Re: New member with a currently-dead 2465


Siggi
 

On Sun, Jul 12, 2020 at 6:21 PM Vincent Mallet <vmallet@...> wrote:

I removed U2556 and replaced it with a new one from TI that I had around
and ta-da! It lives! Super exciting!

Congrats!

I had a couple of questions on the oscillating circuitry:

1/ R2553 is a 10K tied to -5V. What does that do? Why do we need to bring
-5V into the picture?
For the same reason that there's a 680Ohm pullup to the +5 I suspect, e.g.
to bias the inverter just so, to make sure it oscillates. That pullup is
particularly interesting, as it's on the driven output of a gate. As you
found, the circuit seems to be on a knife edge.


2/ Figure 3831-50 “Test waveform setup information” for A5 shows 3
waveforms, including “1” taken at U2556-4 and “2” taken at U2468-5. Both
waveforms seem to be centered around 0V with a +/- 2V swing around 0. How
do I interpret this? Why negative? If I probe U2556-4 (now that it lives) I
get a waveform that looks like what’s in the manual but it swings from 0V
to ~3.6V. Is it because I am supposed to be AC-coupled and ignore the
offset?

That seems weird. There's no way you see a TTL signal symmetrical about
ground so I guess those oscillograms were recorded by the intern?


Now I need to find some probes for this scope so I can try to quantify its
performance. The only probes I have are the ones bundled with my Siglent
DSO ("PP215 200MHz") and I don't know how good or bad they are.
If you read through the "performance verification" portion of the service
manual you'll see no mention of using probes for the verification nor for
the calibration. To verify or calibrate this scope you input the set of
signals specified in the "performance verification" portion of the service
manual and then interpret the results on the CRT. These signals are
generally hooked up with good quality coax cables.

See e.g. this video (https://www.youtube.com/watch?v=oxJQr4dKnys) and this
one (https://www.youtube.com/watch?v=OiAmER1OJh4) for some of the whats and
hows of probes.
If you can compensate your Siglent probes with the 2465, they'll probably
work OK to the limits of their frequency spec. If you can't compensate
them, you'll need probes with a suitable compensation range, but depending
on what you expect to look at, they don't need to match the scope's
bandwidth.

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