Philips PM3305 DSO repair and troubleshoot - Page 1
Forum section: electronics / repair

author: joulethief
author_rank: Newbie
timestamp: January 25, 2021, 07:53:29 pm
content: I am trying to get this scope working. When I plugged it in originally the RIFA cap across the line exploded, that's been replaced, unfortunately that has not solved all the problems.When the scope is switched on the graticule illumination works, the power LED lights and the memory module does its start-up routine okay, however the CRT displays no traces and the PSU makes a loud squealing noise. When the power is switched off half of a trace appears in the middle of the CRT for a brief moment before fading out. I couldn't hear or see any arcing or smell any ozone, but maybe it's happening somewhere out of sight.According to the manual the PSU is a SMPS that supplies 180V, -180V, 38V, 12V, -12V, 5V as well as -1500V to a voltage multiplier.I measured the lower voltages and both 12V and the 5V supplies were about right. The 180V, -180V were around 203V and the 38V was 44V. I thought that maybe something was shorting on the main board, so I disconnected it to see if the voltages would return to the normal level. They got even higher.The fact that some of the voltages are fine is really odd to me, I would've thought that if it was being overloaded all the voltages would be out of whack. Not only some of them? Even more strangely on the back of the PSU board there are blackened areas around the main rectifier, some darkening on the switching transistors and more blackening around a set of 3 diodes for the 12 and -12V. Which implies to me that it's probably the 12V connectors which are shorting somewhere? And in that case why do both of those measure with the correct voltage?The resistance between ground and the 12V pins outside the PSU is about 400 Ohms, I don't know if this is normal or not. And there doesn't seem to be any shorting within the PSU board itself.I'm new to electronics and I'm really lost with this and don't know how to proceed, can anyone help me?Also sorry if this is poorly formatted or in the wrong place.Here's the manual with a circuit of an almost identical PSU:https://archive.org/details/PHILIPSPM32153215USERVICEMANUAL/page/n111/mode/2upThat's the manual for this specific scope, but it doesn't have many schematics:https://bama.edebris.com/download/philips/pm3305/pm3305.pdfPicture of PSU board

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 26, 2021, 12:26:53 am
content: Sorry I haven't been and checked for schematics.On a linear supply secondary voltages that are not regulated will vary high or low with mains voltage variations or transformer mains voltage setting (220/240V).On a switching power supply often the regulation loop is taken from the secondary with the highest power, but can also be a compromised mix of several secondaries.(Over)loading a secondary that is not in the loop will drag it lower while having little effect on the regulated ones.(Over)loading a secondary that is in the loop will have little effect on it (until the supply shuts down from abuse of course) but will cause the secondaries that are not in the loop to rise a little (think in % not in Volts).

author: joulethief
author_rank: Newbie
timestamp: January 26, 2021, 10:41:02 am
content: So there could be something wrong in this part of the PSU?Maybe T201's windings have shorted?T202 has a single winding with taps along it for all of the output voltages, if that's damaged is there any way to tell?