[NLRS] FT-817, FT-857, and FT-897 TX INH

[Dr. Gerald N. Johnson]

The schematics are the same with the 857 and 897 using Q1045 and the 817
using Q1035, a 2SC4514E in all cases. There is 48K of series resistor
from the TX INH pin to the transistor base. There is 100K from base to
ground (to ensure turn off if the transistor or board became leaky) that
takes so little current (60 nanoamps) at 0.6 volts I don't bother
including it in my computations.

In the 857 that I checked, key down or PTT on FM, about 1.35 volts
applied to the TX INH pin turned OFF the transmitter output. It did not
turn the rig to receive, but totally killed the output. When I shorted
out my supply (one made for such service the output rose straight up
about half way on the scope then continued to rise with a rounded slope
for 20 milliseconds to full power. I didn't detect any difference with
different power levels and with that voltage applied there was no SSB
output either.

So how much voltage is too much? The current I measured at 1.35 volts
was 14 microamps. Thats .67 volts drop in the 48K and about that much
Vbe. With 14 volts applied to the pin, there would be about 280
microamps applied to the transistor base. One data sheet I found for the
2SC4514E gives a Vsat rating with 10 milliamps applied to the base. It
says the maximum collector current rating is 200 milliamps and the beta
is typically 150 to 300 for the E suffix, minimum 100 for all currents,
so the base current rating is more than 2 milliamps. There is no limit
shown for base current. So as far as the transistor is concerned it can
hack 97 volts on the TX INH pin. The bypass capacitors probably can't.
And the chip resistor probably can't. But 12 or 14 volts (277 microamps
to the base) from a sequencer is surely no more than 10% of the
transistor rating and would dissipate only 4 milliwatts in the 48K of
resistor (1K at the connector, 47K at the transistor base). The 47K
resistor is rated at 1/16th watt which limits the input to 54 volts. 12
or 14 volts on TX INH is fine. 1.35 may not be enough on some radios if
the beta of the 2SC4514E is lower than it is in my radio. 5 volts or 14
volts should be plenty for all qualities of transistor.

So one can use the TX GND (or PTT ground) to trigger the sequencer while
holding applying +12 to TX INH all the time except when the sequencer
has finished all the sequences and enabled the microwave PA. So the
sequencer has to ground TX INH. I might use a 1.5K pull up to 14 volts
(available from the same interface connector when set for LINEAR, not
for CAT) and let a BS-170 be driven off the +14 by the PA power enable
line of the W1GHZ sequencer to ground the TX INH wire. I don't want to
power TX INH from the gate of that power FET, the current to TX INH
might turn it on, unless I used a small FET as a follower from that low
output to enable the PA. Then all the controls, beacon output, VOX, KOX,
accident keys on FM will not fry but just start the sequencer properly
which will then allow RF power when everything is ready. A dandy radio
these Yaesus for microwave IFs.

73, Jerry, K0CQ