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I would indeed start with checks on the logic board <2>. The Theory is quite clear about signal levels and paths. In both mainframe ALT and plugin ALT the flip-flop U156A is involved and probably refuses to switch between states. If not defective itself the toggle pulses from the time base unit (via connector pins B4) could be missing. (Just guessing).
On Thu, May 14, 2020 at 08:11 PM, Michael W. Lynch wrote:
Good Day to all:
I have a 7633 that works well for the most part, however, it has one nagging
issue. The "ALT" button does not display a trace from one of the two plug
ins. This is regardless of what type of plug in is inserted. I do not
believe that it is the plug ins, since they can be inserted into a fully
working 7623A and function normally. On the 7633, the other Vertical
Functions seem to work correctly. CHOP displays both sets of traces for both
plug ins when 2 - 7A26 plugins are used (so four traces appear). Push the ALT
button and the top set of traces disappears. When the ALT button is pushed,
there are readouts for both plugins and they are "IDENTIFIED" when that button
is pushed, just no trace from the left plugin. Left and Right Buttons will
select their respective plugins, however, when using either one alone, the ALT
switch position on either plugin does not work. Select LEFT button and set
the Left Vertical to ALT and you only get one trace with a dual trace plugin,
however, you get two traces with the plugin switch set to CHOP. So I am
thinking that there is an issue with some part of the logic in the Mainframe?
The plugins seem to work normally when they are inserted into a different
working Mainframe. I hope that I have not mixed up the various conditions
that I have observed.
Thanks to all!