Re: A 2465 teaser...

Chuck Harris

I'm coming to think it is the power circuitry around the
CH1 and CH2 preamps.

I soldered a wire onto one of the pads where the delay line,
attaches to the PCB, and carefully adjusted the hold off,
on my bench scope, so that I got a clean picture of that
output... which is the direct output of the Channel Switch.

I connected the CH1 Gate signal from the back panel to my
bench scope's CH2, and used it for the trigger. The CH2
display of the gate gives me a clear picture of the beginning
and end of each channel's time slot.

I can easily see each of the channels as it outputs from
the Channel Switch in succession. Hook on CH1 and CH2 and
everything. OBTW, that Hook is the clue, as it is *always*
there on CH1 and CH2.

If you look at the power distribution circuitry around the
preamps, you will see that CH3/CH4 use only +5V and -5V, and
share common RF isolation filter networks to clean up their
sources of power.

CH1/CH2 share a common RF isolation filter networks on +5V,
and common RF isolation filter networks on -5V and -8V.

Ok, why is the hook the clue?

Simple, the hook is the vertical preamplifier's way of saying
that it has squishy power, and doesn't like the channel switch
turning its load on and off when CH1 or CH2 is selected.

When the channel switch connects to CH1 or CH2, it is pulling
the quiescent output level of the preamp towards the load
potential of the channel switch... Oh so slowly. Forming the Hook.

It could also be a bad output amplifier on both CH1 and CH2, but
what are the odds of that happening simultaneously?

I think it is more likely that one of the power supply isolation
RF filters that CH1 and CH2 share is squishy, and I will find
one supply has a nice healthy 5us pulse on it. A mirror image
of the hook that shows on the leading edge (left) of the CH1 and
CH2 traces.

I will check that soon and report back.

-Chuck Harris

machine guy @Mac [TekScopes] wrote:

Chuck, I think its from the readout. Most likely the readout DO circuitry. Probably in U485, U475, or U975. There is a complicated switch using emitter coupled logic implemented by U485, U475, and U975. If this switch and logic fails, readout is coupled to the vertical channels of the channel switch. I have never tested to see if it affects only Ch1 and Ch2. But I have seen it. Check waveforms 50 and 51 on U975.

On ‎Thursday‎, ‎November‎ ‎9‎, ‎2017‎ ‎04‎:‎51‎:‎43‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

A hook is a trace that goes from vertical to horizontal
with a little curve between. If you could see the whole
thing, it would look like a shark's tooth, or a lazy
saw tooth pulse.

With display off, it looks like this:




When you turn display on it looks like:




The "(" sections are actually a sharp rise with
a slower fall, and look like random grass. They
appear only on CH1 and CH2, not at all on CH3 and CH4.

The "(" sections after the trigger are at least
0.75 div high, and unaffected by vertical input
condition, or attenuator setting. The initial pulse
hook, is undoubtedly just one of the "herd"... only
it appears with or without the display being turned

I haven't tried replacing the CH1 and CH2 preamp
yet. It would surprise me to find that both failed
in exactly the same way at the exactly the same time.

[Note that CH1 and CH2 are otherwise performing normally,
and in calibration, and that the "(" stuff does not
appear in the CH2 output connector on the back panel.
I would think that would rule out the preamps.]

It would seem more likely to me that it would be
a bad bypass cap on a filter section that feeds both
CH1 and CH2 in parallel. But the power looks pretty
clean, and nothing larger than 1 or 2 mv.

-Chuck Harris

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