Late model Hickok 539C with Bias & AC Line Jacks for Digital Meters
Calibrated & Clean  
See pictures 6 thru 9 for calibration specs and information that includes the added bias and AC line jacks for this 539C. These 4 pages should tell you all you need to know about this 539C.
No trick to use the jacks just plug in a multimeter or other accurate DCV digital meter in the 2mm banana bias sockets, red to positive and black to negative, and it will read the actual DC bias voltage being applied. I set it up so it reads in positive voltage although the voltage is actual negative. Same with the AC line jacks; just plug in a multimeter or accurate digital ACV meter in the 2mm banana plugs and it will read actual AC voltage.
                                                          
Please see the calibration information in the pictures.
 
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