I grew up in Schenectady, NY, home of the famous General Electric company and its scientific and technical innovations led out of their Research and Development laboratory, traceable back through Langmuir to Edison and Steinmetz and the beginning of the enterprise. So when I saw Bill N2CQR's
recent work documented on the
Soldersmoke blog with converting a General Electric late 1970s vintage CB radio from 11 meter work to 10 meter AM work, I was intrigued and had flashbacks to earlier days.
This type of project is especially interesting since as we ascend to the peak of Solar Cycle 25, electron density is way up and higher bands like 10 meters (with their accompanying lower noise floors) have now opened up for longer distance contacts.
Off I went to eBay, where I ended up with a pristine GE model 3-5804D, with 2 crystal configuration but importantly the PLL02A chip from a core Cybernet design that has been the subject of a large number of CB conversion projects over the years of this sort. My find even has the GE front name plate - a must in this case - which apparently likes to detach itself from the radio.
Most of the modifications are CB related - like this one - but we'll be following the prescription for 10m conversion from Jerry K5JC.
The first step is to make sure the radio still works on 11m, and here we encounter our first problem: no response when feeding 12VDC at all. Then I opened the covers and found something strange: the back DC plug clearly marks "-" and "+" polarity:
Yet inside the radio the red wire was going to "-" and black to "+". Wiring error from the overworked GE factory technicians? Or something else?
It soon became clear: someone had reversed input polarity in this radio's past, and had blown the reverse protection diode in shorted mode. In a fit of desperation, the 'golden screwdriver' came out and they reversed the leads inside the radio - but to no avail, since the diode was gone. Here it is:
A quick reinstallation of a new diode (used a 1N4007 which has plenty good rating), and reverse the power leads to their proper positions, and the radio came to life.
Using a forceps clamp stuck into the center conductor of the SO239 jack (there's an efficient antenna for you), one of those types with a kilowatt+ "special" amplifier appears on Channel 6, yelling away into their splattered and horribly overdriven set. Yes, that's the CB band - things clearly worked!
Hooking up a calibrated signal generator with AM modulation, we see that the S meter isn't really that far off; -73 dBm is nearly S9. Good enough for government work, or GE, in this case. Channels are where they are supposed to be tuned.
Then the final check of the transmit section. Hooked up to a dummy load, it drew about 1.25 amps at 13.6V from the power supply, so guessing a reasonable power out. (Power meter was not available right now but I'll check that later.). Employing my KiwiSDR with a 6 inch piece of wire jammed into the center connector, I listened to the audio. I did notice on the waterfall that the frequency was low by maybe 1 kHz but for AM, that doesn't matter. Audio was reasonable quality for this handheld microphone but my ersatz antenna meant that I'm not worrying about it too much yet. Here's a sample (click to play).
So here ends Part 1: the radio works as intended. Next step: modification.
