[Coco] (Marginally Topical) TRS-80 Model I Troubleshooting

[Rob Rosenbrock]

> The computer powers on.  The monitor works but has an extremely touchy
> vertical hold.  But if I power it up with the expansion unit switched on
> I get garbage on the screen.
>
> Next time I have the time to clear the space to set it up and play with
> it, I'll try cleaning the card edge contacts between the TRS-80 and the
> expansion unit, as I gather this is a common failure.  But if that
> doesn't work, is there any other wisdom about likely problems with the
> Expansion unit?  Reseat socketed chips?  Check for bad caps?
>
It's been a long time since I've powered up my Mod I, but I still have it.
Used to know it inside and out, and I will not let it out of my
possession.

Due to the extreme age, a visual check of caps and reseating of all
socketed components would be a good idea. I think the memory is static
sensitive, so follow standard precautions.

Yes, the cable going to the EI is a common problem. The usual (and
frequent) solution is to clean the contacts. Can't recall if that ever
causes what you're seeing. My memory is that the computer would reset
whenever I placed my coffee cup next to it. But, again, we're dealing with
aging hardware.

To narrow it down, try removing the double density controller and RS-232
card if installed. They're located underneath the top panel. Also swap the
two power supplies.

If I remember right, the computer should work properly with the EI
connected but not powered on.

> What about the vertical hold on the monitor?  I'm not afraid of a
> soldering iron, can read a schematic, know what not to touch inside a
> TV, and where to keep my other hand, but don't have a good enough grasp
> of CRT circuits to really know where to find the V. hold parts or know
> what to suspect.  (I do have an ancient copy of TAB's "101 TV Troubles
> and How to Fix Them" or some such.  But it predates even the TRS-80
> monitor by a few years.)

If it's not cured by cleaning the pot, I think you'll need a set of
schematics here. I recall 'fixing' one Mod I by adjusting a pot in the Mod
I itself and not my monitor.