On Fri, Sep 1, 2023 at 4:31 PM dwight via cctalk <[email protected]> wrote:
>
> I do understand the warranty sticker. Say we have the average computer buyer.
> He has a screw driver and mostly knows how to use it. He also went to Radio
> Shack ( long gone 🙁 ) and bought a cheap soldering iron. He is now fully
> equipped to repair what ever is wrong with his box. ( obviously not at Tony's
> level of competence ) He takes the cover off and has no idea what he is
> looking at or what he is looking for. ( I think we have all been there at one
> time ) He sees something with a screw driver slot. He has a screw driver.
> Perhaps all it needs is a minor adjustment. He turns the screw one way, and
> nothing changes. He turns it the other way and, pop, some smoke comes out of
> the other side of the chassis.
You clearly know the TRS-80 Model 1 (or are a good guesser).
There are 4 presets ('things wth a screwdriver slot) on the Model 1
logic board. Two of them are associated with delays on the horizontal
and vertical sync signals. Adusting those can cure a
video-going-off-screen issue.
The other 2? They set the output voltage of the +12V and +5V
regulators. There is no overvoltage protection on the former, turn
that too hgh and you may wipe out all the DRAMs. The latter has a fat
zener across the output, but I am not sure I trust it. And if you turn
the +5V line up you could be looking to replace a lot of ICs...
-tony
