[Boatanchors] SP-600 "S" meter anomaly?

[Philip Atchley]

Hello,

In use I've noticed an anomaly in the "S" meter of my SP-600 receiver.  
When I tune in a station the meter reads signal strength "normally".  
HOWEVER, the meter swings DOWNWARD with modulation, not upward as one 
would expect, and as other radios in the shack do.  Now, we're talking 
about ordinary AM stations, whether local broadcast of Shortwave and I 
know that all these stations can't have a modulation problem. 

Neither do I think that there's a defect in my SP-600 receiver.  On the 
SP-600, rather than measuring the 'filtered' AGC Voltage, either 
directly or indirectly as other receivers do, the meter is placed in the 
Detector circuit.  I suspect, but can't prove, that this anomaly is 
produced by the AC audio signal riding on the detected DC Voltage that 
the meter reads.  You'd think that with the meter inertia, the 
combination of positive and negative swing in the audio signal would 
cancel out, not moving the needle, but this doesn't appear to be the case.

Thoughts anyone?

73 de Phil  KO6BB