7904 vertical/trigger interaction.

Andy Warner

Today, I noticed a problem on my 7904 that I've not noticed before.

Running in ALT vertical and ALT horizontal mode, so the right vertical
signal is displayed using the A timebase, and the left vertical signal is
displayed using the B timebase.
The trigger level of the A timebase was modified by the on-screen amplitude
of the left vertical signal - e.g., I needed to tweak the timebase A
trigger level to maintain a good trigger for the right vertical, depending
on the state of the left input. The converse did not seem to be true (e.g.,
trigger level on timebase B seemed stable.)

In most other ways, this 7904 seems fine (the only other annoyance is that
the readout display gets wiggly if any vertical signal goes way off screen.)

I also should swap the two timebases, to see if the problem stays with the
slot or the module, just typing this up made me realize that. I will admit
that I was looking at some other signals when I noticed this problem, so I
wasn't really in debug-the-7904 mode.

I have not re-capped the mainframe yet, although I have recapped some of
the modules.

I plan to make time to pop the sides off and measure the rails to see if I
am within spec or have ripple, but I wanted to ask here if this behaviour
sounds familiar to anyone.


Hello on my 7904, I have never noticed this issue.

After you swap timebases, you can also try to exchange H and V, to aid debugging.

I would check the 7904 serv manual for the trigger selection circuit

Perhaps a hybrid or IC has degraded.

Bon Chance,