Recapping Tek 465M Scope

[ziplock]

Greetings my fellow techies. Some of you may be familiar with this old workhorse.

I picked up this broken scope for next to nothing many months ago. Only now have I had the time to look at it on the bench. It was not drawing a vertical trace and the sweep was noisy.

Starting with the power supply, I noticed the -5V DC rail dropping down to -3.5V and the ripple was some 800mV! I suspected filter capacitor(s). Yup! Cap directly after the rectifier showing ESR greater than 20Ohm. According to the service manual, there are 11 electrolytic capacitors on the main board / PSU. I check the remainder and of course all but 2 of them test open, shorted, or have high ESR. I guess after 35 years they have finally given up. Time for a recap!

I'm slightly concerned about finding replacements for the main filter caps of the PSU. They consist of two 18000UF/15V 5 pin caps and one 1000UF/75V 4 pin cap. It appears the extra pins are really just neg/common tied together around the perimeter of the can. If I use standard 2 pin caps and tie all the ground traces together with jumper wire, will this be a problem? I figure it should work just fine.

Hopefully making these repairs to the mainboard will put this scope back into operation. Hopefully nothing else was fried when all these caps went out. We'll see!

~Z

 

[Pars]

I would think that would be alright. You might check either on usenet (sci.electronics forums, I forget the test equipment forum), or perhaps check with someone like Tom Gootee. Scroll down to his post in this archived thread from diyaudio for some good resources:

diyAudio Forums Archive - Tektronix 2235 problem

 

[SiBurning]

Haven't got a 465, and the manual doesn't seem to have the power supply details. Don't just try it out and see. Find out for sure what the circuit needs.

Several kinds of 5 pin capacitor-like things exist(ed). There are single, dual, and quad capacitors, as well as t-networks with 5 pins. A typical connection is two pins for each side, and a fifth unconnected pin just so you can't install it backwards. Some single capacitors have 3 extra unconnected pins attached to the inside or the metal case of the capacitor just for compatibility, stability, and heat transfer. The compatibility is with single and dual capacitors that electrically connect 4 pins. Some of the duals let you choose to wire the capacitors in series or parallel, and others didn't give you the choice. There's all kinds of other ways these packages have been configured. Some of these packages aren't just capacitors, but incorporate other circuitry. The t-network is an example.

It's entirely possible, even probable, that a straight capacitor will do, and they chose a fancy package just because it packs more capacitance by volume than a pair. But if that's not true, you probably don't want to find out the hard way.

 

[ziplock]

I went ahead and used standard capacitors for the filter caps and wired them up. All the other electrolytics were a no brainer. She works like a charm now! I'm very pleased.

 

[SiBurning]

Nice. Wish there were real schematics for these things. Never found one. The service manuals just list part numbers. Mine's a 475. Enjoy.

 

[Pars]

I got a manual with schematics here for my 465b. The manuals for the 475 (or the 465m) don't have schematics here?

http://bama.edebris.com/manuals/tek/

 

[ziplock]

The diagrams section was not scanned in for the 465M. Schematics too large? Military thing? Laziness?

Luckily the interface board circuits are pretty straight forward to figure out.

I'll probably buy a hard copy from ebay one of these days..

 

[SiBurning]

Oh... Nevermind.

I have those manuals and didn't see the schematics. Only saw part numbers, not values. Must be more tired that I thought. Might've just missed that one, since there's like 7 different documents for the 475.