Re: Tektronix 475


Tom Lee
 

Sounds like you nailed it -- great job! It's always nice to have a hangar queen to provide substitute parts for quick troubleshooting. High-voltage Zeners can suffer from unstable surface breakdown, leading to "crazy" I-V curves. Sounds like your Zener has not aged gracefully (then again, who among us does?).

Enjoy that 475. It's a fine scope!

--Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/1/2020 14:09, Michael W. Lynch via groups.io wrote:
On Thu, Oct 1, 2020 at 03:30 PM, Tom Lee wrote:

A not uncommon problem that causes the behavior you describe is cathode-grid1
leakage. If you preclude the usual external circuit causes, then it could very
well be a crt problem. If you still have the other crt, you could do a quick
swap back and see if the problem disappears.

There are ways to fix crts with this problem but let’s hold off on that for
now.
Tom,

Thanks for your kind reply. I may have answered my own question. After writing that post, I thought that I might actually be following or understanding Chuck's explanation and that I might have a bad 82V Zener or a bad capacitor. Since I had a parts scope (the one that the CRT came from) I went out and pulled C1373 and VR1374 from that unit and installed them into my "malfunctional" unit. I am testing now, but the issue seems to be resolved. In addition, I put the unknown or "Bad" VR1374 into my Type576 and 577 curve tracers. In both cases, the component would show a normal Zener curve, then suddenly break down into a crazy looking curve (looked almost like a tunnel diode curve). Not sure what was going on, however, both the 576 and 577 showed the same results.
Tom

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