help with TRS-80 Model II?

From: Richard W. Schauer <rws_at_ais.net>
Date: Sat Jan 10 17:45:59 1998

Hi everybody. First off, let me ask- does anyone else have a Model II?
I've been working on mine recently and I can't get it to work. When I got
it the CRT was loose and rattling around the case and the power and reset
switches were broken apart. I got these things sorted out and powered it
up to get a screen filled with hash. It seems to roll around the screen
and no characters are visible, just hash.

I put a test clip on the Z80 and found very little activity. The only pin
toggling is the clock. All of the address, data, and most of the control
bus lines and either low or (apparently) tri-stated. I checked the BUSRQ*
line to see if maybe something was asking for the bus and it was high.
The BUSAK* output is low, however. Shorting NMI* to ground has no effect
whatsoever. After it's been on for a while, turned off and right back on
again, the screen remains blank.

I changed out the Z80 for a known good one with no effect. I happened to
note, however, that the computer generates lots of TVI, and the pattern
changes during reset.

If anyone has any suggestions, they would be most welcome. Also, if
anyone has any schematics for this thing, that would help too.
(Alternatively, if you know that a certain part is the same as the Model
I, III, or 4, let me know as I have the schematics and working examples of
these.)


Richard Schauer
rws_at_ais.net
Received on Sat Jan 10 1998 - 17:45:59 GMT