TRS-80 Model 1 Level 1 repair.
ard at p850ug1.demon.co.uk
Sun Mar 15 11:13:28 CDT 2015
> The satisfaction of fixing a classic. For those interested what it was
> about and what was wrong, here are the details.
As regards the monitors, the older one (the one you now have on the Level 1 unit) is a converted
RCA portable TV. The exact convertion depends on the mains voltage. You see the original TV was
designed for 115V (US) mains and had a hot chassis (not isolated from the mains).
In all cases the tuners, sound section, speaker, etc were removed. Then :
115V model : The video IF PCB was replaced by an opto-isolated video input board (so the
TRS-80 was isolated from the mains). The input side of this circuit took 5V from the TRS-80
230V model : Since the TV chassis was designed for 115V input only, it needed a step-down
transformer for 230V mains input. An _isolating_ transformer was fitted inside the monitor case
so the chassis was now insolated. The video IF PCB was replaced by a simple 1-transistor (I think,
maybe 2 transistors) video amplifier. This model just needs composite input from the TRS-80, the
5V supply is not used.
The later one (which I have never seen inside) was sold in the UK with a moulded green plastic cover
over the entire front, with holes for the knobs to poke through. This gave a 'green screen' display using
the normal TV white CRT.
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