SB-101 Rebuild

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Way back in 2008 I started thinking about how to improve the Heathkit SB-101 design to add extra modes and bands and to take more account of modern band conditions whilst retaining as much of the original architecture as possible. 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 already used the 7360 beam deflection mixer in an SB-301 rebuild I decided to use two of those as mixers in the receive path plus two 6AR8 valves in the transmit paths and one 6AR8 as a carrier oscillator and 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. More recently a four filter IF selectivity module has been completed - see step 8 further down this page.

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-set band pass filters.

Version 1 of the overall layout is shown to the right - the chassis layout is now on version 2 and the CIO module is on version 3 following problems with the component packing density being too high. The right hand side of the chassis layout is unchanged but the left side has been revised to accommodate the latest CIO layout..

Step 3 - version 1 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 correct orientation of the valve holders.

Step 5 - the version 1 prototype of the CIO module is shown above right and version 2 of the CIO module has been constructed and tested. This contains the shared, switched CIO and balanced modulator (6AR8), Rx BFO for CW only plus the CW monitor (ECH81) and the  Amplitude Modulator and audio phase splitter (ECH81). However, the completed version 2 unit is so densely packed that maintenance is difficult and a revised design has been completed on a larger module.

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 - the above photo shows the new rear panel in place with its connectors which, from left to right are: Morse key, External loudspeaker with override switch, Interface to SB-610 Monitor, External VFO, IF output to SB-620 Panadaptor, Interface to data modem / PC, Main power, HF Aerial / ATU, Interface to HF linear, Interface to VHF transverter and Ground connection. The version 3 CIO can be seen on the upper left.

Step 7 - the PA components above the chassis have been assembled with the high power part of the bandswitch now inside the upper compartment. This switch will be linked to the bandswitch under the chassis using a pair of drive drums and spring tensioned drive cord - the detent spring in the PA switch has been weakened to make it easier to rotate. The two vertically stacked dust iron toroidal cores on the left are for 160m and 80m.

Step 8 - Concentric Controls

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-engineering of the standard Heathkit control knob as can be seen to the right. The left hand knob is a standard SB-Line one and the one on the right has had its fixed skirt replaced with a separate knob turned up from 1.125 inch diameter aluminium rod.

Step 9 - Filter Module

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-101.