Fluke 8842A input capacitor?

[FrankBuss]

I recently found out that my Fluke 8842A has an input capacitor of about 650 nF when measuring DC voltage. Couldn't find this in the manual. Is this normal? The manual specifies only 100 pF for AC voltage, if I didn't miss it. Looks a bit high to me. And I couldn't measure it with an external DMM, but it behaves like this when charged and then discharged with an external resistor.

[mimmus78]

There you go!
As you posted this question here instead of the repair subforum what I can tell you is that there is no way to recover from this.
I've seen many of 8842A with this cap busted, it's unobtanium, only thing you can do is to salvage the internal reference for building some voltage reference.

(OK it's a joke, but I cannot resist).

[Tektron]

 My 8842 has 2.5 nF at ranges 20 and lower (unstable and go bit low to 2.3 nF at fast measurement rate) and 76 pF at ranges 200 and 2000 in DCV mode. In the ACV mode it has 0.223 nF at all ranges. Measured with Tektronix TX3.

[FrankBuss]

My 8842 has 2.5 nF at ranges 20 and lower (unstable and go bit low to 2.3 nF at fast measurement rate) and 76 pF at ranges 200 and 2000 in DCV mode. In the ACV mode it has 0.223 nF at all ranges. Measured with Tektronix TX3.

Are you sure about this? I tried to measure the input capacitance with an external DMM (BM257s) as well, but it shows only 100 pF. I guess it is a bit tricky to measure this because it is not just a capacitor. But when I charge the internal capacitor with 3 V and then connect an external 100 megohm resistor, it needs about 72 seconds to discharge to 1 V in the 20 V range, in slow measurement range, which is about 650 nF.

[Kleinstein]

The Fluke 8842 has the option to enable a low pass filter at the input. If the filter is enabled, there is a capacitance of about 660 nF at the input. There is still some 100 K of protection in series. So it depends on the way you measure if you see the capacitance or not.

The manual include a schematics for the input.

[FrankBuss]

Thanks, found it in the manual, section 5-8, Analog Filter. Looks like it is always enabled for slow rate. But it doesn't specify the value, I had to look at the circuit for it at U304. But maybe they expected that customers study the circuit anyway before using the instrument