Tektronix 2235 Oscilloscope woes - diyAudio
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Old 12th July 2004, 05:06 PM   #1
dstaley is offline dstaley  United States
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Join Date: Jul 2004
Location: Central IL
Default Tektronix 2235 Oscilloscope woes

I recently purchased a Tektronix 2235 O'scope but am having some trouble with it. The trace is bright but I can not get it to properly focus. I have no experience in troubleshooting oscilloscopes and I don't have another to try to tackle this kind of project, so I'm looking for some kind of affordable repair service for it.

I have tried the test trace and the scope seems to work except that the trace is so blurred. The square waveform is obvious but not precise. The time scale and both voltage controls seem to work normally.

Is anyone out there able to do this kind of thing, or (cringing) familiar enough with these to know what the problem might be? Is it even worth repairing?

Thanks for any help you can offer!

Douglas Staley
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Old 14th October 2004, 09:39 AM   #2
tekman is offline tekman  Germany
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Join Date: Oct 2004
No need to be dicouraged

The problem is well known - if you have a service manual, look for the resistors in the focus pot. divider chain.

If you have no manual: Opern the scope, no power applied. Turn the focus pot and note where it is on the mainboard. Nearby this pot are a lot (approx 9) 510 k Ohm resistors. Get replacement: 510k 1/2 watt carbon 5% or metal film 1%. Unsolder the original resistors, check with ohmmeter. Change all the 510 k resistor, also the non-faulty ones

For curiosity: most time only a couple of these have failed.

Let us know if this gets your scope back to work
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Old 14th October 2004, 12:12 PM   #3
pukka is offline pukka  Spain
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Join Date: Mar 2002
Tekman, mine's focus isn't specially sharp, but I thought that's due to the CRT age, does that resistor work bring back sharpness?
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Old 14th October 2004, 12:23 PM   #4
tekman is offline tekman  Germany
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Join Date: Oct 2004
2 pukka,

well, if your focus control is best in approx. midrange while turning the focus pot, then there not much hope getting better doing the resistor change.

But if you can't focus well at al and it gets better to one or the other end of turning (cw or ccw), then I would recommend to check the 510 k resistors.

BTW: There are some 2235/2235A around with a metal gaze (mesh) filter in front of the tube -- this also causes some "dizzyness" appearing the beam to have, but it's optical (remove the mesh, if there are no EMV reasons that make it necessary to be in place).
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Old 14th October 2004, 12:28 PM   #5
dstaley is offline dstaley  United States
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Join Date: Jul 2004
Location: Central IL
Default Success!

I replaced the resistors, and all is well. They "did the trick". The scope acted like the pot didn't have enough range, which turned out to be exactly true since the resistors it was in series with weren't within spec.

Thanks all for your help!
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