Re: A 2465 teaser...

Tom Gardner
 

On 10/11/17 06:37, Chuck Harris cfharris@... [TekScopes] wrote:

An additional clue is the leading hook's decay curve changes
slope with sweep speed, but the scope will not trigger on
this signal, nor on any of the random grass that appears on
the trace. That indicates that it is getting injected after
the trigger pickoff.
By that do you mean that when you change the timebase
the decay curve is the same time or the same number
of divisions?

I am pretty certain it has something to do with the display
readout logic.
That was the feeling I got from reading your description.

To see whether it is correlated with the display readout
timing, I would try either of two tests:
1) apply an external signal and adjust its frequency to be
the same as (a harmonic or subharmonic) the display
readout frequency so that any "twinkling" in the traces
is more-or-less stationary. (Start with a timebase of
~200us/div). Then see if the hooks are stationary w.r.t.
the intensity variations. If they are, then the display
readout is involved.
2) without an external signal and a ~2ms/div timebase,
change the holdoff until the twinkling is stationary,
and look for the hooks as above.

If display readout is involved, I would find out what is
happening when the display is switching from trace
to readout. I would observe the Y waveform and blanking
signals after the display sequencer IC and near the
display blanking IC. Trigger on either the blanking
waveform or the Y-waveform.

I would be looking for either the blanking signal to
be mistimed or the Y-waveform to be changing too slowly.
I would suspect the latter, so then it is a case of moving
"away from the output" until you find where it isn't
changing too slowly.

Be careful with probe tips around there; I know they
can short signals and destroy ICs :(


-Chuck Harris

edbreya@... [TekScopes] wrote:
You may want to check all readily accessible hardware mounting and
grounding, particularly around the delay line. If the hooks are constant, then
they must be leaking into the circuit past the front-end, and somehow
localized so some parts are not affected. The one thing that is commonest to
all sections and operations is the grounding. Check that all screws are
tightened up snug (and that none are missing), and jiggle things around to see
if the symptoms can be aggravated.

To see if you've got some kind of ground loops at the front, jump the
apparently unaffected CH3 and CH4 inputs to the CH1 and CH2 - it's easy to
just use BNC cables for this.

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