1 Jul
2012
1 Jul
'12
1:07 p.m.
Hi Guys,
I posted this note on Vintage Computer Forums, and thought I'd see if
anyone could help here too.
T actually an acceptable looking TRS-80 Model 1 monitor I snagged for $3
but I figure the principles are the same regardless.
Actually ,the original Model 1 monitor is somewhat unonventional.
There were 2 versiosn of the M1 montiro sold in the UK. The origianl one
has a white scren and 2 knobs and the video input cable in a horizotnal
row at hte bottom right. The knobs are 'B' (brightness) and 'C'
(contrast). The cable comes through a hole convenientyl labeleld 'V' for
video, buit iot was origianlly (see below) a volume control
The later version, which I know nothing about, still has a white CRT, but
with a green filter over the entire front of the unit. The controls are
still to thew right of the CRT, but about halfway down the case, not at
the bottom.
OK, back to the origianl one, which I do know something about. It's
actually based on an RCA mains-powered portable TV. That's the thing that
had a volume cotnrol. The origninl TV was 115V only and had a live
chassis (the chassis was connected to one side of the mains). The
modification consisitend of not fittign the tuner, IF strip or audio
circuitry (much of that was on a plug-in PCB, forutnately). That PCB was
replaced by a PCB with an optoisolator circuit on it so that the video
inptu (and thus the comptuer) could be earthed. One oddity was that the
input suide of the optoisolator needed a 5V power supply, this came from
the computer.
Now, that TV chassis was, as I mentioend, 115V only. To use it in 230V
countries, there was a mains tranformer fitted inside the monitor. THis
is an _isolating_ transofrmer, so the thign is no logner live-chassis.
With the result that hte video input no longer neeeds an optoisolator.
The PCB in palce of the IFs stirp, etc, is just a simple transisotr video
ampliifier.
It's dead. No raster and no glow in the tube neck.
The board does heat uo
though and there is that smell of old electronics being startled awake
What heats up?
after many years. I've done no tests yet, but I
have Sam's Facts for the
model 1, and they provide a troubleshooting guide for the monitor and say
what voltages should be on cetain components. I suspect something to do
with the AC power supply of maybe horizontal sweep. Some faulty power
transistor maybe?
Quite possibly.
What I would appreciate from anyone who knows, is a link to a page or doc
which explains how composite B/W monitors work. The Sam's document is great
from the perspective of troubleshooting detail but it does assume you know,
conceptually, just what's going on. I don't and I'd like to get some
understanding before I start poking around.
I suspect it's asusmed you know how small monochrome TVs work :-).
Seriosuly , a book on TV serviisng will explain much of waht you need.
Incidently the SAM's fact PDF covers the 110V version while I have a 240V
one. There are some differences, one of which is there appears to be no
fuses in the AC circuits!
The main difference is the isolating trasnformer, shwich si separately
mounted i nthe case.
-tony