Re: Tek 576 noise and looping

peter bunge

No, I have not been working on the 576 curve tracer for 8 years but i have
been curious about the looping for at least that long.
Thanks for your explanation which I tested by comparing curves with slow
and fast transistors. I had some trouble trying to find a couple with
matching gain but the high freq (900 MHz) transistor performed perfectly
with nice curves at low current. On the other hand the slow transistor had
huge loops. I would like to concentrate on the noise problem next. It mat
be finger trouble.
I also tried the DC method but find the dots turn into lines in a "noisy
dot, small line, big line" repeating sequence as I rotate the VERTICAL
control CW. At least that is what I remember. It was totally unexpected and
I intend to go back and investigate it. I was trying to follow one line of
thought at the time.
I am stuck at Step 16, Performance Check/Calibration, Section 5, in the
manual. This step is the "Check Horizontal and Vertical Displayed Noise".
The manual goes all the way from Step 8, where the Initial Settings are
used, with numerous setting changes to Step 16 and it is easy to get them
wrong. I have printed a list of all the setting changes to step 16 and find
that some are messed with but never reset to a specific value (Var
Collector Supply and Step Gen Amplitude for example.
Tektronix really should have listed all settings prior to Step 16 and I
suspect following the manual may be wrong and causing my inability to get
within specs. It does not help that the manual does not tell me what they
are trying to accomplish, or where to look for the problem either.
Does anyone know what the intention of Step 16 is and the correct settings
prior to the tests?
I will send my Setting Notes separately to you and anyone else interested
(contact me off line).
Regards, Peter

On Tue, Nov 12, 2019 at 7:38 PM Kevin Oconnor <@KO3Y> wrote:

Hey Peter,
Have you really been working on this for 8 years? That’s dedication.

I apologize, after looking at your pics I realized you are working on 576,
and I have a 577. The 577 is much simpler in some regards. But the analog
display information is essentially identical. However, as Dennis indicated,
the looping is in all likelihood accurate. Miller effects and hysteresis
along with wire loops can be the cause. Your 36” leads are going to be a
nightmare at low current. Your modest current curves are the best I think
you can expect.


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