Faulty oscilloscope - Page 1
Forum section: electronics / testgear

author: zany
author_rank: Newbie
timestamp: May 12, 2013, 01:28:11 am
content: Hello all,first post here (long time viewer of Dave's videos - thanks heaps Dave!) so please be gentle to this electronics newb.I just purchased a Philips PM3350 oscilloscope and it seems DOA. When powered on it makes a high pitched whine and the crt and lcd screen do not light up. Any thoughts what might be wrong with it? I've got no user manual so don't where to start. Any thoughts greatly appreciated.cheers.

author: c4757p
author_rank: Super Contributor
timestamp: May 12, 2013, 01:30:58 am
content: Sounds like a power supply issue or a short on the main board. Disconnect any connectors from the power supply and separately probe the power rails for shorts, and check the power supply for capacitors that have croaked. (Don't run the power supply without a load connected.)

author: zany
author_rank: Newbie
timestamp: May 12, 2013, 01:37:46 am
content: Hi c4757p,thanks for the fast reply...hmmm... as a newb, I don't feel confident enough to open it up and start probing around high voltage supplies.will call the seller and see what they have to say.cheers.

author: c4757p
author_rank: Super Contributor
timestamp: May 12, 2013, 01:42:32 am
content: Did the seller claim it worked?Quote from: zany on May 12, 2013, 01:37:46 amDepends on what you call "high voltage". Many of the higher quality scopes do a decent job of containing anything above mains. I'd have a go at the service manual - they make for very good bedtime reading - and then give it a try.

author: pickle9000
author_rank: Super Contributor
timestamp: May 12, 2013, 02:00:15 am
content: Here is a link to what I think is the correct manual.http://www.opweb.de/en/model.php?id=4447Sorry not in English

author: pickle9000
author_rank: Super Contributor
timestamp: May 12, 2013, 02:07:40 am
content: Found ithttp://elektrotanya.com/philips_pm8958_pm3350_pm3352_oscilloscope_instruction_sch.pdf/download.htmlThat should get you started

author: c4757p
author_rank: Super Contributor
timestamp: May 12, 2013, 02:24:14 am
content: I don't think the instruction manual for the "RS232-C/V24 Interface for DSO PM3350" is what he's looking for.

author: c4757p
author_rank: Super Contributor
timestamp: May 12, 2013, 02:27:29 am
content: These guysclaim to have it for $5, and in English. I cannot say how trustworthy they are.

author: pickle9000
author_rank: Super Contributor
timestamp: May 12, 2013, 02:45:03 am
content: One more time!I even downloaded it, service manual.http://elektrotanya.com/philips_pm3350_pm3352_sm.pdf/download.html

author: zany
author_rank: Newbie
timestamp: May 12, 2013, 05:08:22 am
content: Yes, there were images with signals for each channel, and the seller labelled it as tested and all functions working.Thanks to all for the responses, and especially for the English manual... been looking for that, had only found the german version. Cheers.Quote from: c4757p on May 12, 2013, 01:42:32 am

author: pickle9000
author_rank: Super Contributor
timestamp: May 12, 2013, 05:24:26 am
content: If you do decide to open it, don't forget to use your eyes and nose first. That said, it sounds like it had a hard fall, there are some high voltages in there (it uses a CRT) so do be careful.

author: Shock
author_rank: Super Contributor
timestamp: May 12, 2013, 05:47:30 am
content: Quote from: zany on May 12, 2013, 01:37:46 amMains voltages are lethal and some people on this forum have experience that makes it seem trivial.If you don't know:How to safely work on disassembled equipmentTo safely identify where the power supply is in a circuitHow to measure and ensure equipment is safe to work onDischarge circuits/capacitors and know what risks are involved with both powered or switched off equipmentIn other words how not to put yourself at risk or dieThen you need to start with that education first.That said if the power supply DC filter caps have not been replaced in that model you could start there.I have a similar model.

author: zany
author_rank: Newbie
timestamp: May 12, 2013, 06:39:19 am
content: Hi Shock,I'm an IT guy who knows about what's what in basic electronic circuits, but servicing mains voltage circuits is another matter... I've just started this new hobby, I reckon it will be a while before I get into this kind of work. Just call me chickenAs pickle9000 suggested, I might take off the cover and use my eyes and nose...

author: ddavidebor
author_rank: Super Contributor
timestamp: May 12, 2013, 08:09:42 am
content: I've a fluke combiscope with the same issue.The power supply was working, except for a line to the crt that is 20% off his nominal value.

author: Salas
author_rank: Frequent Contributor
timestamp: May 12, 2013, 08:41:00 am
content: Quote from: zany on May 12, 2013, 06:39:19 amXY mains rated capacitors located around the on/off circuitry usually in mustard color blow open in old equipment and other capacitors show high ESR frequently. The blue axial ones in Philips scopes in particular. The XY you can usually see it and smell it if damaged hard, the high ESR ones need to have a meter.

author: zany
author_rank: Newbie
timestamp: May 12, 2013, 09:44:10 am
content: Thanks for the information Salas, much appreciated.

author: zany
author_rank: Newbie
timestamp: May 24, 2013, 04:43:13 am
content: Update: Vendor was good enough to take it back for repair/replace, so I sent it back.He has offered a Tektronics 2225 as replacement with pics - seems in good condition. Looks like a reasonable exchange... gotta watch Dave's videos of this unit again.

author: brokenrestoration
author_rank: Newbie
timestamp: April 18, 2014, 11:53:56 am
content: Hi Zany,The high pitched whine is caused by the primary side of the switchmode power supply being loaded down too much,causing excessive magnetic flux in transformer T6007 which is vibrating its cores and windings to generate the sound.The supply is being loaded down because V6112 - a crowbar protection SCR - is loading down (shorting) the +48v & -48v rails on the secondaryof the switchmode.V6112 is firing because of current from R6137 on its gate.There lives a voltage protection circuit on the other side of R6137 consisting of transistors V6136 & V6137.The voltage protection circuit monitors the +5v & -6.4v rails developed on the secondary and fires the crowbar SCRthrough R6137 if the rails are out of tolerance so as to protect the rest of the scope electronics in the eventof the power supply delivering too many volts.I doubt however that the protection circuit has fired.I suspect that leakage current is making it into V6112's gate from R6137 across dust from one of R6137's pins to a nearby supply rail.(V6112 is very sensitive)If I were you, I would firstly clean any crud away from around the vicinity of R6137.Hope this helpsCheersbroken

author: FrankenPC
author_rank: Unknown
timestamp: April 18, 2014, 06:03:27 pm
content: Quote from: pickle9000 on May 12, 2013, 05:24:26 amEven to this day, I'm still nervous discharging the CRT anode. It involves an accident I had as a teen hacking our broken family TV set. My dad never forgave me for blowing a hole in the family carpet.

author: GreyWoolfe
author_rank: Unknown
timestamp: April 18, 2014, 06:48:53 pm
content: Methinks he should be thankful that it was the carpet and not you! Carpet can be replaced.

author: jlsp
author_rank: Newbie
timestamp: July 23, 2017, 01:29:59 pm
content: Thanks Zany for raising the problem and many thanks to 'brokenrestauration' for your perfect solution!My PM3350A scope had this problem too, from one day to the other after many years of good service. I was sceptical first if cleaning would help since the pcb didn't look that dirty. Used cotton swabs with a drop of alcohol, no change. Did a second cleaning round, again no change. It crossed my mind that an awfull lot of reasons can cause to trigger the safety crowbar... but then when switching on again after just waiting about an hour it DID work again! and it still works perfect after weeks!Important is to clean all lines connected to R317 (both sides of R317 and both sides of pcb), and a bit wider to be safe. One line is over 20cm!. For personal safety be sure to wait long enough untill the big capacitors on the pcb have lost their voltage after removing the power cord!Joseph

author: Electro Detective
author_rank: Super Contributor
timestamp: July 24, 2017, 10:52:34 am
content: Quote from: zany on May 12, 2013, 01:37:46 amAWESOME DECISIONYou will see many good years come and go with that type of cautious mindset