Tektronix 475 Oscilloscope channel 2 repair - Page 1
Forum section: electronics / repair

author: silverlego3
author_rank: Contributor
timestamp: February 17, 2015, 10:41:28 pm
content: Hello!So I got my first oscilloscope, a nice Tektronix 475 from my father. The only thing wrong with it, is someone banged the channel 2 knob really well so it broke the plastic and bent the rod that turns the cam inside.I have been folowing EEVblog for quite a few years on youtube and figured people might be interested at taking a look inside this old scope as I try and repair channel 2, and hopefully make it easier for someone else with the same scope to repair. I didnt get too many pictures of me taking it apart but I'll take more as I put it back together.for now here is a picture of the bottom of the scope.

author: Redcat
author_rank: Regular Contributor
timestamp: March 08, 2015, 07:22:57 pm
content: Hello Silverlego,the Tek 475 is for sure a nice old scope. I havn't used mine for a while but i once changed some lightbulbs to LEDs and tweaked the calibration testsignal a bit...Would be nice to see you trying to repair yours. Depending on what rod is broken or bent, it shouldn't be that hard, u can use Channel 1 as a reference...The 475 is well documented and you can easily get a repair manual.Good luck & have fun in repairing and using this scope, I'm looking forward for your repair updates.

author: tombi
author_rank: Regular Contributor
timestamp: March 09, 2015, 06:07:14 am
content: I too spent a good bit of time fixing a very broken 475. I thoroughly enjoyed it and learned a lot!You can get parts from a bunch of places - these guys are a good starthttp://www.sphere.bc.ca/test/tek-parts/tek-knobs.htmlAlso checkout the Tektronix yahoo group. They are great for advice and assistance.Hopefully it is just the fibreglass rod and the knob. On mine the gold switches that switch in and out the attenuators were dicky (still are slightly) and I had to clean them with isopropyl alcohol. The effect is that when you switch ranges you sometimes have to lightly tap the knob to get it to 'settle' onto the right range.I notice that the -8v rectifier on yours has been replaced (based on the way it hangs out of the board - it's easier to solder onto topside).Tom

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: May 03, 2015, 05:15:03 am
content: Awesome to see people repairing these awesome pieces of vintage Tek gear! I'm somewhat new to the field of electrical engineering/"small" electronics myself and I have already decided I am going to collect/repair/rebuild vintage analog gear - mostly Tek stuff. Anyway..The first scope I got was a 465 on a trade for dirt cheap. It didn't work properly - at all - at first, but after taking it apart and cleaning it multiple times and a couple other small adjustments, I was able to get it working perfectly. My most recent purchase was a 468 that "couldn't read a signal". Same deal there - needed a real cleaning. I had to clean the drums and finger switch contacts multiple times to get it working well again ( on both scopes ) - but managed to get them squeaky clean with beautiful traces.Tombi - This may help you -and others- clean the drum switches entirely and get it back to functioning beautifully again: Using DeOxit D5, Isopropyl Alcohol 91%, Camel hair (cheap artist) paint brushes, and a spray bottle:First blasted the drums and finger switches with the DeOxit to loosen up the film/oxidation. Next took the camel hair brush and, keeping it very wet by dipping it in alcohol to soak/rinse, I really brushed the drum - repeatedly - (changing the alcohol as it filled with dirt). After each once-over I hosed it down with the spray bottle of alcohol.For the finger switches - take a small camel hair brush, and using the same soak/rinse method with the brush, rotate the drum to get each switch opened up and gently slide the bristles between the fingers and board. If you feel it to be necessary, (and brush is thin enough) you can close the switch on the bristles and gently pull the brush out. I would only do this if the regular method failed after a few good cleaning cycles.silverlego3 - if you aren't able to bend the rod back gently until you can get it functioning again, you will easily be able to find a replacement attenuator assembly from ebay or one of several parts sites. Definitely shop around a good bit, though, since lots of people like to charge ridiculous prices for parts. Definitely salvage as much as you can, and keep whatever parts you replace. They may be useful in the future! There are loads of people that are far more educated with this stuff than myself, but I don't mind helping out with as much as I can.

author: tautech
author_rank: Super Contributor
timestamp: May 03, 2015, 07:10:51 am
content: Welcome to the forum Addicted2AnalogTek.Thanks for sharing your techniques.Look forward to you posting your repair/restorations.Here's a thread you may find useful:https://www.eevblog.com/forum/repair/vintageclassic-renovation-techniques/

author: PaulAm
author_rank: Frequent Contributor
timestamp: May 03, 2015, 01:14:21 pm
content: The recommended Tek procedure for cleaning those finger contacts is to soak a small strip of bond paper in 91% isopropyl, slide it under the open contact, close it and slowly draw it out. Do that 2 or 3 times carefully.Getting the scope apart to the point you can do that can be a major pain.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: May 03, 2015, 01:53:40 pm
content: Thank you tautech!I need to get a decent vid cam & tripod so I can actually make vids of my projects and use both hands at the same time! Problem is... when I have money to spare I run right to searching for more gear!! hahaha I need to get more accessories, but I just get giddy like a little girl when I find another analog Tek scope that I'd like to buy/refurb. Thanks for the link - I'm sure all the info and people here at EEV will be a major help in my restoration endeavors.PaulAM - Yes, I remember hearing that from ModemHead's vids of the 465 restoration. It's not in my manual for some reason though. My 465 manual says " clean switches using a camel hair brush and Isopropyl Alcohol", so that's exactly what I did. Also it was a lot easier to get the brush tip into the places I needed it to be than it would be for me to get paper and tweezers in there. I just had to watch out for the random hair that comes loose from the brush. I can def see how the paper method would be more proper, and probably more effective for thicker/harder oxidation.I had much practice taking apart my 465, and you're right, it can be a bit of a pain. The delay line gave me the most grief, oddly enough. This 468 appears to be easier, since it's got connectors in place of solder joints. However, most of my transistors and ICs are actually soldered in instead of socketed. I think this is more good than bad, though. Good that the connection can't oxidize or get loose, but a little more tough on the repair person. I just need to find the paper manuals for a deal now..I posted some info regarding the small issues I'm having with the 468 on this other thread:https://www.eevblog.com/forum/repair/tektronix-475-oscilloscope-channel-2-repair/