PM3335 oscilloscope repair - Page 1
Forum section: electronics / repair

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: February 15, 2015, 03:50:00 pm
content: Hi,I am trying to fix an PM3335 Philips/FLuke oscilloscope.After fixing a defective plug, I got the signal on the screen, but with the addition of a 40 khz component.See the attached picture of a 1 khz square signal, with the same on the output of a TDS460.I do not know much about oscillos, but I suspect that it should come from the power supply and I am trying to look atthe electolytics there. But I have no obvious origin of failure up to now.Can you confirm that it is the power supply that should be faulty ?Thanks

author: mij59
author_rank: Frequent Contributor
timestamp: February 15, 2015, 03:57:28 pm
content: The power supply is a very good candidate for the fault.The switching frequency of the power supply according to the service manual is 45 kHz at 220V

author: Smith
author_rank: Frequent Contributor
timestamp: February 15, 2015, 05:02:57 pm
content: Sounds like the problem I had with my pm3055 ( basicly the same as yours without the digital onscreen part). Just check all psu caps on the psu board to be sure.Looks like your psu is the same as mine. You might want to replace the mains x2 cap too, they tend to crack and release the magic smoke. I just replaced mine 2 days ago.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: February 15, 2015, 06:14:28 pm
content: Thanks for the feedback.I have taken out all electrolytics caps from the PSU, and find several bad ones.The big ones from the main (68uF) seems to be OK (on the upper right corner of picture 1), as well as the 6800 uF ones(the grey ones of picture 2).I will change all the other ones, but need to order them as I do not have very much axial electrolytics,and do not see much how to replace them with axial ones.

author: Smith
author_rank: Frequent Contributor
timestamp: February 15, 2015, 06:51:58 pm
content: I just replaced them with radial caps. Just make nice tight bends and use some insulating (kapton)tape if needed. If they are wobbly you could probably use some hot glue.And dont forget the X2 caphttp://i59.tinypic.com/103euzt.jpg

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: February 15, 2015, 08:02:25 pm
content: @SmithThanks for the advice. I will see which caps I can replace with radials. It will save me some funds.I tested the two 6800 uF caps and found, testing them out from the board.A) 7402 uF ESR = 0.12 RB) 7520 uF ESR = 0.12 RAnd for the two 68uF caps of the main, I have altogether (leaving them on the board ) : 140 uF and 0.48 R.It seems that they can still go for a while.The X2 cap of 220 nF seems also to be in good shape, physically, and tested (out of the board ) at 248 nF and 2.7 R.I think I will take the chance to leave them all on the board. I am more concerned by other small electrolytic capsthat are on the other boards, but as a start, I will only concentrate on the PSU.I am not very familiar with very high voltages. I do not want to touch the high voltage, but I would like to test the PSUaside from the rest of the oscillo.Do you think this is a problem ?

author: mij59
author_rank: Frequent Contributor
timestamp: February 16, 2015, 05:34:41 am
content: If you haven't, download the service manual of the scope.Its safer to test the power supply in-circuit.You would probably need a dummy load to test the power supply stand alone.

author: tautech
author_rank: Super Contributor
timestamp: February 16, 2015, 07:42:13 am
content: Nothing special about basic test of a CRO PSU.Good manuals specify exactly where the test points are.ALL voltages MUST be to Service manual spec. (good DMM)Ripple MUST also be in spec.Many use a DMM set to AC for this and it will give a reliable reading of LF ripple.SMPS ripple will likely require another scope to quantify the ripple.PSU specs is always the first step in equipment repair.Get the Tek troubleshooting link from the Repair board, it application principles are universal.https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: February 16, 2015, 08:58:22 am
content: Quote from: mij59 on February 16, 2015, 05:34:41 amThanks,Yes, I have dowloaded the service manual. By the way, searching on google "PM3335" one finds easily the pdf manual inhttp://frank.pocnet.net/instruments/Philips/HR/T&M/PM3335-37_sd_en.pdfbut the quality of the schematics is awful and not readable.I found a better resolution on the original djvu filehttp://bama.edebris.com/download/philips/pm3335/PHILIPS%20PM3335-37-SM.djvuI could then convert it myself to pdf and get a much better pdf file (too big to post here).Below is the PSU schematics.I have a question of the output electrolytic capacitors that I am replacing.I am changing all caps :C6101,C6102, C6103,C6104,C6106,C6107, C6108,C6109,C6111,C6112,C6113,C6114,C6116,C6117,C6119,C61121.Do I need to put exactly the same caps value ? or can I increase the specifications ?For example, can I replace the 22 uF (C6117,C6119,C61121) by 47 uF (with at least same voltagespecifications ) ?And as well replace the 150 uF (C6107, C6109) by 220 uF ?Quote from: mij59 on February 16, 2015, 05:34:41 amYes, I am a little bit concern with the floating wire with 15kV output. On the other hand, owing the position of the board, it is not very easy to test the outputs in-circuit. I will see how I can devise a diverted output for testing.@tautech. Thanks for the link. I had already seen it, and it is actually useful. I have several other oscilloscopes that I can use for testing the ripple of the PSU.

author: mij59
author_rank: Frequent Contributor
timestamp: February 16, 2015, 09:18:10 am
content: There's no need to replace with a higher value capacitor, although it shouldn't be a problem.Its more important with what kind of capacitor you use, low esr, 105 C, genuine brands like Panasonic etc.

author: tautech
author_rank: Super Contributor
timestamp: February 16, 2015, 09:29:41 am
content: You should be able to acess all the critical LV voltages at a PCB interconnect.The Manual SHOULD specify the test points.Time for some study.No need to get anywhere near the PDA lead.Get a scope on the LV rails and FIND whichonehas the ripple.I certainly wouldn't go replacing smoothing caps until you find the rail that is the problem.Then you might consider testing a few others to check they hadn't dried out as well.

author: Shock
author_rank: Super Contributor
timestamp: February 16, 2015, 10:02:32 am
content: C6007 and C6008 the large filter caps have consistently caused problems on these series scopes as well as the X2 spontaneously combusting all due to the age of the components.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: February 16, 2015, 02:41:56 pm
content: Quote from: tautech on February 16, 2015, 09:29:41 amToo late, I already removed all electrolytic caps from the board.In fact, I needed to do that to order the missing parts, and actually, there were quite a lot that were out of specs :C uF V mes C uF ESR Ohms68 16 48 5.868 16 45 19.0220 25 179 1.933 40 29 140.0150 16 82 4.768 63 66 5.0100 10 75 4.3150 16 79 13.0220 10 188 2.6100 25 68 8.7100 25 54 140.0100 25 75 15.0I imagine that all the ones that are listed here need to be changed. As I do not wish to have too many references, I will put 47 uF (which I have ) for all 22 and 33 , 220 for the 150.I have ordered today at Mouser the 220 and 68 that were missing, and expect to get them for next week end. I will tell you then ifthe fix is successful.

author: Smith
author_rank: Frequent Contributor
timestamp: February 16, 2015, 06:12:20 pm
content: The pm3055 service manual shows how to test the power supply (page 163) including needed resistors and power rating. Should be the same as yours.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: February 20, 2015, 11:28:45 pm
content: I have changed most of the electrolytic caps of the power supply board using radial caps instead of axial, as theyare much less expensive. The board is not as elegant as before, but it will be OK.I put back the board, and tested it directly, and it worked !I no longer have the wiggle at 40 khz.I will just have to tune it a little bit now, to see if I can improve it a little bit, but mostly topractice on an oscilloscope that I got for free on a dumpster.Thanks you all for the advices that were very helpful.

author: meeder
author_rank: Regular Contributor
timestamp: March 26, 2015, 11:17:06 am
content: Quote from: Smith on February 15, 2015, 06:51:58 pmThe X2 cap in my PM3055 (50 MHz version) also went out two weaks ago. I still have to replace it though.Made a nice crackle and a *lot* of magic smokeThe rest of the PSU PCB looks like it is in extremely good shape though.