Keithley 602 question

[1audio]

I recentley aquired a Keithley 602 (not A, B or C). No batteries, except the dead PX1. The original batteries are avaiable for $17 ea and it takes 6. Except for battery life is there any reason why I can't use 6 generic 9V batteries? And its not obvious why I cannot replace the 1.35V battery with a 1.5V battery. It came with the original manual and a BNC to triax adapter. Is there any need or interest for me to scan and share the manual? The one posed at Textronix loos the same only with better illustrations. Maybe the schematic could have a better scan if there is interest.

[donlisms]

I don’t remember what the manual says about the 9v batteries.

The 1.35v mercury battery goes into a divider chain that is calibrated to produce specific resistance-measurement currents from 1.35 volts.  There is an expectation that the voltage will always be 1.35; mercury cells were apparently very stable. 

The 1.5v battery will work, but the resistance readings will be off, and will change more because the battery discharge voltage changes more.  The divider chain could theoretically be retuned for 1.5 volts.

Some people apparently replace the battery with a voltage reference or regulator.

[DH7DN]

Hi and thanks for the background information about the mercury cell battery. My Keithley 602 (solid state) electrometer has a dead mercury battery which needs to be replaced by a modern version.

According to the manual, the voltage source is obtained from batteries B201 and B202 for the Ohmmeter Operation (Normal Method).
If the readings of any battery are below two thirds (e. g. < 6V), the batteries should be replaced. If B203 is less than two-thirds full scale, it should be replaced.
B203 is specified as "Battery, 1.35V". B201, B202, B204, B205, B206 and B207 seem to be 9V Zinc Carbon batteries - nothing special about those. My unit has a battery compartment with 9V battery block connectors, probably modified by the previous owner. 

Will also have to check whether the old instrument has other issues.
I am thinking about using a silver oxide coin cell in combination with a high-resistance voltage divider (e. g. in the order of 1...10 GOhm perhaps?).

I'd be interested in a fresh scan of the schematic diagram if possible (Figure 5-6 on pages 5-9 and 5-10) 

[donlisms]

If I recall correctly, there is only one resistor that needs to be changed in the current source range switching to go from a 1.35v source to a 1.5v source.  Similarly other battery voltages.  The downside is that another battery likely doesn’t have the same consistent voltage as the mercury cells did.

As you say, the other batteries are nothing special.  B201 and B202 drive the input stage(s), and B204-7 drive the output stage at +/- 18 volts. 

[1audio]

My current debate- Wheather to make a PCB w/ a LI battery and a low standby current regulator for 1.35V e.g.: https://www.mouser.com/datasheet/2/308/1/NCP170_D-2316658.pdf   Or to change to 1.5V (below) and use a single rechargable LI battery with an internal regulator e.g. https://www.fenixlighting.com/products/fenix-arb-l14-2200u-rechargeable-battery?gad_source=1&gclid=CjwKCAiA0rW6BhAcEiwAQH28IorUnpK5MZ-QkaNyFGmx5qnFdn1w_GrHhc-b04ZLbMJio9gW9ElsQBoCLhMQAvD_BwE  Changing 3 resistors and grafting a single AA holder would be the least work I think. But a PCB to replace all those long gone mercury cells may be useful elsewhere. On paper The standby draw .5 uA gives a standby life of 300 years with a generic small  1500 mAH (not button) LI cell. In otherwords not an issue.

WRT the 1.35 battery, I found this specific set of instructions:
Question :
What battery can I use to replace BA-24 in my 600B or 602 Electrometer?

Answer :
What battery can I use to replace BA-24 in my 600B or 602 Electrometer?
Install an alkaline AA cell in place of BA-24. Of course this will not fit in the battery holder so it will be necessary to solder lead wires on the AA cell and then solder to the holder.
Since the BA-24 was 1.35V and the alkaline cell is 1.5V it will be necessary to change three resistors:

R143 from 3.48kohm to 4.99kohm
R113 from 135kohm to 150kohm
R155 from 1.5kohm to 1.67kohm
The BA-24 cell is used for Ohms function only and can be removed completely if only Volts, Amps, and Coulombs are being measured.

On the other hand, if the Ohms mode is functional, it can also serve as a low constant-current source, 10uA to 1pA in decade steps.

[donlisms]

All that makes sense to me. I hadn’t considered the battery check circuit.

I guess a something to keep in mind is that this meter is all about the extreme ranges, rather than extreme accuracy. Dropping on a 1.5v battery with the circuit as-is just means your resistance measurements would be 11% high, until the battery drains a little, improving with age!  In fact, the battery check reading for B203 could be used to do a little math for a more definite correction. 

[Kleinstein]

An alkaline cell is not such a good solution. The voltage depends quite a bit on the charge state. It is nominal 1.5 V but actually starts at some 1.65 V and empty at some 1.2V, though this limit is more a user choice. An Ag based button cell and adjusted resistors would be the straight forward choice. With so little current there is no need to go rechargible. Except for the high values it is not a good meter for resistance anyway. Even than the low voltage is not ideal for high resistance.

[Conrad Hoffman]

On the original question about the 9V batteries, I don't see why you couldn't just use six regular 9V batteries. What's really funny is that I carefully opened up one of the 9V "big" batteries in mine. What did I find? A small PCB that put a couple regular 9V batteries in parallel! I don't think they were all made this way, but Eveready certainly did at one time. If you have that style, carefully open the cardboard box and replace the 9V batteries!

[donlisms]

It’s just normal 9v batteries as near as I can tell.  I can’t see a reason for otherwise, unless we’re talking about avoiding wimpy batteries from days gone by…