Fluke/Philips PM3285A CRT flicker - Page 1
Forum section: electronics / repair

author: Ja-no
author_rank: Contributor
timestamp: November 21, 2018, 06:39:18 pm
content: Recently I got a Fluke/Philips PM3285A in non working condition. Two of the mains noise filter capacitors were blown and the main power switch was broken (no blown fuses though), I replaced the capacitors and put another switch and the scope turned on and displayed signals.However, the CRT display seems to flicker(?). Here's a video of the issue (input coupled to ground):https://www.youtube.com/watch?v=aGJAJimcj2U&feature=youtu.beI think there might be an issue with the high voltage, but I haven't found similar problems by Googling, so I don't know how to further isolate the issue.CRT controls seem to work fine. I can control focus, trace intensity, and readout intensity.The following voltages can be measured on the motherboard, high voltage is controlled by the 13.9v rail:Marked Value Measured Value+13.9v +13.7v-13.9v -13.7v+5v (analog) +4.93v+5v (digital) +5.17v+25v +25.5v-7v -6.5v+120v +118v-120v -118vThe CRT uses +14,3kV and -2,2kV, which I haven't measured yet, since I would have to borrow a HV probe to try and measure them.I added the schematics that seemed relevant to the problem, but I have the complete service manual, so if other schematics are needed I can add them here.Also, while changing the noise filtering capacitors I noticed that a capacitor (C6208 in the primary converter schematic attached) looked like it might be faulty. Since it couples to the power supply ground, could it be related to the CRT flicker problem?Am I right in thinking this might be a high voltage problem?I can't see anything obviously faulty, has anyone seen similar faults? If so, is this one tied to a more specific part of the circuit?

author: tautech
author_rank: Super Contributor
timestamp: November 21, 2018, 11:49:35 pm
content: Welcome to the forum.Your voltages look a touch low but that could just be your DMM however they're close enough I think.The flicker just looks like a brightness/intensity issue which would point me to the the condition of the intensity pot.Made just some exercise will improve it.....I would try that first as these pots are normally a HV type and hard to find replacements for. On minimum setting the trace should not be visible but should be visible by 1/4 rotation and there is normally a bias trimmer inside to set that behavior with.Alternatively there's a HV resistive divider chain associated with the intensity circuitry or the 2.2KV HV that is breaking down or in the process of going open.Most of the high value Mohm resistors are under some stress and it's a common point of failure in CRO's.But exercise the pot first, it could be a simple fix.

author: Ja-no
author_rank: Contributor
timestamp: December 10, 2018, 03:29:58 am
content: Thank you for your reply, and sorry for the long time to answer but I haven't had time to work on the scope.I checked the intensity pot as suggested, but the intensity does not seems to be controlled by a high voltage pot, instead the intensity pot seems to control a 13.8v signal that is fed to intensity control sections in both the timebase circuit and the x/z amplifier circuit. I am currently reading the circuit descriptions of the relevant sections in order to understand where the fault might have generated. I have also attached the schematics of the corresponding sections, as well as a functional diagram of the scope in case anybody has worked on scopes with a similar layout might have an idea of the section in which the problem may be.I also found another problem: some time/div options would show a signal with a lower frequency than that which should be displayed. However, I think I have isolated this problem to a section of the timebase circuit which injects a x100 amplification signal to the constant current circuit of the timebase, since only the time/div options that should not have this signal seem to display this problem (those with a M100 value f 0 in the table of the las image). The second to last image is a schematic of the relevant circuit; I suspect transistor V9719 since the M100 signal is present and changes acording to the selected time/div option, and the reed relay K9704 tests open when the scope is powered off and when the scope is on the x100 signal is injected independently of the time/div selection.I have posted the schematics that I deem relevant to the problem and not linked the service manual because it it's very long, but here's a link to the complete service manual in case anybody wants to have a look:https://www.allaboutcircuits.com/test-measurement/oscilloscopes/pm-3000-series-3285a-3286a/service-guide/

author: Ja-no
author_rank: Contributor
timestamp: January 30, 2019, 09:22:40 pm
content: I got some time to work on the scope. I replaced C6208 and all the caps of the same type in the primary converter (c6202, c6203 and c6209) and the flicker disappeared. It was a messy process that involved destroying the capacitors with pliers since there is no access to the bottom of the PCB.The problem with the timebase amplification persists, so I'm now working on that.