Powered down HEPI valve drive chassis and ±24V power going to HEPI Interface modules. Replaced R13 from a 2 KOhm resistor to a 33.2K Ohm resistor on D080521 units. This changes the gain of the L4C channels from 22 to 333, per document E1200420-v1. Tested units after mods, gain ~330. Serial Numbers of units modified: S0900178, S0900236, S0900202, and S0900234. Will reinstall units tomorrow morning.