Defective Tek 475 scope

[Yannick99]

Hi everyone,

Reparing my old tek scope, one old electrolytic capacitor was short. So i change all 6, replace the fuse and reinstall everything in place. Got a trace, was looking fine and bang weird noise and fuse blow again.
On the video, with a variac, i rise the voltage slowly and you will see the scope is working. But at the end, they are a short circuit somewhere. Looks like high voltage. I tried same test without the crt. Same noise happen... Any idea ?

https://youtu.be/Anpsb9dRy4o

Thank you

[SaabFAN]

Sounds like there's sparking inside the scope. If you haven't been able to locate it by listening, you should be able to see burns on one of the boards.

It could also be a breakdown in the isolation of the cable that feeds the acceleration-voltage (the big black rubber cap on the CRT - BE REALLY CAREFUL AROUND THAT THING! Even with the scope turned Off, the capacitors can still hold lethal amounts of power!)

[tggzzz]

with a variac, i rise the voltage sloly

Often that's not a good idea.

In an operating circuit the power consumption is constant. Therefore, if you have unusually low input voltage, the input current will be unusually high. High current can cause failures.

N.B. I haven't watched your video on principle: my remaining life is too short.

[Yannick99]

.B. I haven't watched your video on principle: my remaining life is too short.

Was 35 secondes the video. You spend more time to tell me you haven't whatched it. Just no answer me at all next time. Will be fine. You life is short but you have 5920 post here....

Sounds like there's sparking inside the scope.

Yeah that's what it is... Tonight, turn off the lights and saw the spark Problem was the mica isolator between the frame and transistor. The center one. A nice beautifull spark Replace = ok

thank you

[Alex Eisenhut]

Yes, lights off is a good tool! 

[bd139]

That sounds unhealthy.

Might be an easy fix. I had a 465A that did this a couple of years ago and one of the multiplier capacitors under the HT shield had a big crack in it. If it’s the main multiplier box, it’s quite a bit more difficult. They can be rebuilt however.

Be very careful though. Lots of nasty surprises hiding in there. Discharge everything after every time you power it up.

If it’s blowing fuses though, that indicates there are potentially a couple of problems. Also that trace wasn’t healthy. Looks like rectifier has gone / filter cap on the way out.

Variac should be fine with these. They actually specify using one to do the line voltage warning indicator and regulation checks. Haven’t bothered to get mine out on a tek scope as I’m too lazy and it’s too heavy.

[tggzzz]

.B. I haven't watched your video on principle: my remaining life is too short.

Was 35 secondes the video. You spend more time to tell me you haven't whatched it. Just no answer me at all next time. Will be fine. You life is short but you have 5920 post here....

I have scripts turned off to reduce download time, reduce annoying adverts, so I don't even see the still picture unless I want to.

There is a high probability that posting here is a better use of my time than watching most videos.

[Kilo Tango]

have a look under the metal cover on the bottom, at the back. It may look like this.

If so its one of the smoothing caps on one of the high volt rails. You can get replacements easy enough

Ken

[bd139]

It's an LEC (light emitting capacitor) 

[Kilo Tango]

 

I should patent it quick, make a fortune !

Ken

[Yannick99]

I have scripts turned off to reduce download time, reduce annoying adverts, so I don't even see the still picture unless I want to.

ah ok i understand now

It's an LEC (light emitting capacitor)

lol 

Thank for all your advice but it's fixed now.... Problem was a mica isolator between transistor (middle one) and frame. Replace only the isolator

[bd139]

Good find - I would have never worked that out

[gnavigator1007]

Congrats on the successful repair! The title really made me laugh. Something about equipment of this age being called defective as opposed to broken struck me funny