[HomeBrew] S-meter "backwards problem" resolution and answers

[kwylow zinjanthropus]

All:

With respect to the "backwards" NC-57B s-meter solution, I have decided to do the following. I'm also answering a few other solutions that were give and reasons why they won't probably work:

Solution: I decided to completely bypass National's s-meter circuit and tap off the AGC directly. I'm using a separate 6J5 "S-meter amplifier" circuit (ala a 1957 ARRL handbook). All I need to do is tap off of the AGC line and wire the tap into the S-meter circuit to accomplish this. Yes, I know it's redundant to use a tube but hey, it's for a "tube radio" project anyway. I'm just adding to the "BA" factor of this project. Ok, to answer some other solutions:

1) Use an OP AMP and invert. Don't think it will work for the following reason-the National circuit (when turned on) makes the "backwards" S-meter work like a normal S-meter (where the reading goes from left to right with an increase in signal amplitude). Using an inverting amp circuit will just make the meter completely behave opposite of what is intended: you turn on the radio and now the normally left deflected meter will go all the way to the right (max) and decrease in deflection with an increase in incoming signal.

2) Adjusting the spring bias of the meter. Tried yesterday to do this. The meter will deflect only a quarter towards the right and then stops. Reason: the adjustment (which is also hooked to the meter spring) has a stop that causes this action to occur. Therefore, I can't get the meter to behave like a National S-meter. 

Sooo...that's about it. 73 de Cal, N6KYR



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