Philips PM3335 scope repair - Page 1
Forum section: electronics / repair

author: Trax
author_rank: Regular Contributor
timestamp: August 08, 2015, 12:58:49 pm
content: I got a defective Philips PM3335 oscilloscope when i try to power it on it just makes a high pitch noise and nothing else happens.CRT is off and the LCD does not show anything.I guess the power supply is broken?I have removed the power supply PCB and when nothing is plugged it and i power it on it does not make the sound anymore.When the large multi pin connector is connected it does again the noise.Should i just blindly start replacing capacitors? Or is there any know how around here which once are most likely broken?

author: tautech
author_rank: Super Contributor
timestamp: August 08, 2015, 10:14:03 pm
content: Quote from: Trax on August 08, 2015, 12:58:49 pmIt would seem from the described symptoms the PSU is overloaded and failing to start.That points to a shorted component across one of the supplie's rails somewhere in the scope.Measure each rail and the lowest and furtherest from spec might give clues as the where to start looking.Tantalum caps are often the first to suspect, often failing short and quite visibly fried.But it could be a number of possible faults, download the Tek troubleshooting guide in the Repair Docs & links thread and do some reading first.Do you have the Service manual with schematics?

author: dom0
author_rank: Super Contributor
timestamp: August 08, 2015, 10:19:54 pm
content: I'd start to trace and replace bad tantalums. I just connect a PSU across each power supply rail, set it to max. 500 mV, few hundred mA and follow the power traces with a DMM.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: August 08, 2015, 11:36:59 pm
content: The high pitched noise is the Short-Circuit Protection inside the PSU.The first action I would take is to sniff around the unit if there is the distinct smell of burnt electronics.After that, a visible inspection of the boards would be in order. Search for obvious short circuits (loose screw, metal chips, copper - also get a can of pressurized air and blow on each board to remove any loose metal that could cause a short circuit. Accumulated dust on the High Voltage-Parts at the CRT can also cause enough current-flow to trigger the protection-circuit). Also look for capacitors that look abnormal and measure the resistance across them. Fortunately, Philips used Axial Capacitors and leaded solder, which means that you can easily desolder one pin and then measure the DC-Resistance of the cap. In a damaged PM3320A I had, one Capacitor failed by becoming a resistor.As a precaution, I would replace all caps on the PSU-Board, as it is located only 1 or 2cm from the hot end of the CRT and if I remember correctly, the case didn't look like the fan in the back causes much airflow in that particular area.If that doesn't help, I would suggest to continue with dom0's method of applying 500mV to the rails and searching for the short circuit. On the PM3320A, the Philips Engineers put quite a lot LC-Filters on the boards, which makes pinning down the area of the short circuit relatively easy, provided the multimeter is accurate enough.

author: Shock
author_rank: Super Contributor
timestamp: August 09, 2015, 03:58:05 am
content: Take lots of photos.My guess is the SMPS, rectifying diodes, smoothing primary or secondary caps, X2 cap. It seems from the sound it's switching but not at 30-45KHz which should be inaudible. I'd avoid recapping and running around in circles chasing caps on other boards unless you have a short present and can prove that the power supply is going into shutdown rather than not starting due to an absent load. Once you have tested that the power supply is operating within spec (voltage/ripple) it's a different story.But be careful as on that board there is 120, 370, 2000 volts present. The service manual has some info on running the power supply with a dummy load.

author: Trax
author_rank: Regular Contributor
timestamp: August 09, 2015, 06:11:19 am
content: I will try to solder some wires to the PSU connector and measure the voltages.Where can I find the service manual for this scope? So i know what to expect?

author: Shock
author_rank: Super Contributor
timestamp: August 09, 2015, 10:31:02 am
content: Quote from: Trax on August 09, 2015, 06:11:19 amhttp://bfy.tw/1DXK

author: Shock
author_rank: Super Contributor
timestamp: August 13, 2015, 04:49:39 am
content: Any luck? Take some pics and let us know how you get on.