Defective Tek 475 scope - Page 1
Forum section: electronics / repair

author: Yannick99
author_rank: Contributor
timestamp: December 14, 2017, 03:46:05 pm
content: 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/Anpsb9dRy4oThank you

author: SaabFAN
author_rank: Frequent Contributor
timestamp: December 14, 2017, 09:28:32 pm
content: 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!)

author: tggzzz
author_rank: Super Contributor
timestamp: December 14, 2017, 10:55:36 pm
content: Quote from: Yannick99 on December 14, 2017, 03:46:05 pmOften 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.

author: Yannick99
author_rank: Contributor
timestamp: December 15, 2017, 12:19:56 am
content: QuoteWas 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....QuoteYeah that's what it is... Tonight, turn off the lights and saw the sparkProblem was the mica isolator between the frame and transistor. The center one. A nice beautifull sparkReplace = okthank you

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 15, 2017, 03:48:55 am
content: Yes, lights off is a good tool!

author: bd139
author_rank: Super Contributor
timestamp: December 15, 2017, 06:53:59 am
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: December 15, 2017, 08:52:57 am
content: Quote from: Yannick99 on December 15, 2017, 12:19:56 amI 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.

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 15, 2017, 09:33:18 am
content: 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 enoughKen

author: bd139
author_rank: Super Contributor
timestamp: December 15, 2017, 09:34:07 am
content: It's an LEC (light emitting capacitor)

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 15, 2017, 09:48:53 am
content: I should patent it quick, make a fortune !Ken

author: Yannick99
author_rank: Contributor
timestamp: December 15, 2017, 04:18:53 pm
content: Quoteah ok i understand nowQuotelolThank for all your advice but it's fixed now.... Problem was a mica isolator between transistor (middle one) and frame. Replace only the isolator

author: bd139
author_rank: Super Contributor
timestamp: December 15, 2017, 04:56:11 pm
content: Good find - I would have never worked that out

author: gnavigator1007
author_rank: Unknown
timestamp: December 15, 2017, 05:19:28 pm
content: 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