Tektronix T922 weird issue, over-illuminated - Page 1
Forum section: electronics / testgear

author: MadModder
author_rank: Regular Contributor
timestamp: May 15, 2022, 08:55:31 am
content: I have this old T922. Not one of my main scopes, but it was dirt cheap and it has a nice carry handle so I had to have it.Everything is working, except there is this over-illumination kind of deal.When I crank the sweep down so I see the dot moving, there is a large "sun" around the dot, like when turning the illumination up too high.But It's turned way down so I barely can se the trace.When I turn the illumination up the whole screen lights up in bright green.And on top of that there is a "copy" of the beam double the distance from the center point of the screen. And that copy is way out of focus so it looks like a circle.I have not checked the voltages or anything, yet. Focus and astigmatism settings does not affect it. Only the trace itself.The illumination in the pictures is set to a bare minimum so as to see the trace at all with my eyes. The camera is obviously more sensitive than my eyes.Anybody having any ideas about what could cause this?I checked the voltages on the power supply board. According to the service manual, the voltages are well within spec:-8V : -8.002 (ripple <5mVpp)+8V : +7.988V (ripple <5mVpp)+100V : +100.78V (ripple 200mVpp)

author: Fungus
author_rank: Super Contributor
timestamp: May 15, 2022, 01:15:48 pm
content: Quote from: MadModder on May 15, 2022, 08:55:31 amIt's called "blooming". All CRTs do it to some extent.https://youtu.be/Znwp0pK8Tzk?t=280

author: MadModder
author_rank: Regular Contributor
timestamp: May 15, 2022, 02:15:27 pm
content: Well, yes. But I thought it was a bit much, since it's there even when I have the intensity set so low I can barely see the trace.Anyway. Let's disregard the blooming then.The other thing is not blooming, and is a bit more annoying.That circle thing is like a 2x magnification. It moves around the screen at double speed. (looks the same on both channels)Just a test with some music. You can see the "shadow" trace flapping around here.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2022, 03:13:14 pm
content: Quote from: Fungus on May 15, 2022, 01:15:48 pmNo, the thickness of the trace increases because the emitting area of the CRT gun increases at high currents. This is a focus problem. With a worn out cathode, the emitting area can become very large turning the trace into a smear.Blooming also typically involves the current being high enough that the acceleration voltage sags causing an increase in deflection. Oscilloscopes use tightly regulated acceleration voltage to prevent this because deflection accuracy is important. The post deflection acceleration voltage has a much smaller effect on deflection so does not need to be tightly regulated.Quote from: MadModder on May 15, 2022, 08:55:31 amThose are two separate effects and I know exactly what causes them. Nothing is wrong with your oscilloscope.The perfectly circular disk around the spot is caused by secondary emission. When the electron beam hits the phosphor surface, some electrons are blasted away back into the CRT. The high positive post deflection acceleration voltage pulls these back into the CRT where they land to make a disk around the spot.The "ghost" which follows the spot and then precedes the spot across the screen is caused by electrons hitting the scan expansion mesh and getting deflected, or maybe from secondary emission from the scan expansion mesh. Oscilloscope CRTs which lack a scan expansion mesh like the Tektronix 547 and older instruments do not display this phenomena.Both effects lower the contrast of the CRT. Where this is important, like on a Tektronix 7104 or 2467 which use an image intensifier, the scan expansion mesh is replaced with a magnetic lens.QuoteUsually that means that the electron beam is hitting an internal structure and getting scattered everywhere. When you turn the illumination up high enough, the width of the beam might get large enough to hit the focus lens structures inside the electron gun.Storage CRTs have special "flood guns" which are designed to do exactly this.

author: MadModder
author_rank: Regular Contributor
timestamp: May 15, 2022, 04:10:25 pm
content: Huh. Interesting.I've never seen a ghost like that on any other scope.And the focus is crisp. The trace isn't even a millimeter wide/thick.Yes I've read about the flood guns. Remaining positive charges draw electrons to them and the trace can be seen for several minutes. Fascinating.And about older scopes without scan expansion. I've got a Telequipment D31 from 1960, and the contrast is remarkable. Also minimal secondary emission at "normal" intensity. No ghosts either in that one.

author: David Hess
author_rank: Super Contributor
timestamp: May 17, 2022, 02:55:28 am
content: Quote from: MadModder on May 15, 2022, 04:10:25 pmIf they use a scan expansion mesh then the ghost is there. Practically all instruments of 50 MHz or faster use a scan expansion mesh, and many modern but slower ones do as well.QuoteLower acceleration voltages will lower the amount of secondary emission, and different CRT targets will have an effect also. Faster and brighter CRTs use a phosphor protected by aluminization and I suspect that also has something to do with it. The aluminum acts as a heat sink to prevent burning the phosphor, so much higher beam current can be used.The 50 MHz Tektronix 547 was preferred for many years after it was obsolete because of its unusually clear and crisp display from not having scan expansion. Scan expansion also multiplies the size of the spot as much as it multiplies the deflection.

author: MadModder
author_rank: Regular Contributor
timestamp: May 21, 2022, 03:40:39 pm
content: So my 200 MHz Tektronix 475 doesn't have that then?Because I can't see the slightest hint of a ghost trace there, even with a pitch-dark room, or the intensity cranked up blindingly high.

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2022, 07:01:49 pm
content: Quote from: MadModder on May 21, 2022, 03:40:39 pmThe 475 has to have a scan expansion mesh and the schematic shows it and names it. Maybe the implementation was better.

author: MadModder
author_rank: Regular Contributor
timestamp: May 21, 2022, 08:07:37 pm
content: Or maybe the implementation was standard in the 475, and rather built down to a price in the T922.T922 was introduced on the market for $1,220 in 1976, and the 475 for $2,500 in 1973.