EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: waran205 on May 25, 2020, 06:01:42 pm
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Hi all
I purchased a Philips PM3350A oscilloscope for £10 about 6 months ago at a local radio fair, it is in good condition and as far as I can tell works okay, although it has not been calibrated for years.
The problem is that it has a really annoying screen flicker/flash which gets worse when the screen intensity is increased, it is best described as a bright lightning type flash, usually on the right hand side of the display, sometimes a localised dot and sometimes the entire trace/screen. I was just wondering if anyone on the forum has had a similar issue with one of these, or can point me in the right direction of where to locate the fault? I'm thinking it may be a power supply issue, possibly caps - the only capacitor I've changed is the safety cap as it was visibly damaged. Is this scope worth fixing, I don't want to spend a lot of time and money on caps if the HT may be breaking down and may ultimately require replacing.
I really like the scope, but being an early hybrid I'm not sure of its long term reliability.
Any help is greatly appreciated.
Waran
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Hello Friend.
Replace all electrolytic capacitors (be careful not to check these electrolytic capacitors, it may confuse you)
The rest of the components are more reliable in the verification (electrolytic capacitors, No)
Luck.
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I used to have one and it isn't a bad scope, especially for what you paid for it. Because of the age there is a possibility that the intensity control is wearing out and if you rotate it it goes open circuit at some spots and the intensity goes to max. If the trace isn't free running then the trace may be just off the right of the screen and when the intensity goes to max for a second, you see a 'bloom' caused by the very bright dot off screen on the right. Try having a free running trace so you have what looks like a solid line across the screen and rotate the intensity. If the entire line gets very bright and wide, it is a worn control.
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Power supply.
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I had the same problem. I had purchased this used PM3350 scope in 2006 from a research lab in Chicago. It worked all these years very well but developed this problem recently.
As per normal recommendations, i checked all the electrolytics. Almost every single cap was bad in the sense that the values were very much different and ESR was bad. For example 47uf caps were reading only 18uf, 68uf were 22uf, 100uf were like 24uf and 150 was like 29uf. I changed all the caps with the new ones. (Except for 68uf/450v and 6800uf/10v. These were good.)
It didnt solve the problem. The trace was jumping, the entire screen would flicker and sometimes there would be a bright spot at the end of the trace.
The Intensity pot on the front panel changed the intensity smoothly. The flicker was present at any position of the pot. I found these to be completely metal enclosed and sealed so i thought that i must look elsewhere first.
There is one pot on the power supply board R6408 that is used to adjust the reference voltage used in various circuits. I suspected this to be faulty because, when I pressed th AUTO SET button, it would function differently each time I pressed it. I measured the reference voltage. It was jumping all over from 9.74 volt to 9.95 volts. It wasn't steady as it should be. I replaced this pot with a multiturn sealed BOURNS potentiometer and set the reference voltage to 10 volts as stated in the service manual and voila! It solved the problem! The scope display was steady, the intensity was steady at all positions of the INTENSITY pot on the front panel and it stopped flickering.
By replacing the caps in the power supply, the calibration voltage amplitude came back to 1.2 volts(it was 9.6 volts before the caps were replaced and reference voltage was set correctly). But the CALIBRATION frequency remained at 1.92Khz in place of 2Khz as stated in the manual. There is no adjustment for this, so I left it as it is.
Hope this helps!
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Sorry, there is a typo in my reply. The calibration voltage was 0.96 volts not 9.6 volts.
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I have a Philips PM3350 that had high amount of HF noise Replacing all the power supply caps fixed that issue. For the supply I used low ESR type caps. Now the scope does not want to booth up. I found that all the blue PHILIPS 030K0 caps I tested on the plug-in PCBs read low. Does anyone know that the specs are for these PHILIPS 030K0 caps? Are they LOW ESR are type?
Thanks