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Old 2nd March 2014, 07:06 AM   #1
wjcoll is offline wjcoll  Australia
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Default 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.

Cheers,
Wayne (wjcoll)
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Old 2nd March 2014, 07:20 AM   #2
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Somethig wrong with PSU.

SCH in att.
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Old 2nd March 2014, 07:59 AM   #3
Salas is offline Salas  Greece
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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.
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Old 2nd March 2014, 08:01 AM   #4
wjcoll is offline wjcoll  Australia
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Thanks zeonrider and Salas. Off to a good start!
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Old 3rd March 2014, 07:28 AM   #5
wjcoll is offline wjcoll  Australia
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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.

Cheers,
Wayne
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Old 3rd March 2014, 12:47 PM   #6
Salas is offline Salas  Greece
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Old 3rd March 2014, 06:24 PM   #7
tomchr is offline tomchr  United States
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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.

~Tom
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Old 4th March 2014, 03:22 AM   #8
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Capacitors in 2235 have a sealing compund on the (rubber)cap.

Gajanan Phadte
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Old 4th March 2014, 12:43 PM   #9
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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!
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Old 4th March 2014, 02:47 PM   #10
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There is a very, very active user group for Tektronix scopes on Yahoo.
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