PM-3211 Philips oscilloscope repair, dead power supply - Page 1
Forum section: electronics / repair

author: Satbeginner
author_rank: Regular Contributor
timestamp: May 14, 2017, 08:10:48 am
content: Hi all,I bought this oscilloscope as a repair project.It started with blowing the fuse at power on.I found one of the BD237 broken, replaced it with an BD681 (higher voltage, higher current).No the fuse blows after 20-40 seconds.I can not find a clear short on any of the blue capacitors.Even when I unplug the outgoing cables there are no voltages.I disconnected the HV tripler input capacitors, but to no avail.Are there any members with previous experience who can help me forward?Un saludo,Satbeginner (Leo)

author: oldway
author_rank: Super Contributor
timestamp: May 14, 2017, 10:13:48 am
content: A well known issue of some Philips scopes with this kind of power supply is a faulty ferrite transformer.The problem is a short circuit between turns or layers in the HV coil.The switching transistors became too hot and go to short circuit after some time.That's probably the reason why the fuse blows only after 20-40s with new transistor.

author: Satbeginner
author_rank: Regular Contributor
timestamp: May 14, 2017, 11:50:45 am
content: Quote from: oldway on May 14, 2017, 10:13:48 amHi, I checked everything again, and for testing purposes I replaced the fuse with a 150 Ohm resistor.I just won't start oscillating and draws a high current.With the little ferrite transformer removed there is no high current.So it is the single (primary) winding that has a short, because first a transistor dies, and because of the high DC current the little transformer burns......Hmmm, any success rewinding them?I will try to take it apart to count the windings, but not hopeful....Un saludo,Satbeginner (Leo)

author: oldway
author_rank: Super Contributor
timestamp: May 14, 2017, 05:53:25 pm
content: No, that's not the little ferrite transformer who is defective but the big one....A short circuit in the High voltage coil of the transformer.Here a picture of the transformer with the HV coil removed and auxiliary HV board added.

author: Armadillo
author_rank: Super Contributor
timestamp: May 14, 2017, 07:50:18 pm
content: Blueskull should add 1 to his statistic count of shorted transformer.!

author: Satbeginner
author_rank: Regular Contributor
timestamp: May 23, 2017, 07:16:04 am
content: Quote from: oldway on May 14, 2017, 05:53:25 pmHi all,sorry being off-line a while, we had some building going on in the house...Anyway, I managed to re-use the little ferrite transformer, the one that drives the Basis of the two BD237 transistors. (not in the picture, I put it on a small plug-in board)If I temporarily replace the fuse with some power resistors the power supply "sort of" tries to work, generating lower voltages, but as soon as I increase the primary current by reducing the temporary series resistor the BD237's will immediately die again.So it looks like I do have a transformer problem, but........ Mine is different?!?I now understand that some repairs were done by removing the HV winding and add a separate HV supply?In my situation, the transformer is in a sealed, plastic box, so this is a complete No-Go?See the attached picture of my Power Supply.Un saludo,Satbeginner (Leo)