Re: Another A5 board repair attemp - help needed


Siggi
 

On Fri, Nov 13, 2020 at 7:49 AM Rogerio O <rodd414@gmail.com> wrote:

Hi Siggi,
Although it seems it is not here were the problem is, I would like to
learn how to do it properly, so I posted this reply.
I have tackled a wire to pin19 of the DAC and hooked a P6109B to it - this
is CH1 of the scope
Than I am using a P6133 to probe the output pin of both MUXES, on CH2.
The two are 10X probes, right?
Yups.


The scope is a TDS2024B.
I could manage to adjust the trigger to view part of a complex signal with
about 2.5Vpp.
What I can't understand is that NONE of the signals present at the MUXES
outlets seems to vary with time and their voltages never get higher than
120mV.
Why is that?
I can't explain that, except perhaps you're not looking at the right part
of the DAC's output. The DAC is used for other purposes than just
refreshing the S&H analog voltages, notably it does potentiometer scanning.
Also note that the analog voltages only change with control or setting
changes on the scope. Droop aside, they should be pretty much a flat DC
level otherwise. Have you tried twiddling the front-panel controls while
looking at the MUX outputs? Maybe if you e.g. track down the A TRIG LVL
output on the MUX, then twiddle the trigger level control while you look at
the output. What you should see is a portion of the DAC output signal will
change with the pot, and you should see the MUX output track with those
changes.

What do you see if you flip this around and trigger on one of the MUX
outputs while you twiddle the controls?

I remember looking at this on the A5 board of my 2467, and I remember
seeing the probe visibly loading the holding capacitor, at least on some of
the outputs. I seem to remember the various outputs are refreshed
at different rates also, so some may droop more than others. I don't
remember the magnitude of the effect, though, it's been years since I did
this.
If you look at the PCB traces around the MUXes and op-amps (TL074s?),
you'll see that there are "guard traces" from the output of each op-amp,
encircling each MUX output and the S&H components. This is to minimize
leakage across the PCB (or say, due to dirt on the PCB), e.g. these are
high impedance points.

Have fun!
Siggi

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