We were discussing batteries a few weeks ago and I mentioned I used high value capacitors, AKA super capacitors, to replace the 3V lithium disk cell.
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I received an email asking for details.
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All of the receivers have a 5V rail, I used a 4 1N914 or 1n4148 small signal silicon diodes between the 5V rail and positive of the super capacitor to drop the voltage. The nominal voltage drop of a silicon diode is 0.7V, however under extremely low current this can drop to as low as 0.5V. It will vary from diode to diode even in the same family.
I also included a 100 ohm resistor just in case, I like to always have current limiting.
The V+ of the super capacitor was connected directly to the V+ memory backup.
In my first installation I did things a bit different. I used a 1K resistor from V+ 5V to a string of 1n4148 diodes to give a "regulated" V+ between 2.5V to 2.9V. Memory keep alive in the R-2000 is 2V.
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I adopted the first version I described after some thought and experimentation with a R-2000 with a defective tuner optical pulse decoder on the main PCB that defied repair.
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You can tune the radio, it's just a royal PITA that can take an hour to tune to the desired frequency. I programmed in useful frequencies in all 10 channels and treat the radio as a fix tuned radio. I have WWV on 2.5/5/10/15, CHU on 7.850, and, US Global HF Network 4.7240, 8992.0 11.1750, 15.0160 and 11.2320 [interoperations with Canadian forces].
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My super capacitors came from the factory keep alive PCB in Kenwood commercial radios, these PCBs were removed and trash after the PCB with all the programmed channels was installed.
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I've modified 10 Kenwood R-2000 general coverage receivers and 2 iCom IC-28A 2M amateur transceivers. I'll be modifying 3 more IC-28A in the near future.
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My super capacitors are 30 years old and are 4.?F, One R-2000 was not powered up for at least 4 years, probably 5 and half, all the memory channels were still there.
Modern higher value super capacitors might have higher internal leakage. I've ordered some 100F 3V supercaps to experiment with.
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Supercapacitors have odd chemistry and I suspect they will last longer if they are kept biased at their operating voltage. I was given about 30 of the Kenwood keep alive boards, pulled the supercapacitors and they are on a 3V rail powered by my primary 12V ham radio power rail.
They've probably been without power for maybe a week since I wired them to the 3V rail.
My experience with the R-2000 that was powered off for at least 4 years suggests the chemistry is more robust then I initially suspected, but, the supercapacitors are in a dedicated plastic box and I see no reason to remove them from the power. [I'm too lazy to change some things without a good reason.]
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All of this came about because I couldn't find a replacement battery in Lexington and didn't want to wait for mail order, well that plus the memory back up cell in the R-2000 is soldered in, which means you have to remove all the very flimsy (cheap) connectors.
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I was initially going to mount a 2 AA battery holder on the rear of the R-2000 when I was given the Kenwood "keep alive" PCBs and decided to give a crazy idea a try.
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YMMV
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