Help, where should the missing potentiometer be?

[daisizhou]

Hallo every one,When I was checking the PCB circuit board,I found a P4 potentiometer and a 150 ohm resistor missing from the schematic diagram. I don't know where the most suitable position is for it.If you know, please let me know. Thank you

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This is the working principle of this part of the circuit
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**ECG curves are generated by the microprocessor based on data tables. The processor updates the 8-bit D/A-converter in IC1 (channel A), usually 500 times per second. IC-3 converts current to voltage, and IC-2 makes the signal bipolar. The amplitude is adjusted using P1, and the zero point is adjusted using P4.

IC1 D/A-converter part B sets the reference level for the curve generator D/A-converter part A. Part B thus determines the amplitude of the stimulus signal.
The DC value for maximum amplitude is read back by the microprocessor's 10-bit A/D-input El, and can therefore be adjusted precisely.
 The other half of IC2 is used for operating the high-level output or the ECG signal. The circuit has an amplification of 4 times.
The signal is filtered for high frequency noise with F18 before it is conducted to contact J8. C3 ensures bandwidth limitation in IC2. D9 is over voltage protection.**

[tooki]

Please share the entire service manual, since you clearly have it. Snippets of the schematic are nowhere near as useful as the whole thing.

[Kim Christensen]

It looks like the pot P4 might be in series with R4... Hard to tell exactly without seeing both sides of the PCB. Best to trace it and ring it out with an ohmmeter to verify. Looks like a later revision of the PCB, so the schematic you have is out of date.

[floobydust]

It's a METRON QA-45 Defibrillator and Transcutaneous Pacemaker Analyzer. Could not find decent schematics.

[daisizhou]

ok