Author Topic: Defective Tek 475 scope  (Read 724 times)

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Offline Yannick99

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Defective Tek 475 scope
« on: December 15, 2017, 02:46:05 am »
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 :)
« Last Edit: December 16, 2017, 08:59:11 am by Yannick99 »
 

Offline SaabFAN

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Re: Defective Tek 475 scope
« Reply #1 on: December 15, 2017, 08:28:32 am »
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!)

Online tggzzz

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Re: Defective Tek 475 scope
« Reply #2 on: December 15, 2017, 09:55:36 am »
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.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Offline Yannick99

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Re: Defective Tek 475 scope
« Reply #3 on: December 15, 2017, 11:19:56 am »
Quote
.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....


Quote
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
 

Offline Alex Eisenhut

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Re: Defective Tek 475 scope
« Reply #4 on: December 15, 2017, 02:48:55 pm »
Yes, lights off is a good tool!  :-+
 

Online bd139

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Re: Defective Tek 475 scope
« Reply #5 on: December 15, 2017, 05:53:59 pm »
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.
 

Online tggzzz

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Re: Defective Tek 475 scope
« Reply #6 on: December 15, 2017, 07:52:57 pm »
Quote
.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.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline Kilo Tango

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Re: Defective Tek 475 scope
« Reply #7 on: December 15, 2017, 08:33:18 pm »
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
 

Online bd139

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Re: Defective Tek 475 scope
« Reply #8 on: December 15, 2017, 08:34:07 pm »
It's an LEC (light emitting capacitor)  :-DD
 

Offline Kilo Tango

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Re: Defective Tek 475 scope
« Reply #9 on: December 15, 2017, 08:48:53 pm »
 :-DD :-DD :-DD

I should patent it quick, make a fortune !

Ken
 

Offline Yannick99

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Re: Defective Tek 475 scope
« Reply #10 on: December 16, 2017, 03:18:53 am »
Quote
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 :)

Quote
It's an LEC (light emitting capacitor)

lol  :-DD

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

 

Online bd139

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Re: Defective Tek 475 scope
« Reply #11 on: December 16, 2017, 03:56:11 am »
Good find - I would have never worked that out :)
 

Offline gnavigator1007

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Re: Defective Tek 475 scope
« Reply #12 on: December 16, 2017, 04:19:28 am »
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
 


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