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Hi all,

Help

I'm boosting a meter movement to 4V / 1000rpm for a dynamometer and
have struck a small problem. The attached circuit shows the initial buffer
to find the difference between A and B. Point A is static, Point B is 
variable

With no input pulses to the tachometer the meter is showing a reading

Point A is at 10.04V and Point B is at 9.96V. A difference of 80mV, which
equates to 80/210 * 1000 = 380rpm, and that's what the needle indicates

Disconnecting the meter from the circuit, I measure no voltage coming from
the op-amp + and - terminals and the op-amp output is 0V

Removing the 100n makes no difference

With just Point A connected to +, Point A reads 10.04V, so does Point B,
so there is zero potential across the meter with no input pulses

Otherwise, the tachometer and op-amp together work linearly, producing
210mV/1000rpm. The problem is the meter reads 400rpm high because
of this 80mV baseline which is there only when the meter is connected.
The deflection of the meter is caused by Point B dropping from 10.04V
@ 210mV / 1000rpm ie as frequency goes up Point B goes down

Cure suggestions please ?

Separate buffer for each meter lead ?
Higher value resistors ?
A bias somewhere ?

TIA

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