Radio Shack TRS-80 Models I and III available near NE Ohio

Tony Duell ard at
Sat Oct 15 18:25:37 CDT 2005

> > The CPU board stabds vertically at the back, possibnly covered withe a
> > metal shield. Remove that, and you're looking at the component side of
> > the PCB. The model 3 is mostly TTL, with some RAM (4116 DRAMs, 2114 SRAMs
> > for the video), ROMs, and a Z80.
> Right... I did find GIF schematics and pored over them last night.  I
> remembered correctly that there's not anything truely bizarre going on
> in there - worst case, I can trace faults down to individual chips,

At power-on, the video side should be running (there's no CRT controller 
that has to be set up by the CPU), so you probably should have some 
garbage on the screen, unless by chance all your video memory locations 
have initialised to spaces :-).

It should be quite easy (given a logic probe, 'scope, or logic analyser) 
to check that you're getting a system clock, that the video counter chain 
is running, that you're getting addresses to the video memory, that 
you're getting date out of the memory and into the character generator 
ROM, that the character rows are going from the character generator to 
the video chift register, that the shift register is being loaded and 
finally that there's a bit stream coming out of it.

IIRC, there's a connector on the CPU board with 3 screened cables going 
to it that is part of the video harness. The signals on those cables are 
Hsync, VSync, and video. You should be able to see if there's anything 
coming out of the CPU board.

Of course the problem could be in the monitor section. It's very 
conventional, and schematics exist. 


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