--------------
||(
----)||(----+
)||( |
)||(----)---------
)|| |
----)||(----)---------
||( |
||(----+--------- Centre tape of both secondary windings
||(
||(--------------
I once had to determine how the transformer in an old HP printer was
wound. What I did (with the thing turned off and the primary
disconencted) was to connect the highest-voltage secondary conenctions
(the ends of the largest winding, effecively) to a DC supply, the voltage
of which I turned up until a sensible current (about 1A) was flowing. I
then meausued th eDC voltage between the ends of the other secondary. If
it's like the first diagram, you get a measurable voltage between these
wire, if it's the second diagram you get 0. That's easy to check.
The TRS-80 Model 1 TechRef says that it's 2 sparate windings. The first
is 14V at 1A. This is used for the +5V and -5V suppleis). The other
outputs 19.8V at 350mA after rectification (the 2 diodes in the brick),
so I assume it's a centre-tapped 28V winding, rated at 0.5A or so.
There are no smoothing capacitors or transistors/ICs in the brick.
I asusme the diodes can fail. Whether this then damages the transformer,
or whether you can catch it in time I don't know. The manual mentions a
fuse in series with the mains input to the primary winding, this may well
blow to protect the transformer.
I believe the USA bircks were plastic-cased and many of them were glued
together (but check dfor screws under labels, feet, etc). I have heard
that one way to open them is to hold the cables about 1m for the brick
and swing it onto a hard floor :-). Less violent is to either tap round
the join betwee nthe 2 parts with a mallet or to carefully sqeeze it near
the join in a vice.
European bricks are a bit larger (which means they don't fit in the
Expansion Interface compartment unless you remvoe the tray from it) and
metal cased. They are held together by 4 obvious screws. I think the
trnasofmer is potted ad effectivelg glued in to the case by the potting
compound, but IIRC the wiring and diodes are easy to get to if you open
the case.
-tony