Way back in 2008 I started thinking about how to improve the Heathkit SB-
Having already used the 7360 beam deflection mixer in an SB-
This page will plot the progress of this project.
Step 1 -
Step 2 -
The CIO / BFO / balanced modulator assembly is contained in a screened box on top of the chassis to the left of the PA compartment so that the blank space underneath the chassis may be used for the new FM assembly and overall control logic. One of the original preselector tuning capacitors and respective inductors have been replaced with screened pre-
Version 1 of the overall layout is shown to the right -
Step 3 -
Step 4 -
Step 5 -
The lower left photo shows the topside view of version 2 of the CIO module. The lower centre drawing shows the version 3 block diagram of the various functions. There are additional functions and components in this module in order to free up space under the main chassis for the FM module and control logic. There are several preset potentiometers and the crystals all have preset trimmer capacitors accessible via small adjacent holes in the metal base plate.
The lower right photo shows the version 3 CIO module which now includes the transmit SSB filter, ALC amplifier and receiver product detector and AF amplifier.
Step 6 -
Step 7 -
Step 8 -
As the front panel will not easily accommodate additional controls, a small number of the variable controls have been combined using concentric potentiometers. This required some re-
Step 9 -
A new filter module has been constructed on a double sided PC board holding crystal filters for AM, SSB and CW and a simple LC roofing filter for the FM module.
This module mounts on top of the chassis, freeing up space underneath for the mode switch wafers and associated control logic.
Each filter is selected by a SPCO 12v relay at the input and output. Each of the crystal filters in situ shows an insertion loss of around 4dB, passband ripple of less than 2dB and a floor attenuation in excess of 70dB. The input and output circuits both have tuned circuits to correct for extra stray capacitances compared to the original SB-