looking for advise on restoring a 60 year old tektronix scope - Page 1
Forum section: electronics / repair

author: crazyhog
author_rank: Regular Contributor
timestamp: December 19, 2016, 02:22:20 am
content: Hi All,I got an old Tek Type 502 Dual-Beam Oscilloscope for $65; or about $1 per year! I was excited until I got it home and took a look inside. It was very dirty; cobwebs and caked on dust all over. Most likely it has been sitting in storage for decades and everyone has long forgotten about it. Luckily, the weather in San Francisco bay area is pretty easy on equipment.http://w140.com/tekwiki/wiki/502I am new at electronics and I am worried that I am in over my head! Before I invest a lot of time and effort to restore this, any advise on how to quickly assess if the machine is worth repairing?For example, how do I determine if the CRT is still working? I'd imagine if the CRT is toasted, I should toss in the towel. Note, given the machine's current condition (for example, there is a bunch of paper caps that needed replacing), I don't want to power up the whole shebang. Also I don't want to recap until I know the CRT isn't dead.Given how large this machine is and how dirty it is, I am also considering hosing the machine down and wash it with soap water. I'll then let it air dry for 2 weeks. Is that safe? What components are susceptible to water damage? I can deal with oxidation from water with some contact cleaner. However, I am not sure if there are other concerns.Any other parts in these old scopes that are hard to repair? Your advise is much appreciated.I've clean up the machine a bit with a vacuum cleaner and toothbrush. That did wonders and I now feel a bit better about the machine's condition. I've attached some pics. At least from these photos, I think I have a shoot.

author: crazyhog
author_rank: Regular Contributor
timestamp: December 19, 2016, 02:27:45 am
content: The top and bottom pics attached.I really love the look of these tube based machines.

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2016, 02:30:11 am
content: There's an expert or three members on old Teks like yours and I'd strongly recommend you place a link to your thread in this thread (if you know what I mean)https://www.eevblog.com/forum/testgear/vintage-tek-restoration-pictures-by-martin/Further helpful info is in this thread:https://www.eevblog.com/forum/repair/vintageclassic-renovation-techniques/

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 19, 2016, 03:23:45 am
content: Beautiful. It really doesn't look too bad. (I have a RM503 that I've restored and I love it.)Absolutely do _not_ wash it with soap and water! Clean the switch contacts as recommended in the Service Manual. You can safely remove stickers and tape residue from the front panel using a product called "Goo-Gone".A Variac is a good tool to have, so you can ramp up power slowly and stop if it draws too much current (check the mains fuse rating) or the first sign of magic smoke, or odd crackling sounds etc.Now... I was about to mention a potential problem with the big honking mains transformer... and what do you know, take a look. The CRT filament supply is elevated to HV and this can leak back to other windings in the big transformer. And looky here.... I can't tell offhand if this extra transformer is fixing that particular problem but it is definitely something you should look at. I had to fix my RM503 by installing a separate transformer for the CRT filament.

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2016, 03:49:32 am
content: Quote from: alsetalokin4017 on December 19, 2016, 03:23:45 amExcellent spotting.Yes, the OP will need to work through the wiring diagrams to check that out.

author: JuiceKing
author_rank: Regular Contributor
timestamp: December 19, 2016, 09:30:47 am
content: I believe that the transformers are a weak point in these scopes, so test yours before investing too much time and money. I have a 502 from a flea market with this problem, and although there are places you can get a transformer rewound, it's hardly worth it.

author: tggzzz
author_rank: Super Contributor
timestamp: December 19, 2016, 11:29:14 am
content: The Yahoo group TekScopes has a lot of good information in the archive, plus very knowledgable denizens.They will appreciate it if you can indicate what "homework" research you have done before asking a question.

author: crazyhog
author_rank: Regular Contributor
timestamp: December 19, 2016, 11:48:59 pm
content: Thanks all for your suggestions.alsetalokin4017, thanks for pointing out the transformer issue. That transformer you circled is definitely a repair job whacked in there by the previous owner. I'll thoroughly check out the transformers when I restore this thing.I am a newbie and for my understanding, can you clarify what you mean by the follow:"The CRT filament supply is elevated to HV and this can leak back to other windings in the big transformer."In particular, how does this happen? Is it perhaps some secondary winding are shorted out (maybe bad insulation) and cause a change to the transformer ratio? What other transformer failure modes do you have in mind that can cause the HV output to leak back to other windings?For what its worth, I've always thought these beefy looking old Tek transformers really looks business! Much more so then the puny transformers in modern day stuff. Bummer that these old transformers are more fragile then they look!Thanks again.

author: tautech
author_rank: Super Contributor
timestamp: December 20, 2016, 05:11:06 am
content: The cathode in the CRT is often at same potential as the heater filament and depending on the EHT used it could be up to minus ~3 kV depending on the scope. Study of the schematics will confirm if the cathode and heater is commoned, look for a link.