[Heathkit] SB-104, S-Meter display

[Ulrich Graf]

Hello to the list,

>>after modifying the IF of my SB-104 according to the paper of N2EO
(reprint of Ham Radio article, www.mods.dk > SB-104A Improved receiver
performance.zip) if have the effect, that the S-meter, after calibrating the
S-9-point to -73 dBm, does not return to 0 any more. Obviously the new
circuitry has a little less if gain, so that the difference between an
S-9-signal and no signal is less than before and therefor the dc swing is
less too. So after increasing the S-meter swing to show a correct S 9, 0 can
not be reached any more.<<

When you perform the mentioned mods if gain may vary slightly. Since the
AGC-amplifier is dc coupled a different if gain results in different S-meter
deflection. The S-9 point can be easily readjustet, but not so the zero
setting. This can be corrected by giving the differential amplifier Q504 and
Q505 a slight offset. I altered R554 to a lower value. That moved the S-3
"zero" setting of the meter to the correct zero display. This modification
has no impact on S-meter deflection range or AGC range. Just to let you
know.

vy 73 Uli, DK4SX