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SiriusHardware · 8th Mar 2017, 7:25 pm

Nice to see those distinctive innards again. What's the extra chip on the channel table eprom board?

Mine was modified that way as well (to add 27/81) but my self-made eprom board only had an eprom and one transistor on it, the transistor being used to provide the necessary ninth bit of the channel code (as it happened, for all channel codes within the range of interest bits 8 and 9 were always in opposite states, so it was possible to derive bit 9 just by inverting bit 8 as it came out of the eprom)

The only thing that ever went wrong with mine was sudden failure of the potted VCO module (the orange thing just north of the eprom board in your image #2) which I thought was going to be a show stopper until I discovered that a little British company called Spectrum Communications made a drop-in replacement. That was many years ago but I believe they are still around and still making various CB and Amateur radio project and accessory kits, and possibly even still making those replacement Cybernet VCOs.

I would have expected you would have implemented lower FM power by running the output stage on the half-supply available from the bottom end of the 'AM Darlington', an option you alluded to in an earlier post.

It seems to have been done both ways in various radios using the 121 chassis (although your radio above uses the 125 chassis) - I can think of at least one model where the FM TX supply came from the (unmodulated) 'AM Darlington' and another where the FM TX supply came straight from the +13V supply, even though both radios used the same chassis.

Sometimes, people would modify the first type to be like the second type, but there was a tendency in those cases for one of the output coils to overheat and the insulation on the winding would break down, shorting the turns. It could be fixed just by taking the old wire off and winding the same number of turns back on with fresh varnished wire of the same gauge, but we never did work out why only the modified radios did this. Radios with the same chassis which had been natively built that way never did it.

8th Mar 2017, 7:25 pm	#172
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,556	Re: All about CB radio Nice to see those distinctive innards again. What's the extra chip on the channel table eprom board? Mine was modified that way as well (to add 27/81) but my self-made eprom board only had an eprom and one transistor on it, the transistor being used to provide the necessary ninth bit of the channel code (as it happened, for all channel codes within the range of interest bits 8 and 9 were always in opposite states, so it was possible to derive bit 9 just by inverting bit 8 as it came out of the eprom) The only thing that ever went wrong with mine was sudden failure of the potted VCO module (the orange thing just north of the eprom board in your image #2) which I thought was going to be a show stopper until I discovered that a little British company called Spectrum Communications made a drop-in replacement. That was many years ago but I believe they are still around and still making various CB and Amateur radio project and accessory kits, and possibly even still making those replacement Cybernet VCOs. I would have expected you would have implemented lower FM power by running the output stage on the half-supply available from the bottom end of the 'AM Darlington', an option you alluded to in an earlier post. It seems to have been done both ways in various radios using the 121 chassis (although your radio above uses the 125 chassis) - I can think of at least one model where the FM TX supply came from the (unmodulated) 'AM Darlington' and another where the FM TX supply came straight from the +13V supply, even though both radios used the same chassis. Sometimes, people would modify the first type to be like the second type, but there was a tendency in those cases for one of the output coils to overheat and the insulation on the winding would break down, shorting the turns. It could be fixed just by taking the old wire off and winding the same number of turns back on with fresh varnished wire of the same gauge, but we never did work out why only the modified radios did this. Radios with the same chassis which had been natively built that way never did it.