6181A current source mod

[coppercone2]

I found a mod for the 6181A current source where you can add a 250uA range by adding another shunt.

I am wondering if you can go further then this, i.e. add a 25uA range.

https://xdevs.com/doc/HP_Agilent_Keysight/HP%206177C%2C%2081C%20Operating%20%26%20Service.pdf

Schematic on page 51

 I see they put the current signal into a 2N4045

Am I correct in thinking that eventually the problem is that the input impedance of the feedback amplifier is too low to work with such a high shunt resistance? For a 25uA source it would be 90kOhm.

What other problems are there?

[enut11]

I have a HP 6177C and modified it with a 500uA range using a 1800R precision resistor. I tested it down to 1uA and it is surprisingly stable. The test involved running the current through a 10K 0.01% resistor and recording the voltage. The biggest problem I had was lack of resolution on the current pot. I solved this with an additional pot in series with a value of 1/40th the original pot effectively creating a fine divider over a small range.

For your purpose I would suggest you try the 250uA mod first and characterise that before going down to 25 uA. You can easily do a temporary  mod without changing the range switch.
 enut11

[coppercone2]

Hmm yeah I got the special switch from that mod.

I thought someone might have some insight that saves me work doing characterizations of equipment.


I also found like all the other similar HP supplies, the capacitors buzz, I traced down the noise using a silicone tube. I had success replacing them with foil caps. I know its not the ceramic itself, because I bought fresh disk caps and replaced them, and they buzz too on all the harrison stuff, the ones on the little rectifier PCB. Our grid is really noisy.