The 210 has an analog circuit based on a rare, hard to get metal can op-amp used as an amplifier. The op-amp used, at least in mine, has a different pin-out then the usual OP-7 It also has a damping circuit that is designed to speed up its reaction time, and that damping circuit is based on the mechanical and inertia characteristics of the meter coil. The meter coil is part of the calibration feedback loop. It is a rate based "lead-Lag" circuit and that means any LCD is going to show overshoot/undershoot for around a second, or two. The op-amp does drive the meter in "current" mode, meter current is a mA or so for full scale.
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To replace the meter you'll need to install a resistor in place of the meter, measure the voltage across the resistor, add a gain circuit using another op-amp as a difference amplifier to drive the LCD. You'll then need to subtract an offset with another op-amp. To do otherwise you'll loose the calibration. The resistor would have to have the same resistance as the meter coil. The problem is the damping/ feedback circuit is in the ground end of the meter. See how the meter is not directly grounded, that is the issue. Your LCD would have to electrically "float" to avoid injecting small currents into the feedback loop.
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Most people would try to tap it at the output of the op-=amp. In reality you would leave the analog meter in place and tap a weak signal at the high end of R11 with a very high impedance buffer circuit followed by a level shifter / offset adder. It would just be easier to just keep the 210 intact and make a new amplifier in its own box to drive the LCD>
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The 210 was designed for tuning laser cavities, it has a tailored time response designed to aid the users reaction time.
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If you look at the schematic, and I had one in the FTP long ago, adding a LCD looks simple, but it has subtle things that would make doing that interesting in reality, if you want to keep the absolute accuracy. For a circuit with such a low parts count, the internals do some interesting things to make it a really nice LPM.
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I love my 210.
Steve
Last edited by mixedgas; 08-04-2017 at 09:26.
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