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!


Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 10/1/2020 14:09, Michael W. Lynch via 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

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.

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