A 2465 teaser...

Chuck Harris
 

Hi Gang,

I have a 2465 that spends most of its time on my shelf
as a "should fix this soon" hanger queen.

The problem is thus far, I can't figure out what's wrong
with it.

If you are showing simple ground traces, CH1 and CH2 both
have an identical 0.2 x 0.2 division upward facing "hook" on
the left end of the trace. It moves with the vertical position
control, but is completely unaffected by settings of the
attenuator, and does *not* appear on the CH2 BNC on the rear
panel of the scope... nor would I expect it to, preamps no
nothing of the sweep.

CH3 and CH4 show no sign of the hook. They are perfect.

If you turn on the readout, the hook remains, but there are
3/4 division whispy little hook traces dancing randomly along
the CH1 or CH2 trace. Lots of them. Nothing on the CH3 or CH4
traces. Horizontal rate affects how they are displayed, in the
way you would expect.

My first thought was something that CH1 and CH2 have in common,
the CH3 and CH4 do not.

CH1 and CH2 share some filtered power supply voltages, and have
some power supply voltages that CH3/CH4 do not.

CH1 and CH2 share a couple of opamps in the trigger pickoff
circuitry.

CH1 and CH2 pass through the channel switch through different
channels than CH3 and CH4, and the display readout.

The power supply has been recapped, and tested in another 2465,
that doesn't show the problem.

The Channel switch has been swapped with a 2465 that doesn't
show the problem. All the circuits connections and components
around the channel switch were tested for continuity, shorts,
value...

The Display Sequencer has been swapped with a 2465 that doesn't
show the problem... not that I thought it would make any difference.

The Z axis controller has been swapped with a 2465 that doesn't
show the problem... not that I thought it would make any difference.

All electrolytics on the A1 board have been replaced. What can I
say, the scope is gorgeous, I thought someone might appreciate it.

The noise doesn't appear on the power supplies, or the power rails
at the various hybrids.

All of the published waveforms in the vertical section match the
test points (except for a couple which are known to be wrong in
the manual).

All of the resistors and pots around the channel switch have been
tested. I even pulled U475 and replaced it (because I got a
strange junction measurement that turned out to be ok.

I am puzzled. Anyone have any ideas?

-Chuck Harris

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