Re: How to troubleshoot a faulty 7B92A?

Dan G

On Thu, Mar 18, 2021 at 07:03 PM, Thierry Delaitre wrote:

The jumper for mainframe selector is on P835 for "other 7000"
This should be correct for your scope. P834/P835 affect the hold-off
timing at fast sweep rates: approximately 1.2 us when set to "7800/7900",
and 2.2 us when set to "other".

I am not sure if the jumper for time/div variable selector should be on P760
(delayed) or P761 (delaying) for the 7704A ?
The choice of delayed or delaying variable selector is unrelated to the mainframe.
I think most users keep this in the "delayed" position so that the variable
control is available in "normal" mode (i.e. when the delaying and delayed
selectors are locked together).

There is nothing showing on the screen when pressing the beam finder button

When the 7B92A module is inserted into the scope, the trace displayed by the
other timebase is vibrating a bit and it can be observed that the position of
the readout is different for the other timebase/input.

Do you have any recommendations on the approach for troubleshooting a 7B92A?
When the time base plug-in is operated in a horizontal slot, the "sweep gate"
signal, the differential positive-going ramp signal, and a host of other control
signals all have to be ok in order to produce a useful display.

If you are reasonably confident that the 7B92A isn't putting out signals that are
so far out that they may damage whatever mainframe slot you plug it into, then
I would suggest the following tests:

- Install the 7B92A in the Left Vertical slot.
- Set the 7B92A to NORM, AC, INT triggering.
- Lock the delaying and delayed selectors on the 7B92A together and
set them to 1 ms/div.
- Install your other (working) time base in a horizontal slot, and set it to
1 ms/div, AUTO, AC, INT triggering.

With this setup, you should be able to get a horizontal line on the screen,
whose vertical position can be set by 7B92A's POSITION control.
(You will need to move it towards the clock-wise end of the range in
order to get the line on the screen.)

If no setting of 7B92A's POSITION will give you a visible trace, then
push the BEAM FINDER to see where it is.

If you do see a trace that you can control with 7B92A's POSITION, then
change the 7B92A triggering to AUTO. Now you should see
diagonal lines on the screen.

The steps described above will tell us whether the 7B92A is able
to drive the differential output signal at all, and will side-step any
potential problems with the Sweep Gate and other control signals.


Join to automatically receive all group messages.