SB-301 S-meter question

[Glen Zook]

You need to check the "S" meter reading with a well calibrated signal generator.  It should read S-9 for a 50 microvolt signal.  Now if you are comparing the reading with a "modern" receiver/transceiver you will often find that those receivers/transceivers are "way off" in terms of the "olde fashioned" S-9 = 50 microvolt standard.  I have seen "modern" equipment that S-9 happens at under 10 microvolts and a "60 over" reading at less than 100 microvolts.  When 50 microvolts is the standard for S-9 then it takes 500 microvolts for a "20 over", 5,000 microvolts for a "40 over", and 50,000 microvolts for a "60 over" reading.

Way too many amateur radio operators these days are so used to getting "20 over" or better readings and they are insulted when someone is using a receiver that has a well calibrated "S" meter.  Actually, the "S" meter reading is basically meaningless, it is the signal to noise ratio of the signal that determines how well it can be copied.  A signal that isn't even moving the "S" meter but with little or no noise is much easier copied than a signal that is reading "20 over" and the noise is "19 over".

Glen, K9STH

Website:  http://k9sth.com


--- On Tue, 3/10/09, Fred Wittman <wittfa19 at EARTHLINK.NET> wrote:

I've been tinkering with the S-meter on my SB-301 due to weakness.  I found and replaced one out-of-spec resistor (R104-33k) and it came back to life.  However, it still seems stingy compared to my other rigs.  I noticed that the zero set pot actually adjusts the cathode current in V7, a 6BA6.  So here's the question.  What actually effects the sensitivity of the S-meter?  The meter circuit itself reads the secondary grid of V6 (at the low end of T3).  FYI, V6 and V7 both test within spec on my NRI model 69 (basic go/no-go tester), and the overall sensitivity  of the receiver is excellent.  Any ideas?  Thanks in advance.


      

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