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The galvos need to be operated in differential mode. This is determined by jumper settings on the galvo drivers boards. They are usually shipped configured for ‘common mode, non-inverting’ To switch to differential mode only jumper 1-3 need to be connected. This usually means that jumper 4-6 needs to be disconnected. (see MicroMax Series 671 Cambridge Technology manual).
Make sure the driver power is turned off when plugging in the galvos.