Re: 7904A readout bounce


On Tue, Dec 19, 2017 at 5:19 PM, NigelP <nigel-pritchard@...> wrote:
So, further investigation demonstrates more of what I've already noted. Scenario: 7904A
loaded with 2x 7A26, 7B92A, 7D15 and ALT set on both vertical PIs and mainframe set to
ALT...... if I overlay all four traces on top of each other there is NO bouncing of the readout .
As you increment any trace position towards top/bottom of the screen the bounce gets
increasingly larger reaching around 0.1 major division (therefore readout bounce magnitude
is a direct function of trace vertical position). Sweep speed in the region of 50mS/Div +/-
some shows up nicely, but any variation of sweep speed simply varies the level of visual
annoyance. Any combination of ALT setting produces the same result.
I've just tried this on my 7904A and I also get some readout bounce,
but very little. If I position two traces in ALT mode, one at the top
and one at the bottom of the screen, so they're only just not visible,
the readout bounces a very tiny amount (timebase at 10ms/div), much
less than 0.05 divisions. You really have to look carefully to see it.
If I crank the Y position knobs to their maximum possible extents, I
can get the bounce up to about 0.1 divisions, but that's the extreme.

This 7904A hasn't been refurbished other than a slight tweak to the Y
gain and to one of the thermal compensation adjustments. I use it in
my work almost every day and the readings it gives appear to be
accurate when compared with other instruments I use. It doesn't have
traceable calibration but I'd say it was within spec.


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