MegaSquirt MS2 VR/Tach Input Circuit Schematic - 4N25 Opto & MC34072AP Debug

This document is an annotated electronic schematic (filename 'ms2 fail.jpg') showing a tach/ignition input-conditioning circuit, apparently from a MegaSquirt-style engine controller. It includes an opto-isolator (U3, 4N25) with input resistor R12 (390 ohm 1/2W) and capacitors labeled 'Dave C30 0.22' and 'Ed C12 0.001' (noted as 'usually omitted'), a pull-up R13 (4.7K) to VCC, and filter cap C11 (0.01). Output jumper options are shown: OptoOut, VrOut, VrOutInv, and Tsel. The VR (variable reluctor) conditioning section uses a dual op-amp MC34072AP (U7A/U7B) with a Zero-Crossing Setpoint network (R53, R56, both 10K, transistor Q23 ZTX553), a Hysteresis Adjust pot R52 (100K), and feedback/divider resistors R49 and R50 (100K), R42 and R55 (1K), plus input resistors R44 (10K) and R48 (10K). The uploader has hand-drawn two signal paths: a yellow trace labeled 'doesnt work' running from the input through the VrOut jumper to the U7A comparator input, and a black trace labeled 'does work' following an alternate routing through VrOutInv and the U7B stage. The caption indicates it was annotated by JSM on 25 Aug 2010. It documents troubleshooting of which VR conditioner signal path functions.