dbx 165 distortion spec

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cafewalter

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Oct 10, 2014
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I'm fixing and calibrating a pair of dbx 165s.  These are very old units: not 165A, no PeakStop, and they don't even have silk screening on the PCBs.  These have black-can 202 VCAs.  From parts codes I think they are from the very early 80s, but that's just a guess.

As I work through the calibration procedure, step 20 is a THD check.  I am measuring rather higher distortion numbers than the instructions say I should.  For instance, with 0dBu 1kHz at the input, no compression, and +20dBu output, the instructions say I should see < .05%, and instead I measure (with an Audio Precision analyzer) 0.27%, primarily third harmonic.

Were the calibration instructions just wishful thinking?  Or, does the VCA distortion get worse with age (maybe the component values have drifted)?  Is this something I should be trying to repair, or is it "part of the charm"?
 
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