Another Tek 465 repair question

[NickR]

Hello everyone,

I decided not to go to the Testing Equipment forum with my likely to be trivial question, and searching didn't yield any related result.
I have recently bought a Tektronix 465 (not B) oscilloscope. It was in a rather good condition, written-off military equipment (last calibration sticker is from 2004). Power cord was missing, so I made a new one. Probes were missing too and I bought a pair of Hantek P6100 100MHz ones.
After setting up the capacitance of probes and stuff, I was pleased to see the traces nice and clean. However, there are slight (like 1% of amplitude) overshoots still visible from calibrator output.

Other issue concerns me a bit more. When I move up the vertical position and trace crosses 100% mark, it becomes very fuzzy and somehow gets reflected down a bit (like to a 60% value). After that I can move that fuzzy trace all the way up. If I move it back down, it becomes clear again once the top of the signal crosses 100% mark.

What could it be? CRT failure or something with the electronics? I have already got the service manual, but any advices on where to start looking are much appreciated.

[james_s]

Sounds like one or more of the glass support structures in the CRT neck have been broken, does it change if you turn it upside down? Don't write off the tube until you're certain but I can't think of anything else that would cause that effect.

[MarkL]

You could try reversing the wires going to the vertical deflection plates on the CRT.

The traces will now be upside down, but if the problem stays near the top at the 100% line as you describe,  the CRT is bad.  If the problem moves to the bottom, you have a circuit issue somewhere.

[NickR]

Hi guys and thanks for the suggestions.
After I left the scope working for 2 hours, its behaviour has changed a bit. Now it allows me to go little higher than 100% mark on channel A and artifacts are not as severe as before. I tried to rotate the scope as James suggested, but image didn't change at all. Also tried to tilt it gently, no effect on image whatsoever. So hopefully the CRT is intact.

I haven't tried reversing CRT vertical deflection polarity yet, waiting for HV caps to discharge.

[james_s]

It should be discharged within a few minutes, and you don't have to mess with the HV to get to the deflection pins, they're on the side of the CRT neck.