SB-101 Rebuild

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In 2008 I started thinking about how to improve the Heathkit SB-101 design to add extra modes and take more account of modern band conditions whilst retaining much of the original architecture. The obvious areas to address included dynamic range, spurious responses, WARC bands, Frequency and Amplitude Modulation, RF and IF selectivity, CW break-in and RIT.

Having used the 7360 beam deflection mixer in an SB-301 rebuild I decided to use those as mixers in both the transmit and receive paths and one as a balanced modulator.

This page will plot the progress of this project.

Step 1 - remove the original PCBs and replace the original chassis with a new blank aluminium sheet. The original chassis was used as a drilling template for the mounting and PA compartment holes.

Step 2 - plan the layout of the main components so that everything may be accommodated with sufficient space for cooling etc. Note the space for an extra IF filter. The BFO and balanced modulator assembly fits into a die cast 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. One of the original preselector tuning capacitors and matching inductors have been replaced with screened band pass filters.

Step 3 - paper layout fixed in place and ready for drilling.

Step 4 - Most of the chassis has now been drilled and deburred and thought can be given to correctly orienting the valveholders.

Step 5 - the CIO module is now nearly complete. This contains the shared switched CIO and balanced modulator (6AR8), Rx BFO for CW only plus the CW monitor (12AT7) and the Amplitude Modulator (12AT7).