TRS80 Model 4P problem

From: Keith Whitehead <>
Date: Tue Feb 17 18:28:39 1998

>Why does actually getting booted up cause the monitor to be unusable?
>Trying other disks in the drive, if they are not system disks, causes other
>messages to appear, perfectly readable.
>Any ideas?

The model 4 support 2 video modes 64x32 (?) and 80x24 (?)
What happens is that the 4P starts up in low-res mode but then boots up
into the higher res mode when a model 4 boot disk is inserted (if you had a
disk to allow the 4P to boot into model 3 mode the video would be stable as
the 3 used the low res mode only).

The problem can be fixed quite easily.
The way the system works is that there is a crystal oscillator which
generates the clock signal for the video circuitry (and pretty much
everything else through dividers !), it uses this for the low res mode.
However it uses a phase locked loop to generate the clock for the hi res
mode because there is no easy divide from the crystal frequency to the
video clock needed, and what has happened is that the PLL has drifted out
of its capture and hold area.This is probably caused by component
tollerance changes, most noteably the capacitors.
However there is a trim adjustment near the front left hand side of the CPU
board which will enable you to adjust the loop back into sync.

I have the service manual at home and can get more info if you need it such
as the component designator etc.


+----------- Keith Whitehead -----------+
| Physics and Chemistry Depts |
| Massey University |
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Received on Tue Feb 17 1998 - 18:28:39 GMT

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