Beginner trying his luck at fixing a Philips PM3350 - Page 1
Forum section: electronics / repair

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 01, 2015, 09:27:24 pm
content: So, today my PM3350 arrived. Just in time to be productive over the easter holidays and also not being stuck with the tiny DSO203, which is great to take quick measurements out in the field, but at home its kinda sub-optimal. Partly because I only have a stand for it made out of LEGO and partly because 2 channels aren't enough to perform all the measurements.Now to the PM3350 and its quirks:First it didn't work, because it probably was a bit abused during transit - PowerSupply-Board and the Papst-Fan were loose. One of the retaining clamps for the PSU-Board even sheared off and rattled inside the casing.That fix was easily done by just pushing everything back in place.Another rattling-test revealed nothing else, so I turned it on and after some playing with it, I got familiar with the operation and also discovered two problems:The First one should be easy to fix, as it probably is just a worn out potentiometer that controls the illumination of the grid.The second one is a bit harder to fix, I think: It becomes noticeable at 0.1s/div and really obvious at 0.2s/div and 0.5s/div Timebase - The horizontal movement of the electron-beam is not uniform! It jitters to the left and right while moving across the screen. I later also noticed this jitter at higher timebase-setting when I fed a signal from the DSO203's signal generator into one of the channels and the scope triggered on it. It's also visible in the digital mode: The waveform is jumping a little from left to right in live, as well as in locked-mode.I'm guessing that there is something wrong with the sawtooth-generator that generates the horizontal deflection-signal. My first suspect would be old caps on the PSU and the Generator itself.Is that a good first guess, or am I fishing in the wrong sea here?Here's a video showing the Problem:https://youtu.be/IbRcqXXzNHw

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2015, 12:22:52 am
content: You should exclude the PSU from all doubt first.It must meet the Service manual spec.I see your issue with the jitter and yes it might be a problen with SLOW timebase speeds, but in reality how often do we use slow timebase settings on a CRO?Is the jitter still present with a waveform applied?Is observed jitter affected by either waveform amplitude or Trigger level?DSO, yes it would need fixing.Can you get a stable Cal waveform?

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 02, 2015, 09:01:36 am
content: I checked all the voltages, they're clean. As I would expect from a PSU with fresh caps. Almost all have been replaced by the previous owner.As there wasn't anything I could really check on the time-base board, on a hunch I checked the X-Position knob, because the Grid-Illumination also seems to have a mind of its own even when the knob isnt touched. And BINGO! Turning it to various positions increased and decreased the jitter.Measuring the corresponding pin on the digital Mainboard, I discovered glitches as bad a 400mVpp, visible on both the DSO203 and even my Multimeter! So I have two dodgy potentiometers.Getting to the pot on the control-panel was pretty easy, but the bastards seem to be sealed, so no quick cleaning with some Kontakt-Cleaning-spray and lubricating it with Kontakt Oil, I guess.Upon closer inspection, I also disovered that the pot had been unsoldered at least once, which also ripped out the via in the board, so I'm going to repair that as well.

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2015, 09:09:48 am
content: Quote from: SaabFAN on April 02, 2015, 09:01:36 amMight be time for some pot "swimming lessons" in a cup of IPA.Rotate while wet frequently and dry well in the sun.If you have a good idea how it's constructed, a small hole drilled in a carefully selected spot might allow a better cleaning job. Cover before re-assembly.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 02, 2015, 09:24:53 am
content: I was more thinking in the region of just replacing it, but I have found an old bottle of Caramba Oil. Like WD-40, that stuff can reach everything^^The shafts appear like they have been greased once, btw.Can you identify this thing? Google hasn't been too helpful so far.It has these markings on it (Viewed from the side):10KA 50190096HQ V4

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2015, 09:56:12 am
content: 10 K ohms "A" taper, that's logarithmicnotlinear.Service manual might have the manufacturer in the BOM, or maybe a code that you will have to x-check against the manufacturers list.Might be a Bournes pot, is that a logo next to "V4"?Rip it off and measure it up, looks a very std form factor type to me.Should be no trouble to find a replacement from the better suppliers.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 02, 2015, 10:24:52 am
content: I think, I fixed it.The Pots are more or less open on the bottom, a few Drops of Caramba and the jitter is basically gone!I will take the board out of the scope again to let it sit for a while upside down with some more oil to get the jitter entirely out and ensure proper lubrication again.Btw. the Service Manual only has "PP17" and "20% 10k" listed next to "R7010", which is the component-number in the circuit. Alldatasheet gave me an old datasheet for modular potentiometers, which apparently can be fitted with custom made attachments.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 02, 2015, 02:17:36 pm
content: Yep, the oil fixed it.Always remember the doctors orders: Be generous with the oil!Now I have to do a little bit of calibrating. The square wave of the calibration-oscillator has slight overshoot and undershoot with my new probes (150Mhz Probes btw.) and the peak-2-peak voltage is displayed as being a little bit low. Also the digital waveform has a slight offset. But with the service-manual this shouldn't be a problem.Or are there some really evil traps?

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2015, 05:35:51 pm
content: The probe Cal output is ONLY for 10:1 probe compensation, it is not precise enough for either Frequency or Amplitude adjustments.It is likely the overshoot you see is because the trimmer on your probe needs adjustment.This is only needed on 10:1 setting.Correct termination to the scope of fast rising edges dictates the accuracy of the displayed waveform.Check this thread:https://www.eevblog.com/forum/testgear/show-us-your-square-wave/Internal scope adjustments should only be done with accurate voltage and waveform sources, those that can supply fast clean pulses at max BW for your scope.If you must have a fiddle, mark any adjustments you make.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 02, 2015, 06:22:47 pm
content: I figured as much. So I refrained from anything too complicated and limited the adjustments to the ones I could do without expensive calibration-equipment. The main goal here was to eliminate the 0.2div offset of the digital Channel A and B compared to the analog channel, which can be done mostly without calibration-equipment because of built-in test-procedures. Anything further than that I leave out until I get the Rigol and a decent signal-generator. Service-Manual says I need one capable of creating clean square-waves up to 10Mhz and adjustable Amplitude. Hopefully I can build one myself. I already spent more than enough money on equipment for my liking

author: dom0
author_rank: Super Contributor
timestamp: April 03, 2015, 11:06:17 am
content: Quote from: tautech on April 02, 2015, 09:56:12 amNope. Can't say if it's linear or log by the taper marking. Only possible if you know the manufacturer, because there is no standard ; some mark A for linear, some for log and vice versa. So best way is always to measure it if it's lin or log.Anyways, I'd be very surprised to find a log pot in that spot.

author: Shock
author_rank: Super Contributor
timestamp: April 03, 2015, 11:15:25 pm
content: I tried looking for these pots before with no luck. Ended up buying a wrecked PM3355 unit, I don't have any spares left though unfortunately. Make sure you don't break any.If you really need one, increase your chances by checking in the service manual and cross reference it with other service manuals like I did, mine was:PP17 20% 10K 5322 101 30546There is an IC on that front panel PCB that performs math for the LCD display, they are made from unobtainium also, if I recall correctly that board is sensitive to static discharge.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 04, 2015, 06:27:21 pm
content: I probed the i2c-Bus of the scope when I tried to localize the problem. And based on what I saw, the LCD, which is a little bit damaged on my scope, is controlled by 3 chips that are updated in regular intervals. I didn't hook up a logic analyser, but I bet it should be entirely possible to reverse-engineer the protocol and hook up some DIY-Display if necessary.What I currently worry about is the ROM for the main Processor. For most ROMs the manufacturer guarantee valid data for only 20 years and that scope is almost as old as I am (28 years). So I'm thinking about getting a backup of the content of that ROM somehow. And even if it is only to ease my mind.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: April 04, 2015, 06:45:48 pm
content: I have a spare LCD and LCD parts. If I can help, send me a PM.

author: lpowell
author_rank: Newbie
timestamp: April 21, 2015, 01:42:18 am
content: Just picked up a PM3350. Does anyone know where to find the service manual free?

author: tautech
author_rank: Super Contributor
timestamp: April 21, 2015, 10:31:20 am
content: Quote from: lpowell on April 21, 2015, 01:42:18 amhttp://elektrotanya.com/philips_pm3350_pm3352_sm.pdf/download.html

author: lpowell
author_rank: Newbie
timestamp: April 21, 2015, 11:39:42 pm
content: Thank you !!!