Did I bust my analog scope? - Page 1
Forum section: electronics / general

author: dzarren
author_rank: Contributor
timestamp: March 26, 2017, 03:04:13 am
content: I have a Tektronix T932A, and it was working really really well for what i was doing with it for the time I had it.I recently discovered the whole oscilloscope music thing, and ive been putting through music files that show cool shapes on the scope for the past while.I had some issues, but overall it was working well, but suddenly everything got quite a bit more blurry.Now everything that is displayed has a strong halo around it, and the Dot that appears in the center of the display when in XY mode is surrounded by a fairly large disk. In regular ch1 ch2 mode, it seems to display alright, the lines are still sort of crisp, but there is some strange ghosting in the back ground.But it looks now that the XY mode is totally screwed for some reason, and everything has a huge halo around it. I have a picture i can link to, and I'll also link to a video i shot. In the video, you can see that everything has a huge halo around it.Even on the ch1 ch2 mode, you can see in the background there is a ghost image of the wave.Did i fry the scope? This scope was my pride and joy, i love it very much, i really hope i didnt break it or ruin it's display characteristics.picture.

author: ProBang2
author_rank: Frequent Contributor
timestamp: March 26, 2017, 03:45:40 am
content: "W2AEW" to the rescue...

author: Rerouter
author_rank: Super Contributor
timestamp: March 26, 2017, 03:54:24 am
content: when you press "beam finder" does the display change? what your showing there looks exactly like my scope screen when that button is pushed. (may be bad contact)

author: ProBang2
author_rank: Frequent Contributor
timestamp: March 26, 2017, 03:59:13 am
content: Your beamfinder magnifies the trace to a blurry shadow?

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: March 26, 2017, 04:58:51 am
content: Maybe the brightness pot has just broken a wire or the wiper went open?It looks like the control has defaulted to maximum brightness. The halo looks like second emission from the electron beam. When electrons strike the phosphor some of them send out secondary electrons from the screen that are re-attracted back to hit the phosphor again. This makes the halo.The ghosting also looks like that which happens when some electron beam current reflects off of the sides of the CRT to then hit the screen.

author: Rerouter
author_rank: Super Contributor
timestamp: March 26, 2017, 10:28:07 am
content: It looks exactly like that if i either have intensity cranked to full in XY mode, or if i push beam finder, it creates a "bloom" around the waveform like that,Its a pre loved and patched as needed tek 475 so mine could well have issues of its own, (spent today cleaning the cam switches, and replacing some capacitors)

author: Martin.M
author_rank: Frequent Contributor
timestamp: March 26, 2017, 10:33:24 am
content: ah the Tek dust sucker Design...T932A will arrive here in next time, then I can help you with the repair.Martin

author: alsetalokin4017
author_rank: Super Contributor
timestamp: March 26, 2017, 11:44:49 am
content: Quote from: basinstreetdesign on March 26, 2017, 04:58:51 amYes, it would be helpful to know if turning the Intensity control has any effect at all. And, when the OP says that "XY mode is totally screwed" does that mean it doesn't show deflection, or just that the trace is blooming severely?

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2017, 06:52:18 pm
content: It looks like an intensity problem to me also.The sharply bounded halo around the spot is from secondary emission. At high brightness, the electron beam will also cause shadows from internal structures like the scan expansion mesh.If the intensity is stuck on high, it is a good idea to deliberately defocus the beam to prevent damage to the phosphor screen while you work on the oscilloscope.

author: primegaps
author_rank: Contributor
timestamp: March 26, 2017, 08:20:23 pm
content: I don't think it is an intensity thing, its hard to see because i do it really quickly in the video, but i adjust the brightness up and down a few times during the video.I find that the brightness is really really strong when i am in XY mode compared to regular mode. I find that if i adjust it to a comfortable level in xy, the ch1 ch2 mode is not visible at all.You can see at 6 seconds i turn down the brightness because it's too focused and i dont want to burn it. I turn it back up at around 16 seconds to bring it to visiblility after switching out of the XY mode.When i push beam finder, it just squishes the shape from the left side and the right side is not really affected that much. but the beam finder button is not stuck.When I say that the XY mode is screwed, Im not being precise. It still works fine, but the display is showing those halos.but the intensity is definitely not stuck on high.thank you for reading and any other suggestions

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2017, 11:56:48 pm
content: The ghosting shown in the video is from the scan expansion mesh and completely normal but maybe not at the levels shown and you did describe something changing suddenly and the display getting worse.What I would do is check that the geometry, astigmatism, and focus controls still work and then that the voltages at pins 3 (+33 volts) and 4 (+12.5 volts) of the CRT are correct. Also inspect and reseat the connector on the back of the CRT; maybe there is a bad connection.Then I would try to measure ripple from the high voltage inverter but this could be difficult. Maybe start by looking for ripple at the output of the z-axis amplifier which is point 4 on schematic 1.

author: w2aew
author_rank: Super Contributor
timestamp: March 31, 2017, 04:38:46 pm
content: The intensity is intentionally reduced in XY mode on most analog scopes, including this one. This is because the beam often moves *MUCH* slower in XY applications vs. normal scope usage. It looks like this function is faulty on your scope.If you look at the block diagram of the user/service manual:http://bama.edebris.com/download/tek/932/Tek%20T932-T935.pdfpage 93, you'll see a signal line called "X-Y Intensity Limit" that comes from the Source Switch on the Trigger block to the Z-axis Amplifier on the CRT and Vertical block. I would start my investigation there. See schematic on page 95.