TRS-80 Mod I Video Displayu

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sat Jul 19 17:01:01 2003

> I've got a TRS-80 Mod 1 with a bad display. There is a horizontal scan
> line in the center of the screen but no vertical sweep.
>
>
>
> Is anyone familiar enough with these displays to give me repair hints?

I have the service manual for the original (no green filter, controls in a
horizontal row at the bottom right) somewhere, and I'll dig it out if you
want to get inside.

However, a word of warning. This is by far the most unpleasant monitor
I've ever worked on. It's a converted RCA portable TV (the conversion
consisted of removing the tuners and IF plug-in PCB and fitting a video
input PCB in the IF PCB slot). This means that the 110V model is hot
chassis -- the video input PCB contains an optoisolator so the computer
is not directly connected to the mains. Since you need to work on the
vertical deflection circuit, you'll be working on the 'hot' part. If you
don't have an isolating transformer then don't even think about working
inside this monitor with the power applied!

The 230V model is a bit more pleasant. The TV's PSU was strictly 110V
only, so it needs a transformer for 230V mains. Radio Shack used a proper
double-wound isolating transformer, which means the chassis of the 230V
model is isolated from the mains. The video input board on that model is
just a transistor buffer stage, no optoisolator.

Let me know if you want me to dig out the service manual and talk you
through the vertical deflection circuit.

-tony
Received on Sat Jul 19 2003 - 17:01:01 BST