[Collins] 75A-4 S-Meter History Question

[Jim Walker]

I was wondering if there is someone who can explain to me what the
unusual meter is on two 75A-4s recently sold on EBAY.  They are both
closed items: SN# 5567 EBAY: 190023487461 and SN#5518 EBAY 230024162332.

 

Both receivers have an S meter that the lower scale is graduated in 0 -
S9 and marking for 20, 40, and 60 dB over S9.  The top scales starts at
10 and is graduated 20, 40, 60, 80, and 100 dB.  

 

The only way I could see this scale to be of any use is if there were
some switch to change the meter scales and reference like that on the
SP-600.  Can anyone tell me if this is a last production run meter,
something for the military or some other special application?  What is
the top scale used for?

 

TNX,

 

Jim KZ5AW