Very annoying fault on my old PM 3232 oscilloscope - Page 1
Forum section: electronics / repair

author: RichardAtRedditch
author_rank: Newbie
timestamp: September 11, 2016, 03:05:02 pm
content: Hello.I have a Phillips PM 3232 oscilloscope – an old model now, of course, but I would like to rid it of an annoying fault.The X-axis is not staying in place on the screen, but seems to be triggering at different times. The visible effect is that a waveform is smeared left and right to about 2mm left and right of the mean position. The calibration signal has a bright, well-focussed top, but the Y movements are diffuse and dimmer, although vertical. With the triggering set to the mains position (which isolates the trigger inputs from the Y amplifiers) the same effect happens – an AC sinewave is likewise smeared out in the X direction.I have checked all supply voltages, both for voltage and ripple. These are in full accord with the service manual. I have also checked the transistors in the time-base unit, all to no avail.Has anyone come across this fault before? Any help will be most welcome indeed!Many thanksRichard

author: setq
author_rank: Frequent Contributor
timestamp: September 11, 2016, 03:15:43 pm
content: If it's one Philips bothered to socket the transistors in, give them all a wiggle. This fixed a similar issue on another unit I had about ten years ago.My current PM3217 trigger is iffy on fastest sweeps and shows this as well when DTB is used. Not managed to get around to looking at it yet.

author: RichardAtRedditch
author_rank: Newbie
timestamp: September 20, 2016, 06:00:34 pm
content: Hello Setq. Many thanks for your prompt response. I'm sorry that I haven't acknowledged it sooner, but work took over and it is only recently that I managed to get into the workshop.Yes, my 'scope is the one with transistor sockets and I have wriggled those transistors on the timebase board. Unfortunately, that has not helped, and the fault is still there.Attached are photos of the calibration output, and a 50Hz sinewave with the timebase set to trigger on the mains. The pics are a bit small, and the calibration one should show the verticals smeared out better than it does. The 50hz one does so, I think (The light part on the LHS of the trace is a camera artefact). It would be nice to get the scope working - I don't want just to put it in the skip!Thanks againRichard

author: Greg981
author_rank: Newbie
timestamp: September 20, 2016, 06:28:18 pm
content: It probably wont help, but my next move would be to give all the controls a darn good wiggle.

author: singapol
author_rank: Frequent Contributor
timestamp: September 20, 2016, 07:12:31 pm
content: I think this is a focus (crt) adjustment. Please read the service manual to adjust the focus control to get a sharp focus or point and then another adjust pot at but set to X/y trigger setting for a round spot but you need to set the intensity first.

author: tautech
author_rank: Super Contributor
timestamp: September 20, 2016, 07:35:00 pm
content: Not come on this exact type fault before but looking at the wide vertical component of the waveform as compared to the much narrower horizontal portion I would suspect trigger jitter.Check all Ecaps in the trigger circuitry.

author: StillTrying
author_rank: Super Contributor
timestamp: September 20, 2016, 08:05:53 pm
content: Your 50Hz photo looks to me like it has caught 1 single sweep, but still showing the blurredness(is that a word) in the X direction, which would mean it's astigmatism, but I don't know where that knob would be, try pulling the focus knob or something.

author: voltz
author_rank: Frequent Contributor
timestamp: September 20, 2016, 10:36:15 pm
content: Have to agree, it looks more like a focus/astig problem with the beam rather than a X trigger issue.Turn of X and Y to get a sharp round defined dot (at reduced brightness to really get it sharp) Can you see a sharp dot, or a flat blob?

author: StillTrying
author_rank: Super Contributor
timestamp: September 21, 2016, 12:02:31 am
content: The service manual is available.http://elektrotanya.com/philips_pm3232_service_manual.pdf/download.htmlhttp://www.electro-tech-online.com/attachments/philips_pm3232_service_manual_-et-pdfAstig. appears to be a 470K pot in the power supply, described on page 72. Watch out for the 6.5 killer volts even when off.

author: singapol
author_rank: Frequent Contributor
timestamp: September 21, 2016, 01:17:33 am
content: First you need to adjust the focus pot to get sharp defined spot then the astig pot to get a round spot (both at X&Y setting). They interact so need to adjust back and forth to optimise.

author: tombi
author_rank: Regular Contributor
timestamp: September 21, 2016, 01:52:28 am
content: Don't know Phillips scopes at all but have worked on another old CRO.Could try setting it to trigger off the line or an external input. If that changes things it could be a problem in the triggering circuit.Could well be astig/focus or bias however.Thought I'd suggest this as it is an easy check. Otherwise I leave you in the hands of the very capable folk who've already responded.Tom

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: September 21, 2016, 03:45:33 am
content: Have you verified the out put of the power supply card?I had one with bad caps. The first thing to verify is that all output are well in range without excessive ripple.

author: RichardAtRedditch
author_rank: Newbie
timestamp: September 21, 2016, 09:54:41 am
content: Hello everyone!A big thank you to everyone who replied promptly to my query. It's great to know that there are really helpful people still left in the world.Specifically - Jaques, yes, all PSU voltages are OK and the ripple is within limits.StillTrying - I have a copy of the service manual - unfortunately not a particularly good one, but just about adequate to read the component numbers on the circuit diagram.TautTech - "trigger jitter" - a lovely phrase! If nothing else works, I think I will remove the time base panel and replace every component in the trigger circuitry.I have holiday coming up now, but I will post results IDC.Richard

author: tautech
author_rank: Super Contributor
timestamp: September 21, 2016, 07:15:16 pm
content: Quote from: RichardAtRedditch on September 21, 2016, 09:54:41 amTo tick it off the list, do check Astig as member voltz has suggested.

author: oldway
author_rank: Super Contributor
timestamp: September 22, 2016, 03:11:23 pm
content: I have a PM3233 who is the same as the PM3232 but with two delay lines in the A and B channels.I don't recomend to try to repair your oscilloscope by replacing all the components of the time base board.This is not a good way to repair.You should first make a diagnose and be sure of what is faulty.My philosophy of repair is to replace only what realy is needed.I have more than thirty years of experience in industrial repair and this philosophy has always be proven to be right.Replacing a lot of components may introduce new problems and not solve the first fault.To make a diagnose, you will need another oscilloscope.

author: RichardAtRedditch
author_rank: Newbie
timestamp: October 22, 2016, 03:11:41 pm
content: Hello everybodyI'm now back from a little time away, and have been able to look at the scope. I'm very pleased to say that it is now working with a beautifully sharp trace! There was a capacitor on the timebase board that was faulty, but the main fault was in the focus circuit where two resistors had doubled in value! Putting all these right was not difficult.Any way, a big, big thank you to everyone who responded to my crie de cour. I do appreciate being put on the right path.Richard