Hello guys,
--- Summary ---
--> Something burnt down in my oscilloscope power supply from ~1994 (-->psu.jpeg)
--> I can't see any obviously burnt components.
--> Now a strange clicking noise comes from it (mp3 as attachment).
--> What could cause this noise?
--> Voltage spikes compared to earth but not compared to ground pin on power supply (--> spikes_turned-on.png)
--> Spikes continue a little while after power is switched off
--> Sometimes everything seems to work (without load), and sometimes not --> None of the "essential" components burnt?
--> I can observe many things (see detailed desctiption) but don't have enough experience and knowlage to combine them into a conclusion
--> Any help would be so nice!
--- Detailed description ---
A few weeks ago, a very strong smell like burnt semiconductor came from my HP 54540A oscilloscope and all the lights and the display were flashing randomly.
I removed the cover and the smell seemed to come from the power supply (HP part no. 0950-2369), from the region marked red in psu.jpeg. I looked over it and saw no obviously burnt transistors, MOSFETs etc.
I probed the transistors, MOSFETs and diodes with a multimeter and none of them seem shorted.
I removed the connector going from the power supply to the mainboard and turned the supply on.
Now a pretty loud and high-pitched clicking noise comes from it around every 25 ms. I recorded it with my laptop microphone (mp3 as attachment to this post).
From what kind of compoment could this come from?
I also looked at the outputs with another oscilloscope. What is really strange is that I can turn the power switch on and it works just normal (--> normal.png) and sometimes there are these spikes, around every 25 ms like the noise (--> spikes_turned-on.png). One spike can be seen in detail on spike.png.
I don't know if I'm wrong, but I conclude from that that it's none of the really "essential" components that is burnt. Am I wrong with that assumption?
I noticed a few other things:
--> Another thing I noticed is that the spikes and the noise continue a little while when I turn the power switch off (--> spikes_turned-off.png).
--> There is always one of these spikes when I turn the supply on. Sometimes this is the only spike and the power supply seems to continue normally (--> powerup-normal.png)
and sometimes the spikes continue and the offset of the output approaches zero (--> powerup-fail.png)
--> Connecting a 100 ohm resistor across a 5V pin and ground seems to trigger the spikes (also turning them off while the power switch stays on), but not always...
I notice that I observe many different hints, but since I study computer science with a lot of EE lectures since only half a year, I don't have enough experience and knowlage to combine them into a conclusion which part(s) might have failed.
So any help and hints which components to look at closer would be very much appreciated!