Electronics > Beginners

1X probe acts weird (needs compensation?)

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beowulfenator:

--- Quote from: ArthurDent on September 13, 2018, 06:17:41 pm ---Try putting a capacitor in series with the x1 probe on CH2-4 and see if you can simulate the same condition. You probably will need a fairly small cap to get the same waveform. If this can be verified it means there is a cap in series with CH1 but not CH2-4 the way you have it now set. If the 4 channels are set exactly the same but CH1 give a different display, it may be a dirty switch/relay that shorts out the cap to go from for A.C. to D.C. coupling.

--- End quote ---
You're right! A capacitor in series makes other CH2-4 behave like CH1. At 100 nF and above there is no effect. At 1nF the effect is greater and at 10nF it is less pronounced. Channels are configured exactly the same (DC coupling).

So do you think it's because of a problem with the CH1 relay? A dielectric gap between relay contacts that should normally be shorted?

David Hess:

--- Quote from: beowulfenator on September 14, 2018, 08:12:46 pm ---So do you think it's because of a problem with the CH1 relay? A dielectric gap between relay contacts that should normally be shorted?
--- End quote ---

Or a crack somewhere later in the high impedance circuit before the FET buffer.  The whole thing is part of a hybrid.

Compare the DC response of channel 1 with the AC response of any of the working channels.  If they are identical, then the problem is probably with the relay.

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