Steven Hirsch wrote:
All,
I could use some suggestions. I'm trying to troubleshoot one of my
IIe motherboards, but with little success. The symptoms are a bit
odd. If I do not have an AUX slot video board inserted, it works just
fine (albeit in 40-column mode only). With any board (I've tried
several) in the AUX slot, the display becomes total garbage - and it's
not limited to an "every other character" issue. My first thought was
a problem with one of the custom gate-array chips and started
substituting from a second, known good board. This got me nowhere.
Next step was to sub every socketed chip on the motherboard! Didn't
get me anywhere either. Now, I'm down to suspecting either memory, or
one of the soldered-in 74-series logic chips. However, it's hard to
see how it could be memory, since the machine works fine with nothing
in the AUX slot.
Any ideas?
The AUX slot board supplies latched video data during ph1 low period.
PH1 is on pin 36 of the AUX slot. If for whatever reason it is stuck
low, the AUX card will always drive the video data bus resulting in
garbage.
Bad buffer on the AUX card (LS374) will do the same.
If you don't have a scope, buy at least a logic probe. Cheap and quite
useful.