[Milsurplus] AN/ARC-44 (long)

[Jack Antonio]

I put in some time in the last few months on an RT-70, and
also a BC-1335, and was thinking about getting a radio that
would talk to them.

The AN/ARC-44 covers 24.0 to 51.9, so I decided to
get one, and see what the radio is about. I haven't seen
much discussion about the set, so thought I'd share what I've
found.

It is an interesting little set. The radio I acquired is in fair 
condition. Definitely well used, but it had "MOCK UP" stenciled on the 
side. Thought maybe that it was a nonfunctioning unit used for 
installation tests or similar. From what I was able to gather, the radio 
was used mainly in helicopters in the late 50s through the 60s.

The radio takes relatively common voltages, +28, +150 and +300. It
uses 115V 400Hz to run the internal fan, and another use I'll explain
later.  All subminiature tubes, with the exception of 2 5763 in the PA.
No solid state devices. Layout is straight forward, most of the 
non-tunable modules are mounted on the sides, which swing down. The
modules are held in place by cam-lock fasteners, and are easily removed
for service.

The set uses an RT unit, and three control heads.  The primary head
(SB-327)controls power, receiver volume and frequency.  Another head
(SA-474) is a switch panel that provides a receiver unsquelch, homing 
control function, and 3 extra unused switches.

The third head(SB-329), is an audio panel that contains the microphone 
preamp, the receiver audio output amplifier, and switching to support 
three radios and an intercom function.  Provision is also made in the 
heads for multiple control locations, mainly in a pilot/copilot setup.

Connectors were and are a bit difficult to find, they are a 34 pin
Winchester style connector, and new prices today are pretty high. I
did find an online source at a somewhat more reasonable price, but
they are out of stock, and they have been expecting them by the "end of 
the week" for three weeks now.  Thanks to the list members who have and
are still scrounging some(still need some more).

Power up was relatively straight forward. As my custom, I applied just
filament voltage, checked to see that tubes all lit up, and that the
various relays pulled up when they should.

I then wired up the frequency select lines, and discovered maybe why
the radio was a "mock up". The radio uses one motor to drive
two mechanical drive trains, one to tune each MC step, and another to
tune the 100KC steps. There are two "relay/clutch" assemblies that 
control the motor, and apply torque to the proper tuning system. The
MC unit never cycled, the 100KC unit never stopped.

I found the MC relay/clutch disconnected, and that the 100KC relay had 
been seriously worked on, the leaves were very much bent. It looks like 
a previous tech had spent a bit of time trying to repair it, but was
thinking the problem was spring tensions.  Can't blame him much, as
that is what it first appeared to me also. Took a couple hours, but
finally discovered that what I think is the clutch pressure adjustment
nut was loose, and was binding against the relay frame. Then I 
straightened the leaves and all was well. It took some head scratching 
to get the gear positions worked out but finally the tuning mechanism 
was working OK.

After that, applying 150V to check out the receiver revealed a leaky cap
in the squelch that needed replacing. Applying 300V to the transmitter, 
and keying, resulted in anywhere from 5 to 10 watts at various spot 
frequencies in the radios range. Receiver sensitivities are
in the 1 to 3 microvolt range. Without the fan, the radio is warm
but not unusually so. I imagine that in a helicopter in southeast Asia,
the fan would have been absolutely necessary.

The receiver is fairly broad, when listening to 29.6, it will hear the
repeater at 29.62 well.

The radio also has a retransmit relay built in, and I purchased a
second unit to investigate that.  The second unit came up pretty easy
but I discovered what may have been another weak point in the radio.

There are 14 crystals in the MC selector system, and between the two
radios(28 crystals), I found 5 dead crystals, and 4 more that are more 
than 25 kc off frequency. Even given the age of this radio, that seems 
like a pretty high failure rate.  Fortunately, none of the defective 
crystals cover a ham band, so replacement is not critical.

One interesting thing in the receiver. The radio is an FM set, but in
homing mode, the limiter/discriminator is replaced by a standard AM
IF amp and detector. However, this homing IF amp is modulated by a
small amount of 400Hz to make a CW signal audible.  However, without
400 cycles, it recovers AM pretty nicely, and was able to hear the
"stuff" on 27 MC, 3 or 4 channels at a time due to the wide
bandwidth of the receiver. I haven't heard any AM at 29.0 or 29.1 yet 
though.

At this point, I am still trying to scrounge up some connectors for the
second set, as well as finding a pair of DY-107/AR dynamotor units. I
am planning on hooking up the two radios to investigate the 
retransmission function.

Hopefully looking forward to a QSO on 29.4 one of these days.

Jack Antonio WA7DIA/4