I fitted the additional video RAM, enabled RGB on the jumpers, and as Larry expected still no luck with the v2.5 monitor ROM.
Update on my searches for info on this found the following on comp.sys.zenith.z100 from 1987, looks like I have a way forward with the upgrade to the V2.5 ROM
====================
boot ROM version question
I have an "old" motherboard with version 1.2 boot ROM, and would
like to upgrade to a newer boot ROM version. What is the highest
boot ROM version (& part #?) that will work with the old motherboard,
and how should J101/J102 be set?
I bought a 2.x ROM from Heath over a year ago, but my system
would not boot with it (trying every combination of J101/J102),
so I sent it back for a refund. As I remember, the system would
beep once and that was it. Since I had S101 set for auto boot,
I don't know if it would have gotten as far as the "hand prompt"
or not.
Thanks in advance for any information.
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To Upgrade Boot Monitor V1.X to V2.x (2.9 is latest) requires changing
EPROMs, moving J102, and changing/torturing a PAL chip. The exact
details will be forthcoming in a week or so in the ZUpGrade Series.
This should give me enough time to refind the info...
Cheers,
Gern
Zupgrade part 4 is here:
https://groups.google.com/forum/#!s...comp.sys.zenith.z100/1OirJiaerZg/rPUx6lsXImUJ
Key part for updating from the V1.2 ROM is:
Upgrade From Version 1.x:
If the current MTR-100 version is 1.x then 3 changes must be made:
1) The ROM Address Selector PAL IC at U161 (part number 444-129) must
either be replaced with a ROM Address Top 32K Selector PAL IC (part number
444-129-1, about $7.00) or the 444-129 PAL IC must be 'converted' to
perform the new function.
The 444-129 IC can be converted into the function of a 444-129-1 IC by
connecting input pins 2 & 3 high to Vcc at pin 20.
a) Remove the IC from the socket and bend up pins 2 & 3.
b) Bend pins 2 & 3 together and solder them both to a small wire one inch
long.
c) Carefully solder the other end of the small wire to the top of pin 20.
Do not bend pin 20 or impede the pin from normal insertion into the socket.
d) Insert the converted IC into the socket. Pins 2 & 3 must remain above
the socket, but pin 20 must go into the socket normally.
2) Move Jumper J102. This allows the use of a 16K ROM.
3) Replace the ROM at U190 with Heath part number 444-276-1 MTR-100 Version
2.9 [*]. Upgrade completed.