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Radio Broadcast Technical Consulting and Sales
 
10032 Island Drive, Brainerd, MN  56401

 


Technical Tips from Mark W. Persons

QEI 691 Modulation Monitor Repairs


Not one, not two, but three QEI 691 FM Modulation Monitors lined up for calibration after repair on the bench.

Special note:  Because of the difficulty of working on these units, we are no longer doing repairs and calibrations on them.  Best to contact QEI for that.  http://www.qei-broadcast.com  

These monitors, from the 1970's and are crammed with DIP IC's.  With the front panel down, only half of the electronics is visible.  The other half is in the back and underneath. 
The original design called for single-turn trim pots.  You can modify these monitors to have 10 or 15 turn trim pots so the calibration is less likely to change with time and vibration.
A spectrum analyzer using the Bessel Null method of measuring modulation from an FM exciter should be used to calibrate monitors to 100%. 

This is the best possible method for this kind of calibration. 

Here is a relay (on the left) modification to the 691. 

The relay shorts all three meter movements when the monitor is turned off. 

This helps protect the meters from damage from vehicle vibration.

Special note:  Because of the difficulty of working on these units, we are no longer doing repairs and calibrations on them.  Best to contact QEI for that.  http://www.qei-broadcast.com  


The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   Ham WMH

Questions?  Email Mark Persons:  teki@mwpersons.com         

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page last edited 02/22/2016