Re: Tek 576 noise and looping


Hi Peter,
The big loops in one photo, which is showing a worst case scenario, are due to the Miller Effect which takes the very small capacitance (Ccb) between the collector and base and multiplies it by the gain of the transistor (Beta).
As a result the sweeping collector voltage (dV/dt) generates current loops (Ib) because a tiny capacitance is multiplied by a factor of 100 and shows up as loops in the base steps. Ib = Beta x Ccb x (dV/dt).

The solution is to shut off the collector sweep and switch to the DC position which supplies a fixed voltage to the collector. Now by manually sweeping the DC slowly from 0V to your maximum voltage you will eliminate the loops. Of course on a 576 all you see is a dot which, as you slowly turn the collector voltage up, the dot will move. With a grease pencil (or equivalent) mark the progress of the trace as you increase the collector voltage and you will get a sort of useful set of curves.

A MUCH SIMPLER SOLUTION is to get a 577-D1 (Storage version). All you have to do is turn the storage on and slowly increase the DC collector voltage and the screen stores the resulting curves without loops as you increase the DC collector current slowly. This is just one of the many things you can do with the 577-D1 storage that you can't do with any other curve tracer.

Dennis Tillman W7PF

-----Original Message-----
From: peter bunge
Sent: Monday, November 11, 2019 12:58 PM

I suspected that I could not attach photos.
Any one interested please see the file I uploaded with examples of looping on pages 5 and 6.
File is: Tektronix 576 Curve Tracer Repair Log 1.pdf
It also covers several repairs and my current attempt to find out about the noise.

On Mon, Nov 11, 2019 at 12:19 PM Mlynch001 <@mlynch001> wrote:

"I'm not sure if pictures show up, let me know.
[image: 2N3568 low current.jpg]"


Attachments are not allowed within our posts. However, you can post
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Michael Lynch
Dardanelle, AR

Dennis Tillman W7PF
TekScopes Moderator

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