This shows the elements of the meter. Only the green and red assemblies plug in.
The preamp has a voltage and an ohms mode, selected by a 4-pole form C relay. In either case two form C reed relays control the output in four decade steps.
In voltage mode one relay selects an input attenuator of X1 or X1/100. The preamp is configured as a non-inverting amplifier. The second relay changes the feedback for X1 or X10 gain. The overall gain is therefore selectable as X10, X1, X1/10, or X1/100. The input impedance is 10 megohm.
In ohms mode an operational amplifier uses the first relay to select an output voltage of either -3.32 or -0.332 V. The second reed relay in conjunction with the Kilohm-Megohm relay selects the resistances of 332, 33,2K, 330K and 33.3M between this voltage and the inverting input of the preamp. The resistor under test goes in the feedback path.
The 5-15 relay in the secondary relay assembly selects between 150 and 500 ohms between the preamp output and the inverting input of the meter amplifier. This gives scales of 150 and 500 mv, since the meter is 1 MA FS.
The NE536 J-Fet op amp for the preamp is a modification made to the preamp in 1981. It is mounted on a carrier board with pins matching the original Burr Brown modular amplifier. I plan on replacing it with a OPA190.
The other elements are described in these links:
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