Oscilloscope Resuscitation - Page 1
Forum section: electronics / repair

author: Gabbox231
author_rank: Newbie
timestamp: November 05, 2022, 06:43:52 pm
content: Hi everyone,a while ago I went on a search for my first scope, and with my tight budget I had to go with an old analog one. I locally found a Philips/Fluke PM3055, I don't mind the digital controls and the Delayed Time Base is a nice feature. Luckily, while arranging the sale with the seller, it stopped working so I was able to talk down the 180€ price to just 50€.As I expected it had a shorted capacitor on the 5V rail, which I promptly replaced. Being there I replaced all the electrolytics and the RIFA cap on the power supply. I think it was a little overkill, as I spent around 70€ for capacitors. Through my research, I found out Vishay BC is a descendent of the Philips Electronic Division, so they are still producing all of those weird axial capacitor series, so it wasnt that hard to find exact replacements. (I can post the part numbers I bought if needed)The oscilloscope now lives, however, with it being my first time with a CRO, i have a few questions.The trace has got some pretty noticeable halo as you can see in the attached pictures. Is it normal? I checked the main power rail as the manual suggests and it shows 10.00V, spot on. What's that other "ring" that appears next to the trace?Furthermore, I would like to get it calibrated somewhat for my hobby use, probably using the instruments at my university. The timebase is a little bit off. However I would like to dig a little at the causes. If oscilloscopes going out of cal is caused by ageing components, wouldnt I need to replace them? From the service manual I see there are some electrolytics on both the time base and vertical boards. What do you suggest? My only way to test for capacitance is with my Aneng AN8008 DMM. I have no way of checking ESR directly. I know I may sound like a bit of a perfectionist, but I worry ending up trying to calibrate something that is not calibratable.Regards

author: Ian.M
author_rank: Super Contributor
timestamp: November 05, 2022, 06:59:04 pm
content: Turn down the brightness ('INTENS') *IMMEDIATLY*, hopefully before you burn the CRT phosphor!The halo is caused by excessive beam current. If it doesn't respond to the INTENS control, you have a fault that must be fixed.N.B. a high intensity static spot can not only burn the CRT phosphor, it can crack the CRT by localized spot heating of the faceplate glass, which lets in air destroying the tube.

author: Gabbox231
author_rank: Newbie
timestamp: November 05, 2022, 07:45:16 pm
content: Thank you Ian for your reply.I can turn down the brightness without problem. The halo disappears when lowering intensity. I though it wasnt normal to have it with higher brightness. When using a fast sweep time I'm forced to increase intensity, otherwise the trace is too dim, and I get a bit of halo still.What about the calibration? If I set the Y Pos and X Pos both to halfway, the trace is around 1 div off to the right and half down

author: Ian.M
author_rank: Super Contributor
timestamp: November 05, 2022, 08:30:04 pm
content: Its usual to have to turn up the intensity for fast sweeps, especially if the time between trigger events is much longer than the sweep duration, and yes the halo does come back, so shade the screen from direct room light so you can get acceptable contrast with less intensity increase. Generally, you should only use enough intensity to get a clear trace with enough contrast to read easily.I wouldn't worry about the X Pos and Y Pos centering provided there is enough range on both to reach each corner with a stationary spot, and once centered (on screen) it doesn't drift significantly.

author: jdragoset
author_rank: Frequent Contributor
timestamp: November 05, 2022, 08:39:52 pm
content: The Halo is from secondary electron bounce from the phosphor screen and then returning.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 07, 2022, 12:45:16 pm
content: The timebase for the horizontal sweep in old scopes is not perfect.They area all off a bit. I think they were never perfect when new. Whatever oscillator is used, it is not a high accuracy unit. Usually not even a temp compensated or an OCXO. Just an Xtal. So just do the best you can with the horizontal timebase. If there is an adjustable cap, clean it.Even in my newer scopes the timebase leaves a lot to be desired.I don't know if you scope has a provision for an external timebase. This can help immensely.Also amplitude of signal is not entirely accurate in old scopes,. I can also vary by freq changes.Just go through the calibration in the manual and you will end up with an acceptable calibrated scope,The amplitude of the displayed signal at the advertised freq limit of CRT scopes can be only about 80 % of the real amplitude. This is a frequent occurrence and normal..Usually you can get a pretty good DC calibration, Amplitude of a sine wave is harder and not anywhere near perfect.

author: David Hess
author_rank: Super Contributor
timestamp: November 07, 2022, 06:52:17 pm
content: Quote from: Gabbox231 on November 05, 2022, 06:43:52 pmThe halo is caused by secondary emission. Electrons bounce back from where the beam hits the phosphor and then are dragged back to the phosphor around the spot by the high positive post deflection acceleration voltage. The secondary electrons have an upper energy limit which is why the boundary of the halo is so sharp.The "ring" is called a ghost and is caused by the electron beam hitting the scan expansion mesh on its way to the screen. As the beam moves across the screen the ghost follows, catches up, and then precedes the beam.QuoteThe actual timing resistors and capacitors are precision parts and should be left alone. The timing capacitors may look like electrolytic capacitors, but are often oil filled film capacitors in an axial can.Adjustment of the timebase is made by adjusting the horizontal gain or sometimes the sweep charging current.Quote from: Ian.M on November 05, 2022, 06:59:04 pmThe halo and ghost are present even when the beam current is not excessive.

author: Ian.M
author_rank: Super Contributor
timestamp: November 07, 2022, 07:08:11 pm
content: True,Quote from: David Hess on November 07, 2022, 06:52:17 pmI think my second sentence there was confusing. I meant if the halo and trace brightness together don't respond to the intensity control, there's a fault.QuoteTrue as all the primary electrons must hit with close to the same energy, but unless you are pushing the sweep rate vs the holdoff + time to next trigger, so getting a very poor duty cycle for the beam actually scanning the trace and thus insufficient brightness, its usual to keep the beam current low enough that you don't notice the secondary emission halo unless you turn off the room lights.Also, detail on fast edges is always problematic, as the spot has to move much faster vertically to trace the edge so its brightness will be much lower, maybe almost non-visible, so if you are glitch hunting (within the bandwidth of your CRO), its not unusual to up the intensity temporarily, living with the visible halo on horizontal(ish) portions of the trace, to see if the edge is an even line or if it has a bright patch caused by the beam retracing part of it due to a glitch.

author: flindersit
author_rank: Newbie
timestamp: August 23, 2023, 12:20:03 pm
content: On another issue can anyone please advise the correct Varistor to use, i note its not in the service manual i am unclear what one to buy markings are 1.5NS 3KV 624 D3 in a yellow package. i was thinkinghttps://au.rs-online.com/web/p/varistors/7858815?gb=bthanks for any help/.