Electronics > Repair

HP4140B picoammeter

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Keith956:
I wonder if anyone's familiar with the HP4140B picoammeter? I have one which works, but has a rather large offset current in the lower readings.

After about an hour's warmup, with nothing connected to the triaxial input connector, the reading on the 1e-12 range is approx -0.350. The readings on the 1e-11 and 1e-10 ranges are -0.017 and -0.002 respectively. This seems odd, as it seems the 1e-12 reading is double what might be expected - although this may just be a coincidence.

I have tried the adjustments - checking the +/- 15V power is within spec, adjusting the pA meter amplifier offset and filter offset as per the manual, but while this gives nice 0.0 readings on ranges 1e-9 and above, the small offset on the lower ones remain.

Looking at the circuit of the A1 amplifier board, relay K1 is off on the most sensitive range. So the only leakage path would be R52 (the 99G resistor) and its parallel capacitor C22, R94 and Q26.

I have removed the covers on the A1 board and as far as I can see the covers have never been removed, R52 is clean and dust/grime free.

Any ideas what to check next?

coppercone2:
might want to spray it down with isopropyl alcohol from a can, and the front connector too. its sensitive enough that your eye might not be good enough.

what I would recommend though is to get some 1 gigaohm + resistors and hook them up to the meter to see what it reads out when you try to measure a resistor. they are cheap

floating electrometer inputs are going to be bizarre, you need to test it with a source or load

https://www.jensentools.com/mg-chemicals-824-450g-99-9-isopropyl-alcohol-16-oz-aerosol/p/484-433

that device is ancient and you just can't be sure something that old is clean. maybe get 2 cans because that is a whole lot of unit

wn1fju:
For what it's worth, on my 4140B, after a one hour warm-up, I got the following readings:

-0.546 on the 1e-12 range
0.008 on the 1e-11 range
0.001 on the 1e-10 range

With normal cabling out the front triax connector, and without a special test fixture, I think accurate readings on the 1e-12 range is kind of wishful thinking anyway. 

coppercone2:
also keep in mind the contamination could come from within, i.e. a capacitor that sprayed a little and clogged itself and the finest of mist got on something

wn1fju:
Some further observations on my HP 4140B for "Keith956".  I pulled the A1 card, opened up the covered input stage, and gently cleaned the board and the range resistors with alcohol.  After a one hour dry time, I ran through the offset and gain adjustments.  The (open input) reading on the 1e-12 range was now about 0.050, not the -0.546 I obtained previously.

Moreover, according to the op manual, once the reading is below 100 fA, or 0.100 on the 1e-12 range, the zero button on the front panel will work.  After I pressed it, I took the readings on all the ranges and got:

0.000 on the -12 range  (drifts to about 0.005 after a few minutes)
0.003 on the -11 range
0.001 on the -10 range
0.001 on the -9 range
0.000 on the -8 range
0.001 on the -7 range
0.000 on the -6 range
0.001 on the -5 range
0.000 on the -4 range
0.001 on the -3 range
0.000 on the -2 range

I think I will quit while I'm ahead!

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