Re: A 2465 teaser...
I'm coming to think it is the power circuitry around thetoggle quoted messageShow quoted text
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
I will check that soon and report back.
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.