Re: {Collins} 75S1 S-Meter not Sensitive after S-8

Thanks, Glen, for the history of 50mV. Nice to know, not that we will get asked on any quiz show!

I never had any signal above S9 on my '3B so it never mattered.

Happy Holisdays.

----- Original Message ----- From: "Glen Zook" <gzook@xxxxxxxxx>
To: "W9HAK" <W9HAK@xxxxxxxxxxxxx>; <collins@xxxxxxxxxxxxx>
Sent: Thursday, December 06, 2012 3:51 PM
Subject: Re: {Collins} 75S1 S-Meter not Sensitive after S-8

Actually, the 100 microvolts for S-9 included a "matching pad" for the 50-ohm input of the receivers. This "matching pad" introduced 6 dB loss which meant that 50 microvolts were actually applied to the receiver. During the period of time when the A-Line receivers were manufactured, and well into the manufacturing period of the S-Line equipment, most of the commonly available signal generators did not present a true 50-ohm load. As such, a 6 dB pad was used to match the variable impedance output of the signal generators to a 50-ohm load. There were also pads available to match the impedance to a 75-ohm load.

Signal generators like the Measurements Model 65 and Model 80, a number of the military signal generators like the URM-25 series and TS-497 series, and a number of signal generators manufactured by others all had "matching pads" available.

When signal generators became available that had a true 50-ohm impedance, then the "matching pad" was no longer needed and the applied voltage for S-9 was 50-microvolts.

The standard for S-9 = 50-microvolts was introduced by the RME (now IEEE) in the mid 1930s.

Glen, K9STH


From: W9HAK <W9HAK@xxxxxxxxxxxxx>
To: collins@xxxxxxxxxxxxx
Sent: Thursday, December 6, 2012 10:31 AM
Subject: Re: {Collins} 75S1 S-Meter not Sensitive after S-8

Good Morning All,

I find in my notes about the about the 75S-
receivers that R-16 is a test select resistor. At
the time of production there were two S9
standards. 100 uv or 50 uv for S9.

In addition, the various tolerances could be
accounted for with the selection of R-16.

Just thought you would want to know.

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