This is a replacement PCB for the HP3400A Chopper Board (A6).

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UPDATE Rev 1.2 - 8th June 2023

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Rev 1.2 changes a few resistor value markings to match HP's in the filter section to address difficulty tuning the 1V/400Hz calibration (R617, R618, R619, R620) and also tunes the op amp gain (R622, not critical), and DC output matching (R624, R631, not critical) the BOM has also been updated for the appropriate parts.

The hole for the standoff bracket has been moved down 2.5mm to improve alignment.

If you have Rev 1.1 do not panic, you can simply swap out R617, R618, R619, R620 with the ones in the updated BOM if you have trouble calibrating the 1V/400Hz adjustment.

Thank you to Daniel for the feedback which prompted these changes

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All component values are marked on the PCB itself making assembly easy.

Photos are of Rev 1.0 not the current version, so there a few differences between the photos and the current Rev 1.2 PCB.

The PCB is based on the 1980's version of the original HP updated board, but with a few less critical parts modernised and the layout improved, some parts are the same as the original HP design.

This can be used to replace the original HP chopper PCB, especially the early ones that used the Neon chopper method.

In early units with neons a couple of modifications are required to the PCB socket connections to remove the obsolete connections, this is done by moving the Clear wire from R4 (the chassis mounted "Cal" adjustment) to directly on the PCB socket pin 10, and removing all of the resistors between the socket and the chassis adjustments (R3, R4 wire, R8, R9, R10).

The service manual mentions removing parts from the power supply PCB also, but that is not really required, there is no harm in leaving that original, this PCB omits those connections anyway.

I have done a video series for the repair of my unit, and the last part covers this PCB: https://www.youtube.com/playlist?list=PL2z3V9RkHQE12roTSKiF60icefxviRo72