Here's a story which may be of interest to anyone with a remote-controlled
U.S. Tower crankup (or anyone interested in circuit modifications):
Last year, when I got my U.S. Tower MA-770MDP 72' motorized base-rotated
tubular tower with remote control, I was surprized to find that the "remote
control" is really nothing more than a metal box with three 120 VAC toggle
switches, six 120 VAC neon indicator lights, a teminal strip, a 1A fuse, and
an AC power cord. The real remote control parts are all located at the
tower, including a pair of 120 VAC limit switches, two 120 VAC time delay
switches, two 120 VAC multi-pole relays, and a fused 120 VAC main power
relay. The power switch in the remote control box simply sends 110 VAC down
the control cable to activate the main power relay at the tower. The
"Raising" and "Lowering" switch toggles one of two motor activation relays,
both of which are wired through settable time-delay switches. Evidently,
this is done because if the motor direction is reveresed too quickly, it can
continue to run in the previous direction, with potentially disasterous
results! When either the "Full Up" or "Full Down" limit switch at the tower
is triggered, the active relay is cut off and 120 VAC is applied to the
appropriate lead in the control cable, which is in turn connected to the
appropriate neon indicator light in the switch box. There's also a
"Local/Remote" switch that uses a 120 VAC signal to enable/disable operating
the tower from a switch mounted at the tower end.
I guess all that stuff is supposed to be worth the almost $1,000 extra they
charge for the remote control option. They're certainly very high-quality
parts, and a lot manual work was done to mount, wire and install them, but
$1,000? I don't know...
But what really bothered me about all this was that I had already pulled all
my cables through three separate 250 foot runs of buried conduit before the
tower arrived. In conformance with local building codes and good engineering
practice, I had taken great pains to put the three types of cable in
different conduits: one for 240 VAC, one for legal limit RF, and one for
low-voltage (12 or 24 volt) control signals. It makes good sense to keep the
AC, RF and low-voltage cables separate, as a short between them could put
lethal voltages on the rig and controllers, a hazard to both operator and
equipment.
Now I was faced with the fact that one of my 8-wire (#16/#20) control cables
was carrying multiple leads at 120 VAC. True, the high voltage is there only
when the tower is being raised or lowered, but that still bothered me. Worse
yet, the 120 VAC signals were not compatible with the brand-new pair of
Polyphaser rotor cable surge protectors I had installed at both ends of the
tower motor control cable (and all my other control cables.) The first time
I tried it, I blew four 1A fuses in the tower remote control box before I
realized that the Polyphaser surge protectors, rated for only 82 VAC, were
shorting the 120 VAC signals to ground! Luckily, earlier I had tried the
remote control box at the tower, bypassing the surge protectors, so I knew
it should work. Boy did I kick myself when I realized what was going on. To
get up and running, I bypassed the surge protectors with terminal strips.
Later on, I bought some 200 VAC MOVs from Polyphaser and stuck them on all
the cables leads in the remote control box to provide some sort of lightning
protection (albeit, not ideal.)
This year, I resolved to fix the problem once and for all. I considered
replacing the 120 VAC relays and time delay switches at the tower with low
voltage models, but I suspected that either exact replacements would be hard
or impossible to find or they would be very expensive. Instead, I decided to
build a relay board to replace the function of the old AC switch box in the
shack. Using four 12 VDC 10A relays to switch 120 VAC, I was able to mimic
all of the switch functions of the remote control box (main relay on, raise,
lower, and remote/local.) Using two 120 VAC relays to switch 12 VDC, I was
able to turn on a set of low-voltage LED indicators for full-up and
full-down.
I mounted the relays on perf board using cable ties, added a couple of
8-position terminal strips, and wired it all up using "ugly construction"
(point-to-point wiring.) It's very sturdy. I installed the relay board in a
plastic Tupperware leftovers container using circuit board standoffs and
rubber grommets for the cable entry points. I secured the Tupperware
container inside the relay box at the tower using cable ties, connected
control cables running to the shack and tower motor relays (basically, the
new relay board is inserted at a break in the control cable at the tower
end), and tapped into the tower's AC supply in the relay box with a short
fused (1A) cable. I built a new switch box for the shack in a plastic
project box with low-voltage wirewrap switches salvaged 15 years ago from an
old mainframe, six LED lamps with built-in resistors, a low-voltage power
connector to the shack's 12 VDC supply, and an 8-wire Cinch-Jones jack/plug
pair found at a hamfest. The whole thing cost less than $50 in parts, almost
all of which I got at Radio Shack.
It works like a charm. Now the control cable conduit has nothing but low
voltage signals in it, and the AC is restricted to the appropriate conduit.
Best of all, I could hook up my two Polyphaser rotor cable surge protectors
again! If I ever sell the tower, the modification can be removed in a few
minutes.
I highly recommend this mod to anyone with a U.S. Tower remote control.
73, Dick, WC1M
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