The keying transistor in the FT-1000MP is specified to handle 12 volts. The
Alpha amplifiers have about 24 volts on the key line. That said, I measured
the breakdown voltage of the transistor in my MP, and it's more than 50
volts.
I've run both 76PA and 87A amplifiers using the band data connector without
any
problems for at least 5 years. With the MP keying relay turned off it is
very nice.
The mechanism here is called 'avalanche breakdown'. When the transistor if
off
(not keying the amplifier) if the voltage applied to the transistor voltage
exceeds
the avalanche voltage, it will conduct anyway, and usually fail due to
overdissipation.
Avalanche itself does not cause the transistor to fail, it's the excess
power dissipation
that does it.
To measure the withstand-voltage, I used an old Heath capacitor checker with
a 1 megohm resistor
in series to the transistor (making sure not to accidentally reverse the
polarity!).
But you could use any variable DC supply with the right voltage capability.
A DMM across the resistor tells if the transistor is starting to conduct.
The resistor
limits the current to the transistor to a safe value should it decide to
avalanche.
Then the test voltage is increased up to about 50 volts while watching the
DMM. I quit
at 50 volts, deciding that was sufficient safety margin compared to the 24
volts presented
by the Alpha amplifier.
Incidentally, the avalanche voltage is temperature-dependent. At higher
temperatures, it
decreases, so run this test with the rig at maximum expected temperature, or
test
at room temperature to a higher test voltage.
-- Tom, N5EG
John writes:
> I also noticed that the owner was switching his Alpha amp via the 'Tx GND'
> socket and associated relay. I'd have thought it should be possible to use
> pins 2 and 3 on the BAND DATA output to achieve the same end -- is this
> correct?
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