First steps testing and troubleshooting bench power supply

[bitseeker]

If you have a DMM with sufficient bandwidth on AC, you can use that to measure the amplitude of the noise as confirmation.

[guymo]

I don't think my DMM is up to the task. Haha, I should probably buy a really good DMM to help me fix this ancient and cheap PSU... and on down the rabbit hole.

Meanwhile I found this thread https://www.eevblog.com/forum/repair/linear-psu-where-does-these-spikes-come-from/  with similar looking spikes on a PSU output. The conclusion there was not 100% convincing but that it came from noise on the mains. Hmmmm.

[bd139]

This is how it always starts and the first thing you after this is buy a bust power supply to fix the bust power supply with and then your scope will blow up so you'll buy another one to fix it

Looks to be around 166KHz which is in the reasonable range for switching supplies and lots of interference. Might be picking up French TDF time signal https://en.wikipedia.org/wiki/TDF_time_signal

Ergo, can you stuff the case back on it if it's open and check the noise on the output. If it's still noisy, I'd go around and see if you can find any wall warts and appliances and try and turn them off. I forever end up chasing odd noise problems inside analogue scopes only to find that it's picking up my lamp's power supply transients etc. If that makes no difference then we're almost certainly dealing with oscillation.

I would replace C5 and possibly the trimmer in that case as in the manual that says it's critical for the purpose of oscillation control.

[guymo]

Law of unforeseen consequences:

My in-laws are coming today so I have to put all the electronic stuff away. While packing up I came across a Pomona shorting bar. It occurred to me that this could be used  to connect one end of the output to ground. Yes, it fits, so I connected it and set up the scope for "one last look" before the weekend. Hooked up a 100uF cap across the output and measured, and what do you know, no spikes. Both channels of the supply are behaving themselves, reading a flat line on the scope (a bit "fuzzy", +/-1mV at most).

Obviously it could be that there was some external cause of the spikes that is not there this morning. But it seems more likely that it was my fault, and that lazily grounding one end of the supply with a 50cm test lead caused problems. Could that be down to inductance in the cable? I don't understand inductance.

So I think I am back to where I was the other night, in need of replacing the output caps.

I'm really grateful for the help and support I've received here. Just not being completely alone in puzzling over this is a great help, but the specific advice and navigation has been fabulous. Thank you! No doubt something else will go wrong and I'll need to revive the thread later.

[bd139]

Damn those in-laws. Mine have to put up with it

That might actually have been the issue. If it is, phew! Good find! 1mV or so of noise is pretty good. My Thurlby TS3022 is around 2.5mV noise. I suspect that needs refurb as well but I can't be bothered at the moment

Inductance is hard to visualise and think of really. There are two domains to think of it in; the time and frequency domain. In both domains it is exactly the opposite of the capacitor. Fundamentally in the time domain, if you stick a voltage across it, the current rises from 0 to infinity exponentially at a rate defined by the inductance. When you remove the voltage source, the magnetic field collapses and generates a high voltage to try and force the current to flow again (this is incidentally basically how switching power supplies work). In the frequency domain it's a frequency dependent resistor. The higher the frequency, the higher resistance (generalised as reactance in these things). Typically outside of RF, which is purely concerned with high frequencies, they are used to get rid of high frequencies like oscillations which is why you see a ferrite bead on a transistor leg occasionally. That's a little inductor that stops high frequency oscillation.

You have three modes that could have caused noise which I will detail:

1. The loop acted as an antenna (electric field coupling). This could be a resonant length of some RF flying around in your house. This would develop a voltage across it. At that size this is possible but unlikely I would suggest. Anything resonant at 166KHz is usually pretty big as the wavelength is 1.8Km!
2. The loop acted as an inductor (magnetic field coupling). Try this as a demonstration. Take your oscilloscope's probe, stick it on 10X if it's switchable and clip the ground wire to the tip creating a loop. Set it on the lowest volts/div setting and go and wave it around laptop chargers, televisions etc and see what happens. Basically the magnetic field lines of force will cut the loop and induce some current. Because the probe impedance is so high it'll result in a visible voltage. Some EMC testing tools actually use things that do this on purpose.
3. The additional inductance of the wire created an accidental resonant oscillator somewhere with a capacitance inside the device.

Have fun and let us know if anything else goes wonky