Another faulty Philips PM3050 scope - where is the short? - Page 1
Forum section: electronics / repair

author: 1980s_john
author_rank: Contributor
timestamp: May 10, 2025, 08:29:57 pm
content: Hi,I already had one working one, so when a cheap faulty PM3050 appeared on ebay I bought it. I first checked the caps on the PSU board, and replaced the 0.22uF Rifa, two 150uF and one 100uF Philips blue can caps. Both scopes are the later type with cream front panels. I powered the faulty scope on and it made a high pitched squeal, which I understand means the PSU is shutting down.I decided to compare the resistances between the power rails and ground on the two scopes. Here is what I measured on the good scope using a multimeter, note these are lower of two readings with DMM lead polarity switched between connecting black lead to ground or red to ground.Measured on the back of the PSU 20 pin connector, with all boards attached, in ohm (nb will vary depending on meter used):Pin 1 (+12V) 435Pin 2 (-17V) 507Pin 4 (+5V) 222Pin 6 (-6.4V) 1.2kPin 7 (+48V) 5.5kPin 9 (-48V) 21kPin 16 (+17V) 1.2kPin 18 (-12V) 535On the faulty scope all the resistance measurements were similar except for -12V, which measured 20 ohms, so I think this has a short and causes the PSU to go into protection and not start up.I disconnected the PSU from the other boards and rechecked resistances, the PSU -12V line measured 25kohm, but the rest of the scope still measured 20ohm, so the fault seems to be somewhere in the body of the scope.Before I start disassembling all the boards (getting to the analogue input board at the bottom looks like a right pain) has anyone had a similar fault please?Regards,John

author: tinkerer124
author_rank: Newbie
timestamp: May 20, 2025, 05:38:36 am
content: I have two of these scopes in the older model, with the dark front panels. Both had the same issue on startup, likely had been sitting 20+ years before I plugged them in. What I did to fix both was I unplugged all of the cables from the PSU board except for the AC inlet connector and switched the PSU board on with no load. No whining sound was heard and after letting it run for 20 minutes I plugged everything back in and it has never had an issue since then. To my best understanding the cause of this was the smaller electrolytic caps needing to "reform" after being unused for so long and applying voltage with no load to drag it down allowed the caps to "heal" themselves internally. If that works then it suggests you might need to replace many or all of the electrolytic caps at some point.Schematics are seemingly unavailable so you may have to trace the issue yourself if it is indeed a board in the scope causing issues. Capacitors are a more likely cause of shorts/low resistance paths than most other components, so I would start there.Good Luck!

author: ondraN
author_rank: Regular Contributor
timestamp: May 20, 2025, 07:40:49 am
content: About a year ago I got a PM3050 in poor mechanical condition. The plastic was brittle and broken. I took it apart. There are nice axial electrolytic capacitors, so I soldered them and was happy. But my joy ended the moment I started measuring them. Almost all of them had a much lower capacitance than the data on them and a large series resistance. Almost half of them had a large leakage current. So in the end I was glad that I didn't try to get the oscilloscope to work.

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 20, 2025, 08:29:32 am
content: A side note on these scopes:On the back, there's a little compartment where you can stuff the power cable.If you have the scope on your shelf, close that opening with gaffer tape or plug it. In my scope, a mouse decided to make my PM scope it's living room. That did not end well for both scope and mouse.

author: 1980s_john
author_rank: Contributor
timestamp: May 22, 2025, 08:28:07 pm
content: Hi,I traced the 20 ohm short on the -12V rail to the front panel. I swapped in a panel from my working scope and it powered up fine and displayed a trace (not tested further). It looks like the LCD backlight (a tiny incandescent bulb) may have shorted, I replaced this on my working scope with an LED strip, but the result wasn't that bright, so I'll use the bulb from the first scope and see how they compare.(I'll also replace the mains switch on the first scope as this has just decided to no longer latch on - a common fault with Preh ME5 type switches, which Philips used in many of their 1980s computer monitors including the Commodore 1084).Regards,John

author: 1980s_john
author_rank: Contributor
timestamp: July 28, 2025, 09:56:50 pm
content: Hi,I've finally swapped out the filament bulb for an LED, the bulb measured 20 ohms out of circuit so an odd failure which led to a dead scope. Both scopes are now working fineRegards,John