Boonton capacitance model 72bd don't work

[Halide]

Hi,
I have a problem with my boonton capacitance model 72bd. It's show all the time the ~6pf value and i can't set the zero pF. I checked the voltages and all is good. I don't have a service manual and I need a sugestion what I should do next.   

[doktor pyta]

Hi,
I expect model 72B to have identical front end.
Take a look at the full manual located here:
https://www.google.pl/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwi43NWqmtvZAhVmDZoKHS9XCXQQFggqMAA&url=http%3A%2F%2Fradiosurplus.it%2Fpdf%2FBOONTON%252072B%2520Instruction.pdf&usg=AOvVaw3CE2996BQ1elifkdeJLrWk

[Halide]

Hi
Thanks for this manual. Did you ever fixed this device and mayby you can prompt which part can be damaged?

[doktor pyta]

Nope,

I'd check :

1. supply voltages according to the schematic,
2. the signal at A101 input ("analog signal"); then You could determine if the bug is in the analog part or in the digital voltmeter part.
3. AFAIK C404 is the differential variable capacitor used to adjust zero reading when the input is open. This capacitor could be damaged or touched by someone.

[Halide]

Thanks for fast reply. Currently I am out of my lab and I can't check it. When I come back I immediately chek all this points.

[korlatos]

Have you turned the Zero control far enough? The Zero control can actually travel beyond the region where you can move it easily. I attached the excerpt from the manual where this is discussed.

[Halide]

Today I have a time to look at this capacitance meter. Korlatos you have right, someone  turned the Zero control too far. Thanks everyone for help.