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Accurate Low Amp Current Probe Advice

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KungFuJosh:
Well, this is also interesting. I replaced the final 4 caps (220uF x4 and 1uF).

Before replacing those, after degauss/zero button would show around 0.25mV on the DMM.

Now it's slightly better.

So far:

Big caps (1000uF and 2200uF) affected the stability/drift drastically. Went from ~5mV PP to 0.2mV PP.
100uF caps...nothing noticeable.
220uF and 1uF: much closer to zero after degaussing.

Now to check the stability again.

Thanks,
Josh

Martin72:
The TM501 unit houses the power supply for the plug-in units.
Ultimately, the capacitors would also have to be replaced there.

KungFuJosh:

--- Quote from: Martin72 on November 10, 2024, 08:24:42 pm ---The TM501 unit houses the power supply for the plug-in units.
Ultimately, the capacitors would also have to be replaced there.

--- End quote ---

I agree, those should be replaced too. I'll get there eventually. 😉

nctnico:

--- Quote from: Martin72 on November 10, 2024, 08:24:42 pm ---The TM501 unit houses the power supply for the plug-in units.
Ultimately, the capacitors would also have to be replaced there.

--- End quote ---
I'd do a frequency analysis (FFT) of the noise (it it isn't mains related) and try to correlate that with noise in the same frequency band(s) from power supply lines or other places in a circuit before attempting to make seemingly random changes to equipment. It is very possible there are external sources which cause the noise. For example: in my own lab I can see interference around 30MHz coming and going at random times.

I'd also be wary about changing capacitors for ones with a much different ESR without checking the specification for the original capacitor. Tektronix may have designed the circuit for a specific ESR range. For example: in the TDS500/600/700 series scopes there are a few electrolytic capacitors which can't be changed by any other type like ceramic or tantalum because the ESR is too different.

Martin72:
When the capacitor was replaced, it was more a matter of drift than noise, at least that's how I understood the posts, and that's how an improvement was achieved.
So if he's already replacing electrolytic capacitors, then he might as well replace the electrolytic capacitors of the actual supply.
If I remember correctly, all supplies in the system are linear and not switched, so the ESR question hardly arises in this respect.
However, I agree with you about other capacitors in other circuits of the system.

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