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Technical Tips from Mark W. Persons

Harris Stereo-5 Audio Console upgrade

Here is a Harris Stereo-5 Audio Console on the bench for repair and update.  This console dates back to the 1970's and has integrated circuits instead of discrete transistors used in early offerings from Harris. 
One easy and important modification is to balance the gain in the two stereo microphone inputs.  Those can easily be several dB off because of component tolerances, especially because of poor tracking on the stereo audio level controls on the front panel. 

Remove 30K ohm resistors R6 and R12.  Replace them with 100K trimpot controls.  You can see one (it is blue in color) just below the left integrated circuit microphone preamplifier (U2) for the channel 1 input.  For preamplifier U1, the original 30K resistor (R3) remains in the circuit. 

The channel 2 input preamplifiers are to the right and not shown in this photo.      

The goal is to get the VU meters to read exactly the same on microphone inputs when using a single microphone going into left and right channels.  Adjust the blue control until that happens. 

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   ham W0MH      

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