Tektronix 2221 screen distortion

[mjohnm]

I bought a Tektronix 2221 DSO on eBay. Everything appears to be working except for the distortion of the screen in the lower right quadrant. Is this easily fixable and if so how?
Many thanks,
John

[bdunham7]

I bought a Tektronix 2221 DSO on eBay. Everything appears to be working except for the distortion of the screen in the lower right quadrant. Is this easily fixable and if so how?

Put a single horizontal line on the screen (CH1 only, set input to ground, timebase to <1ms/div and trigger to auto) then use the vertical position knob to scan the line up and down.  If it wrinkles as it passes over this area but looks OK otherwise, then you have a damaged CRT.  If it does anything else, report back.

[David Hess]

That looks like the distortion typically caused after an oscilloscope gets dropped and a lose piece inside CRT dents the scan expansion mesh.  Look at the corners of the oscilloscope enclosure to see if there is any evidence of impact.

If you did not see that distortion in earlier photographs, then it was probably damaged during shipping.

[mjohnm]

Thank you for your reply. As you expected, it only wrinkles in that area. I'm guessing this is unrepairable other than replacing the CRT tube. Is that correct? Thanks again, John

[bdunham7]

Thank you for your reply. As you expected, it only wrinkles in that area. I'm guessing this is unrepairable other than replacing the CRT tube. Is that correct? Thanks again, John

Unfortunately, yes.  And spares for that model are scarce.  I lucked into one last year and was able to repair a scope, but I waited a year to find it.

[bob91343]

If you can't find a replacement CRT, don't throw away the unit.  It's very much still usable, and one day you may luck into another of the same model to make one or the other a source of parts.

[David Hess]

A replacement CRT could come out of any of the 2235 series oscilloscopes.

[Allen49]

I just read an article on the internet about replacement of resistors that cause screen distortion. I will try to attach it here. hope this helps. I could not post the saved web page.

[CaptDon]

99% chance the C.R.T. is damaged. Any chance of localized magnetization? Can the expansion mesh become magnetized? Fortunately I have never had to deal with this problem even though some of my portable scopes have seen some insane abuse on railroad locomotives. I have always wondered why when I lend out a lab scope to the junior technicians that the scopes come back to the lab missing, probes, power cords, feet, knobs? (Or the scope never comes back at all)

[inse]

The question is whether a 60MHz DSO with 4k/2k memory depth and 20MS/s is really worth the effort, though I’m not a fan of tossing things easily.

[fmashockie]

That's a real shame if this is due to a bad CRT and it seems like that is the case.  Fortunately, this models have extensive service manuals so at least look thru the CRT drive circuitry to check.

Check QServices.tv for spare parts.  They have a ton of spare parts for vintage Tek equipment.  The problem is (at least for me here in the US) that they located in Greece so shipping time can be long and expensive.

[cf4044]

It seems to me it might just be magnetized, or maybe even influenced by something magnetized outside of the tube, maybe even a screw in the chassis.  I would try, if practical, first to operate it with the tube outside of the chassis and see if the distortion changes/vanishes.  If not try a good degaussing as there are sometimes parts inside the tube that can become magnetized. I have a 2215 that had once gotten some distortion of the trace, though very different from what you have. It was caused by leaving it on a shelf next to a box containing magnets dismantled from large speakers. I used a simple hand wound coil perhaps 15cm diameter fed by a 12V transformer and randomly scanned it around the front of the scope and removed most of the distortion though not quite completely. If I had the time and inclination I would probably dismantle it and degauss the tube, chassis and other parts separately.