Modern software tools are available to optimize the output filters for a better result. Also VNA tools are available to verify the results.
The primary difference between the older and newer band filters is that the newer filters place a rejection notch closer to the HFO frequency. HFO leakage must have been a problem.
SB-104/HW-104 older 20M band filter design. Notch near 26MHz
SB-104A/HW-104A newer 20M band filter design. Notch nearer to 22.895MHz.
IMHO the HFO frequency traps should be on the D or C board.
Service Bulletins
SB-104-64(April 21, 1978) - Increased ALC Meter Action
C-827 on the ALC/Filter (K) circuit board should be changed to a .68 uf
tantalum capacitor [PN 25-200].
This change will increase attack time on the ALC loop and decrease the
possibility of "flat-topping."
Updates (SBM-104-2)
ALC Board as modified following above instructions
SB-104 => 85-1425-x
PC Board
X-Ray
Schematic
HW-104 => 85-1630-2
PC Board
X-Ray
Schematic
SB-104A => 85-2056-1 / 85-2187-1
NOTE:
The only difference between 85-2056-1 and 85-2187-1 is how L815 is mounted. Board 85-2187-1 has holes on the PC board for L815.
Board 85-2056-1 requires one lead of L815 be wired to the switch.
