Its easy for a bit of excess voltage drop in fuses and back panel
connectors (I seem to recall some comments in the past about those back
panel connectors being a real problem in some Scouts) to drop the
voltage at the radio. Some Kenwood power cables with fuses and crappy
fuse holders have done that for a decade or longer. That's the worst
place to have a voltage drop problem because the PA current really drops
the supply voltage to the regulators.
The classic three terminal regulators (7808, LM-340-8) tended to
regulate down to about 2.25 to 2.5 volts with a light load. They almost
never needed more than 3 volts drop at full rated current, but they did
vary from chip to chip.
Something to check is whether the early Scouts and the late Scouts used
the same output voltage. Any update that raised the regulator output
voltage so the low level stages had more power capability, keeping the
input limit as it was might have made the low level stages work better
without improving the low limit on supply voltage.
Once you get a 6 cell lead acid battery down to 11 volts with a light
load, the voltage versus time curve for a constant resistance load is
going practically straight down. I need to run one on my truck battery.
Its 5 years old in a couple months.
A step up switching regulator to supply 13.8 from 11 to 13.8 volts could
be one solution, if its quiet. Its efficiency should be fairly high, on
the order of 85 or 90%.
I've built a regulator for my station supply that regulates with
regulator voltage drop to a minimum of about 20 mv (at 30 amps load or
less). This circuit regulates in the negative line, though there is an
analogous circuit for the positive side that I've not tried that uses a
different regulator chip and P-FET instead of the more common N-FET.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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