[Amps] Calibrate a panel meter?

[Bill Turner]

I have a Simpson 923A panel meter, 1 amp full scale. It is reading 
low by about 6%. When it has 800 ma through it, it indicates about 
750 ma. I'll be using it in my 8877 amp at 800 ma and I'd like it to 
be more accurate.

If it were reading high, I could just shunt it, but reading low...???

Is there a way to calibrate it by adjusting anything inside? Moving 
the magnets around, etc? I'm sure the factory calibrates them somehow 
but I've never read anything in the ham literature about how to do it.

Suggestions appreciated.

73, Bill W6WRT