A few more details of the process..
These are examples of the three major models that were manufactured. The basic electronic circuit design was hardly changed from model to model - although physical board layout changes occured. The "A" model had two separate batteries contained within the same case and introduced the overload (circuit breaker) and pluse DC miliamp circuitry. The popular "B" model had even more safety circuitry, a modified rectifier, upgraded meters and still another new battery configuration. About halfway through the original manufacturing run, a removable "plug" was introduced as a port through which one could replace the battery. In most meters I have seen however, the battery was soldered directly to the circuit board and couldn't be removed/replaced through the hole anyway. I assume battery replacement in the field just didn't work out....
During the past several months I have purchased several PSM-6's and 6B 's from several sources. In every case the batteries required to operate the Ohms function were discharged. I quickly decided to re-engineer the circuits to work with modern, easily available batteries. The original battery was a center-tapped mercury oxide contraption 4 1/2 inches long and 3/4 of an inch in diameter. It produced the improbable voltages of 1.34VDC and 12.06VDC within the same tube! Here is what it looked like:
Mercury Battery Pack for AN/PSM-6 Multimeter - Last manufactured in late 70's
Mercury batteries have the desirable charactistic of maintaining a constant voltage output as they discharge. When they finally "run down" they go immediately to zero! Mercury (or Mecuric Oxide) cells were banned by the EPA in the 80's and the loss of that option caused real problems for users of PSM-6 multimeters as well as photographers around the world (1.35 VDC Mercury batteries were used to power the electronics in most early camera metering systems).
This is the overflow parts locker. Contained herein are most of the defective military and Fluke Multimeters that have been designated as parts units. There are also some meters here that are awaiting refurbishment. The main parts cabinet is located in the lab. If you see something here you think might assist you in a project you are working on, visit the contacts page and tell me what you need.
THE OHMS FUNCTION
Fortunately for us, unlike some older camera metering systems, the PSM-6 meter dosn't require a constantly stable DC source - we have a manually controlable "zeroing" circuit to compensate for the fluctuations in output voltage. The trick is to redesign the circuitry to compensate for higher initial voltages and the fact that new alkaline batteries will not behave during their lifetimes like mercury batteries.
That work is done. I am happy with the design changes made to facilitate modern batteries.I personally modify each meter and test all functions before they leave the shop.
I estimate that the ohms function on the most used scales (x1, x10,x100 & x1,000) will last for at least 60,000 individual measurements. That is considerably more than the original design specifications.
The high voltage battery configuration required for the less often used x 10,000 Ohms scale should last for over 12000 measurements.
Testing in progress... 30 VDC test as monitored by the Tektronix DMM.
25 Volt center test
Note the PSM-6 case in the center background. This meter case contains a certified movement only.
It is used during the testing process to determine the relative accuracy of a specific movement.
This movement can be used for both PSM-6 and PSM-37 tests since both meters use movements that have 50 microamp full scale deflection characteristics.