[ARC5] BC-946B Errata

[Brian Clarke]

Hello Martin,

 

Bill and I look forward to seeing some pix of your aircraft mock-up.

 

The audio bandwidth of the BC-946 is definitely not broadband, say 10 kHz as would be required for any kind of fidelity. Broadcast standards in the MF band in Australia limit the bandwidth to 4.5 kHz, ie, between C# and D above top C on the piano. The BC-946 has even narrower bandwidth – more like 1.5 kHz, as appropriate for navigation purposes. This means that the selectivity is very good, whether listening to countries with 9 kHz or 10 kHz (the older Australian standard) channel spacing. The narrow bandwidth with a 237 kHz IF gives high Q and hence high sensitivity. I well recall being able to hear 2UE and 2GB on my BC-946 in Auckland, well before they became 100 kW behemoths.

 

The fact that you can hear good bass in your headphones is testament to two factors: the seal between the headphone muffs and the skull surrounding your ears; and the 13 Hz lower cut-off (-3 dB) designed into these receivers for navigation purposes (0.006 uF into a 2 M Ohm grid resistor). But try driving a loudspeaker directly from your BC-946 and you’ll come up against the limit imposed by the small amount of iron in the audio output transformer.

 

You  could hardly call the BC-946 a Hi-Fi BC receiver unless you had advanced presbycusis; rather it was used as a tuner with the output of the detector fed to an external amplifier. Mr Green (now SK) of ’73 Amateur Radio Today’ advocated stripping the audio-processing goodies out of the original chassis, bolting the chassis to a 19” panel and then adding a push-pull 6V6 audio output stage. You could improve on that by replacing the 12A6 with a 12C5 wired as a phase splitter to feed the 6V6s.

 

73 de Brian, VK2GCE

 

On Friday, 21 June 2019 3:46 PM, you said:

 

Hi Brian (and Rich), Looks like we have the same idea! You may have wondered how I’m getting on assembling my 2 Tx-3 Rx set. With some significant interruptions behind me I’ve now got the control box done and all the rest is just waiting to be hooked up on my “rack” that I’ve constructed to look a bit like an aircraft installation. Regarding BCB Rx, I have got one with a 12V dynamotor- and it has wonderful bass response on a pair of modern headphones. I can see why they were used for Hi Fi BC radios in their day- lovely wide passband.

Thank you very much again for all your help and also putting me in touch with Bill. I’ll keep you informed!

Regards, 73, Martyn ZL3CK 

 

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