Re: A 2465 teaser...

Chuck Harris
 

The glitch is the same regardless of the vertical or horizontal
positions. The attenuator and input AC/DC/50/GND switch makes no
difference.

The only thing that makes a difference is CH1/CH2 vs CH3/CH4. You
can even have all 4 traces on the screen, and CH1/CH2 have the
glitches, but CH3 and CH4 are clean and perfect.

When I look at the input to the delay line, I can see CH1 with its
hook, and CH4 with its perfectly straight trace. If display is on,
both show so much garbage that you can't tell what you are seeing.

The input to the delay line is the output from the Channel Switch
hybrid.

Because CH2 output on the back panel is clean, I think I can assume
the preamps are out of the problem area. Since the problem shows
clearly on the output of the Channel Switch, and the Channel Switch
is a known good hybrid, the problem has to be in the care and feeding
of the Channel Switch....Somewhere.

Unfortunately, there are no test points between the output of the
CH1/CH2 preamps and the Channel Switch inputs... they are all on the
underside of the board, and running the A1 board out of the chassis
is impractical.

-Chuck Harris





Tom Gardner tggzzz@... [TekScopes] wrote:

It would be interesting to know if the "glitch" amplitude depends
on the relative positions of the CH1/2 trace on the screen, with the
inputs grounded and with different DC inputs.

I would identify just about all the analogue switches (e.g. 4053s)
I could find, and see if any of those are being switched too slowly
and/or had too high a resistance and/or whatever was driving their
signals had become too high resistance. Maybe there's a mechanical
failure somewhere in that chain.

I would use another scope to find where the glitch /doesn't/ exist
- trigger on any control signal vaguely associated with the glitch,
potentially even the readout vert/horiz signals diagram 48 and 49
- work back from the Y output deflection voltage, through U600, U400
- then to U100/U200
- then control signals VS1/2/3/4 from U650, checking particularly
if there is any difference between VS1/2 and VS3/4

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