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wjcoll 2nd March 2014 07:06 AM

Dead Tektronix 2235A
Hello everyone.

I've only just joined, so this is my first post and typically I'm looking for help.

I'm an absolute newbie when it comes to the use of oscilloscopes. I'm a life-long self-learner and thought it was time to educate myself. Have some basic knowledge, but not a lot.

Anyway, I did what many before me have done and bought a used scope off eBay for not too much. Did some homework and waited until I scored a Tektronix 2235A. The seller said that apart from powering up, he hadn't done any tests. Caveat Emptor!

Scope arrived the other day and it did power up and looked like it might be a winner. Then, about 15 minutes later it blew the main fuse, a 1.25A 240V. I'm in Australia (240v here). I let things cool and tried another fuse; no good, blew again. Only had a 1.5A left, so tried that.....bang.

Doing some homework I've found that they're susceptible to power supply problems particularly if they've been sitting around unused for a long time, like I suspect this one has.

I've sourced the manuals (including service and schematics) but need some advice on where to begin and if there's a common easy fix.

Any help would be greatly appreciated. As I said, I'm a newb' so simple explanations in words no more than two syllables would help.:D

Wayne (wjcoll)

stoN_Cold 2nd March 2014 07:20 AM

1 Attachment(s)
Somethig wrong with PSU.

SCH in att.

Salas 2nd March 2014 07:59 AM

Welcome to the forums. Also look for blown X type mains rated cap around the power switch / mains entry area. They are usually blocky plastic with several safety standards printed on them.

wjcoll 2nd March 2014 08:01 AM

Thanks zeonrider and Salas. Off to a good start!

wjcoll 3rd March 2014 07:28 AM

Hi again guys.

Salas, thanks very much. You were right on the money. When digging a little deeper I found a 0.068uF X2 cap which had totally died. I also found a blown thermal resistor as well.

Have ordered the parts from Mouser and we'll see how we go.

Thanks for the help.


Salas 3rd March 2014 12:47 PM


tomchr 3rd March 2014 06:24 PM

If the 2235 is anything like the 2465B that I have, you may look into changing the electrolytics in the switchmode supply as well. In my 2465B, all the 'lytics were dried out. Some of them wet themselves and made a mess on the PCB.


gmphadte 4th March 2014 03:22 AM

Capacitors in 2235 have a sealing compund on the (rubber)cap.

Gajanan Phadte

dirkwright 4th March 2014 12:43 PM

Yeah, you did good so far. A simple but careful visual inspection of all the parts in the power supply is the first place to look. I agree that the caps are best replaced with new ones as well. Good luck!

jackinnj 4th March 2014 02:47 PM

There is a very, very active user group for Tektronix scopes on Yahoo.

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