Distorted CRT trace on 2235 - is it a bad CRT? - Page 1
Forum section: electronics / testgear

author: KC3FEY
author_rank: Newbie
timestamp: September 14, 2015, 03:24:46 am
content: Picked up an old Tek 2235 on Ebay. It will not display a flat CRT trace, and the distortion changes as the vertical position is moved up or down on either channel. Both channels exhibit the same distortion.Is this likely a bad CRT or should I be looking elsewhere?I had to manhandle this unit to get the cover off. The trace actually straightened out on the right side after some jostling.

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2015, 04:31:17 am
content: Welcome to the forum.This thread is probably better in the Repair board if you don't mind.There, in the sticky threads you will find a good troubleshooting pdf by Tek and links to find the Service manual.Post a link to that manual so we are all on the same page.It's much more likely the problem is with the PSU or Vertical amp circuitry, faulty CRT's are quite rare.

author: grumpydoc
author_rank: Super Contributor
timestamp: September 14, 2015, 06:35:04 am
content: Quote from: KC3FEYQuoteThose sound a little worrying, is the distortion the same at different time base settings? Is it identical for each channel at various vertical deflections using the vertical offset control?I hope that I'm wrong but it looks and sounds like a damaged CRT.Quote from: tautechIn equipment of this age quite a lot of things can happen. Rough handling can break internal glass supports or damage delicate structures such as deflection plates, they can outgas and suffer from cathode poisoning. That said I'd probably agree that most 'scope CRTs are actually OK, even in instruments which are 30 or 40 years old.

author: bktemp
author_rank: Super Contributor
timestamp: September 14, 2015, 06:44:11 am
content: If the curve changes when changing the timebase it is most likely caused by a voltage ripple on the deflection voltage.If not it could be either the deflection plates beeing mechanically damaged or the deflection voltages having a large offset.

author: grumpydoc
author_rank: Super Contributor
timestamp: September 14, 2015, 07:19:51 am
content: Quote from: bktemp on September 14, 2015, 06:44:11 amThe most common is that the 840/1000uF caps on the +/- 8.6V rail go dry/high ESR. That usually manifests as a fuzzy trace as the ripple is asynchronous with the scan.Sometimes if it is large you can trigger on it to show an approx 40kHz ripple (it tends to be picked up in the input amp so pre-trigger) but it will then look like a normal signal and stretch with the timebase settings etc. It can also sometimes be affected by the input attenuator (so, becoming larger as you move to more sensitive V/div settings) but as it generally gets picked up after that point it can be variable.The clincher as to whether this is PSU ripple is whether it is affected by the sweep speed.

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2015, 07:29:57 am
content: Quote from: grumpydoc on September 14, 2015, 07:19:51 am+1

author: KC3FEY
author_rank: Newbie
timestamp: September 14, 2015, 08:43:35 pm
content: Thanks for all these replies. I will dig through the troubleshooting guide and start working throw the workflow there.I thought I might save myself the trouble if it was clearly a CRT issue, which it seems likely is not.I'll continue this in the Repair Thread (which I had not noticed before) as I am sure I will have more questions (like, where the heck is the disconnect diode?)

author: grumpydoc
author_rank: Super Contributor
timestamp: September 14, 2015, 09:18:58 pm
content: Quote from: KC3FEY on September 14, 2015, 08:43:35 pmFrom what you posted I'm not sure I would come to the same conclusion.

author: KC3FEY
author_rank: Newbie
timestamp: September 14, 2015, 10:06:35 pm
content: grumpydoc: "Likely" is too strong a word. I should have said that it is not presumptively a CRT problem.BTW, the non-linear pattern is not affected by sweep speed.

author: grumpydoc
author_rank: Super Contributor
timestamp: September 14, 2015, 10:29:48 pm
content: Quote from: KC3FEY on September 14, 2015, 10:06:35 pmIn that case it is starting to sound very like a broken CRTAll is not lost, you might be able to find a parts doner with a working CRT or you might be able to use a CRT from a 465 - and you'll probably find a used/for parts 465 a lot cheaper than a 2235Seehttp://www.sphere.bc.ca/test/tek-parts/tek-info.htmlin the section "Bizarre CRT Traces"

author: rf-loop
author_rank: Super Contributor
timestamp: September 15, 2015, 05:19:51 am
content: Quote from: KC3FEY on September 14, 2015, 10:06:35 pmTurn horizontal shift <---->. Look if nonlinearity follow exactly trace or if it stay horizontally fixed place related to tube front surface.Also this shape of trace line is bit weird. If it have been in strong magnetic fields some quite small things can happend and demagnetizing may help some but typically trace shape is not just like this now. (look also that possible small geometry compensation magnetic bits are ok. In some other scope model I have seen compensation magnetic bit (small rod) have broken and some pieces dropped inside outer tube shield and in this case trace was really really weird. Also some outer tube magnetic shield can may magnetize example if some one have done some "kids play" using strong magnets and this magnetic/electric field protection over tube is so close electron beam that if magetized it may do also quite "sharp" geometry distortions and with this kind of problem horizontal speed do not change shape as here is)If really suspect magnetizing problem this tube shield can remove (1. Tube remove, then shield remove and then tube back... very easy in this model but need be careful. It is highly fragile including deflection plate connection pins side of tube, and also tube front class joint need care due to its age (very careful work!) and after then look what is effect. Also NEED KNOW safety rules for this work, electric and explosion (implosion) safety. Rules read exactly in service manual where is not any unnecessary word.But there can be also many other reasons, even in very rare case tube horizontal plates may somehow damage but - rare, really rare. I have handled truck load of tek scopes over tens of years and never seen this kind of problem due to deflection plate damage. (but also know gthat these can exist). These Tectronix tubes are really as state of art, even in these sheap basic models.

author: oldway
author_rank: Super Contributor
timestamp: September 15, 2015, 11:06:44 am
content: This kind of fault may be caused by a defective internal metallization of the tube. (phosphore metallization)Some screen regions become negatively charged and repulse the electron beam.I have already replaced a Tek CRT that had this problem.