[Amps] Bird Termaline 694 Wattmeter Query

[w2cqm at juno.com]

I have a 1000w metered  Bird Termaline #694 oil filled wattmeter that is not reflecting accurately the power output  level. When contrasted against  another  calibrated  1000W  device,   the Bird registers about 800W. The non-inductive load measures  a perfect 50 ohm input so the problem lies  elsewhere.   The factory  manual indicates that there are no means available "in the field"  to adjust the internal circuitry for proper calibration. I'm not certain if that is a veiled invitation for the need to send it back to the factory for big bucks servicing.  Radio amateurs are well known for their ability to overcome a whole range of obstacles,  consequently, this posting is made in the hopes of finding a solution if any  to the calibration problem. Any assistance would  be greatly appreciated.  Ron W2CQM/3