SB-301/310 Receiver Improvement
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I collect and refurbish/redesign Heathkit amateur radio equipment because, although old by modern standards, it was well designed in the first place and is ideal for home constructors - you can get at everything without needing a microscope or a Ph.D. in mechanical engineering. The first thing that usually happens is that the outer case is removed and washed in the bath in warm soapy water to remove years of grime that gets trapped in the crackle paint. Then the real work starts but I digress.
The classic Heathkit frequency conversion using a tunable first IF of 8.895 - 8.395MHz followed by a second fixed IF of 3.395MHz provides a good but not perfect platform for modern day HF band activities. However, the performance of valve receivers like the SB301/310 can be significantly improved if you are willing to expend some time and effort. I made the following, fairly major, changes to the design:
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Overall, the second IF gain was increased, the front end gain reduced, the two single ended mixers replaced with balanced mixers with low noise and improved dynamic range and the AGC range extended.
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The 6BZ6 RF stage was retained but the gain was reduced using a partially decoupled cathode resistor. Additional muting was added by raising the cathode potential of the RF stage on transmit. A three position, 0/10/20dB 50ohm RF attenuator was installed on the front panel.
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Both mixers were changed to the gated beam deflection 7360 which is now very expensive (about 26GBP in the UK). Balanced oscillator injection to the deflection electrodes was provided using broadband transformers - the VFO injection to the second mixer was buffered with a solid state amplifier as 10pk-pk is required. Both mixers use a balanced anode configuration and have preset balancing controls. The band oscillator output to the rear panel for an associated transmitter is now buffered. If you cannot obtain the 7360 then there are alternative beam deflection mixers with different base wiring or alternatively a balanced VHF triode mixer (ECC88 or possbily the ECC81 / 12AT7) will provide some improvement over the original pentode mixers although the mixer gain will be somewhat lower.
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An extra IF stage was introduced and all three amplifiers used partially decoupled cathode resistors and AGC control to the control grids. A buffered IF signal is available on the rear panel for an external monitor scope.
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The AM, AGC and product detectors, BFO, BFO buffer and audio amplifier were replaced with solid state circuitry to reduce the space and power requirements.
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The AGC line was changed so that it could be used for rapid muting and unmuting the receiver for full break-in CW operation whilst retaining an adequate but separate time constant for SSB/CW reception.
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A wide IF filter and FM detector module have been installed and the bandwidth switch extended to four positions.
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Keyed AF tone and own-Tx monitoring are available on CW with the level adjustable from a front panel control.
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The WARC and 160m bands were added and all front end tuned circuits were constructed using one inch square screened IF assemblies from old TVs and radios.
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A multiway connection lead has been installed between the receiver and matching SB401 transmitter to handle the revised transmit/receive changeover.
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All relay switching has been changed to solid state in the matching SB401 and grid blocked keying installed. The changeover is now relay free and completely silent.
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The receiver mode switch has been extended to include RTTY and FM.
The original chassis was emptied and removed with a hacksaw so that just a 1/4inch (6mm) horizontal lip was left all the way round. A new top chassis plate was installed using 18swg NS4 half hard aluminium. The original VFO was remounted and the overall signal path re-arranged to be in a straighter line with the BFO screened and moved further away from the 3.395MHz IF strip. The phono connectors were replaced with surface mounting Belling&Lee TV connectors - my personal preference to phonos although purists would probably use BNC. All coaxial connectors are now individually screened from each other using thin tin plate soldered structures. Additional lettering was added to the front panel using white Letraset and a clear spray-on retaining varnish.
The overall results were well worth the effort. The receiver can listen through dots at 25wpm which is great for contest operation. The break-in is click free and comfortable for long periods of operation. The overall noise figure is about 10dB, the first IF rejection is in excess of 80dB on all bands and the second IF rejection is in excess of 100dB - now difficult to measure. The dynamic range has improved and most of the 'ghost' signals on 40m in the evenings have now disappeared. AGC operation commences at about 2uV and continues to at least 100mV.