EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: kblue on June 02, 2023, 09:09:25 am
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Hello!
I recently got a Fluke/Philips ScopeMeter 97 (Philips PM97 to be exact). I already replaced the four problematic capacitors on the display board (two of them already leaked a tiny bit). The new ones don't fit quite perfectly, but SMD work isn't necessarily my forte either. The device works perfectly in all measuring ranges. There are only two problems left:
a) One of the Fluke 1:10 probes (the gray one for channel B) makes any channel it its connected to... noisy. With other probes (Rohde+Schwarz) everything is fine on channel B. Which would rule out channel B of the PM97 as a problem. Can such a Fluke probe be repaired in a meaningful way? Probably not, right?
b) The other "problem" that makes me suspicious concerns the backlight. As soon as I turn on the backlight, a high-frequency sound is heard. It's kind of like a coil of some graphics cards when the FPS goes too high. This occurs equally with battery operation and mains operation. This doesn't seem right, does it? Mayhaps my replacement SMD Caps are no good (even if measured good). As far as i know, some of them are for the EL Backlight. I could order some quality panasonic SMD caps for another replacement.
Best wishes from germany and thanks for any suggestions in advance. Have a nice day!
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1 dont think the probe is repairable ... i would switch for other
2 i would try to get an idea of how the backlight driver / dc-dc conveter is made, surely or maybe some bad caps, bit it could mean it is working outside it's frequecy specs ? try to get the working frequency if it's okay or not, i have seen in past the coils vibrating at high frequencies (what you ear)
in some case: sillicon heped to eliminate vibration, some rosin injection in the core helped too etc .... i dont see the internals
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The audible noise with the backlight on is likely coming from the small transformer T1301, which is driven by transistor V1307 which oscillates to generate approx 100V for the EL backlight. There is a feedback loop via a pair of 47V zener diodes for simple regulation. If you have the service manual, then this should be possible to find in the schematic and assembly drawings. Did you change the capacitors in this part of the PCB, e.g. C1302?
As suggested above, some silicone sealant or nail polish applied to the transformer coils might help reduce the audible noise if the EL driver is otherwise working fine. This is a pretty common issue as these EL drivers age.