EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: eurekaiv on November 04, 2020, 08:30:35 pm
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I recently picked up a Hameg 203 analog scope for relatively cheap and it's working pretty well outside of this weird issue on the triggering/time-base section. The wave sorta jumps off to the right, then flickers back until it matches up with the main input signal. My hunch is this is a capacitor failing but please see the vid for the symptoms. If it is a cap failing, where do I look? I would like to avoid shotgunning 40 caps into this $50 scope.
I'm not well versed in the engineering side of things but I do fix electronics constantly and I've built probably a dozen tube amps myself so I can handle a soldering iron if someone can point me in the right direction. Thanks in advance for any help.
https://youtu.be/RIaMor9EJJ0
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Hello and welcome to the forum.
What are your knowledge of oscilloscopes?
It is possible that this is simply a wrong setting of the trigger level or transients in the input signal.
Have you tried manual trigger and also automatic? It's the same thing ?
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I have basic scope knowledge... essentially, plugging in a signal and looking for that signal in a circuit, and an understanding of how to measure that basic signal based on the size/volt/time settings. Beyond that, none. I have a small Siglent digital scope but I've been looking for a smaller analog one that wasn't too expensive for awhile, just because I like them.
This seems to happen at any setting on the time scale but it's easiest to see when the wave is spread across the screen somewhat as in the video. I don't think the auto/manual trigger switch made any difference but I'd need to double check to say for certain.
I forgot to mention that when I got it, it was having trouble getting the trace up but I was able to narrow this down to a dirty switch for the component tester on/off. I also tried cleaning the switches and vernier control for the time dial, that helped eliminate some noise when switching (though it was working at that point) and left me with only this repeating strobe-like effect that you see in the video.
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It looks more like a bad contact, bad solder, than a breakdown due to a faulty component
I will describe what I would do if I had this oscilloscope in my hands:
1) cleaning all switches and switches with a good product to clean the contacts ... see the topic deoxid!
2) visual inspection of electrolytic capacitors and solders, especially near the power supply .... repare bad solders.
3) Check if this problem can be caused by tapping the circuit board with the insulated handle of a screwdriver while the oscilloscope is in operation.
Try to locate where.
After this, we will need another scope to look for waveforms in the trigger circuit
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I see it's an HM203-4.
Set the hold off to 0 (completely anti-clockwise.)
Test the oscilloscope with the calibration square wave.
Is the focus really that bad?
Is that the best you could adjust it?
Try with the +/- trigger button if the fault continues in both switch positions
Schematics only in german version of service manual.
https://cdn.rohde-schwarz.com/hameg-archive/HM203-4_deutsch.pdf
I watched the video again, the trigger variation is cyclical, it repeats on a regular basis, it looks like ripple in a power supply.
Check the power supplies, measure the ripple of the power supplies.
A classic failure in this oscilloscope is a bridge rectifier with an interrupted diode and operating in half-wave
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I just double check the AT/NORM function as suggested and it’s working perfectly fine today. Apparently my cleaning routine from yesterday needed some time to sink in. Thanks a ton for the replies. These will all be useful if I start having trouble again.
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Just for your information, the problems associated with the ripple of the power supplies appear or disappear depending on the voltage level of the electrical network.
It is therefore possible that you no longer notice the fault today simply because the voltage of the electrical network is higher than yesterday.
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So I’m still having this problem even though I thought it had gone away. I did check the power supply for ripple and I have perfectly smooth DC out everywhere I would expect. I did replace the bridge rectifiers since one was testing a bit different than the other but results are the same and +/-12V looks great. Any other suggestions?