Looking through the 301 manual's alignment instructions for the s-meter, I immediately had a question about the signal generator levels all refering to "db". db is a relative measurement and levels need a reference. A reference to what was the question in my mind. So it was a bit confusing. After a little research, it became clear that the levels in the manual are dbuV (decibels per microvolt). The issue with using dbuV is that it doesn't necessarily specify into what impedence. Most modern equipment has settled on using dbmW (decibels per milliwatt into 50 ohm resistive load). With the help of information from the following sources, I was able to equate the levels in the manual to levels that my sig gen could use.
(which in turn, has references)
( a really old reference, but not old, considering the era that the 301 was designed in)
So, the next issue is that yaesu meters (from this era), in general, seem to be "tight" on the s-units. This includes my FT-101E. This is somewhat understandable since at the time, the IARU hadn't yet adopted the -73dbm level for S9 that Collins Corp. had been using for years (per references above). I am not going to discuss the 3db or 6db per s-unit debate. For this 301, I decided to see how close I could get it to the IARU standards.
I ultimately ended up with being mostly successful:
S4 = -103 dbm
S7.5 = -83 dbm (This was just a check because I could put in 20db of attenuation quickly) S8 should be -79, S7 should be -85, so close enough...
S9 = -73 dbm (the "gold" standard number)
S9 + 10db = -58 dbm (meter reads 5 db low)
S9 + 20db = -43 dbm (meter reads 10 db low)
Above S9 + 10db, the meter reads low, but I'm okay with that because any signal above S9+10db is "59" anyways... The weak signals are the ones that I would like to give accurate reports for.
Because of the circuitry design, the meter likely can't be fully calibrated to IARU without changes to the circuit (I am not that ambitious to do this at this time), I ultimately toggled between S4 & S9, checking at S1 and S9+5 and once that was done, I checked at the higher levels +10, +20, +40, +60 and live with the lower readings. One additional note, in the yaesu cal, they use 80 dbuV for S9+60 (full scale), however 80 dbuV actually is ~S9+45 in the IARU world.
We'll see, if I don't like it, I can always change it.
The rig is on right now, listening to 40m nets at the moment.. Sounds good and the meter "is a movin"
(next up, the clarifer center on/off is out by quite a bit, the alignment seems simple enough)
That's all for now.
-Jim
