Tektronix 222 CRT alignment issue - Page 1
Forum section: electronics / repair

author: 99wombats
author_rank: Contributor
timestamp: March 30, 2019, 09:09:12 pm
content: Hi all, I just got a Tek 222 off eBay, all seems to be working fine except the CRT display is a bit off - it’s showing more of a trapezoidal shape overall instead of rectilinear, wider at the top and narrower at the bottom. Tweaking the various alignment pots inside the scope has improved positioning a bit but not fixed the main issue. Any thoughts? I’ll try and post some images tomorrow..

author: pbarton
author_rank: Frequent Contributor
timestamp: March 30, 2019, 10:39:09 pm
content: Check this link out....https://www.eevblog.com/forum/testgear/tek-222-portable-scope-schematics/Scroll down until you find the image of a 222 CRT Xray.That could be your problem.

author: 99wombats
author_rank: Contributor
timestamp: March 30, 2019, 10:52:48 pm
content: Thanks, yeah I did see that.. not 100% clear but looks like if the deflection plates have got knocked out of place that could cause the issues I’m seeing. Guess this sort of thing isn’t really user serviceable!

author: Andy Watson
author_rank: Super Contributor
timestamp: March 30, 2019, 11:46:42 pm
content: Do you have another oscilloscope that works ?I would first check the high-voltage supply for the deflection amplifiers - is it sagging? Then check the voltages appearing on the deflection plates.

author: pbarton
author_rank: Frequent Contributor
timestamp: March 31, 2019, 10:01:21 am
content: I don't know how practical it is, but if you could swap the X and Y plate connections on the CRT neck, then the CRT trace will be rotated by 90°.If the 'trapezoidal shape' aberration remains static, then your problem is with the CRT deflection plates.

author: 99wombats
author_rank: Contributor
timestamp: March 31, 2019, 01:00:32 pm
content: I do have another scope, I’ll try the suggestions so far, hopefully without destroying it! It’s still pretty functional really so will go carefully as would be a shame to make things worse..! Here’s a picture of the alignment trace..

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 31, 2019, 02:06:59 pm
content: If you open the unit, there are trimpots to adjust the alignment.I've included the relevant pages from the service manual.Give that a go and see if you can correct for it.

author: 99wombats
author_rank: Contributor
timestamp: March 31, 2019, 03:42:47 pm
content: Yep I’ve tried fiddling with all the adjustment pots - can do shifts and scales for x and y separately, and a kind of rotation-stretch, but nothing that fixes the trapezoidal shape. It’s a bit tricky to do the x and y swap, the boards are pretty packed in and using angled headers at the bottom of the case to connect things together so it’s tricky to get access :/ also I’m a bit scared of the CRT!

author: LapTop006
author_rank: Unknown
timestamp: April 01, 2019, 10:55:49 am
content: Quote from: 99wombats on March 31, 2019, 01:00:32 pmLooks like the Death Star trench run.

author: pbarton
author_rank: Frequent Contributor
timestamp: April 01, 2019, 01:27:02 pm
content: OK, so moving all the X and the Y leads to rotate the display is difficult. How about just swapping the polarity of the Y connections?This would put the top of the display trace at the bottom of the CRT and vice versa.If the original 'trapezoidal shape' aberration remains static then your problem is with the CRT deflection plates.You have the choice of either swapping the polarity of the Y connections at either the tube neck or where the two Y wires leave the PCB.

author: 99wombats
author_rank: Contributor
timestamp: April 01, 2019, 02:28:26 pm
content: Just tried switching pos and neg on y deflection as suggested, and sadly distortion remains in the same orientation so looks pretty conclusively like the plates have moved inside the tubeho hum, like I said it’s still usable and yep I can pretend I’m blowing up the Death Star if I get bored :p

author: cvanc
author_rank: Frequent Contributor
timestamp: April 01, 2019, 02:47:35 pm
content: Are the tubes completely unobtainable? (I assume so, sadly)

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 01, 2019, 03:03:47 pm
content: Pretty much, unless you just buy another scope.But that kind of defeats the purpose of replacing the tube then.....You could always try dropping it on the opposite side to try to get the plates inside the CRT to bend back. Not much to loose at this point.

author: 99wombats
author_rank: Contributor
timestamp: April 01, 2019, 04:47:42 pm
content: Haha I did give it a couple of tentative taps but bit reluctant to commit :p