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Radio
Broadcast Technical Consulting and Sales
10032 Island Drive, Brainerd, MN
56401
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Radio
Broadcast Technical Consulting and Sales |
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Technical Tips from Mark W. Persons |
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QEI 691 Modulation Monitor Repairs |
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Not one, not two, but three QEI 691 FM Modulation Monitors lined up for calibration after repair on the bench. |
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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. |
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The original design called for single-turn trim pots. I modify monitors to have 10 or 15 turn trim pots so the calibration is less likely to change with time and vibration. |
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I use a spectrum analyzer and the Bessel Null method of measuring modulation from an FM exciter to calibrate modulation monitors to 100%. This is the best possible method for this kind of calibration. |
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Here is a relay (on the left) modification to the 691 that I carry in my service vehicle. The relay shorts all three meter movements when the monitor is turned off. This helps protect the meters from damage from vehicle vibration. |
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The stories go on and on.
Stop in again sometime. I'll leave the soldering iron on for you.
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page last edited 11/29/2009
