[ARC5] ATD Transmitter Longwave Tuning Unit Repair

[David Stinson]

For those with an interest in the Bendix ATD
transmitter- Repair of a scarce, possibly rare
200-500 KC Tuning Unit.
https://s3.amazonaws.com/files.qrz.com/s/ab5s/ATD_RAX.jpg

The Longwave tuning unit for the ATD transmitter
is not often seen.   This one is Serial #8:
https://photos.app.goo.gl/4ohMv7rxtwLeDm3o6
https://photos.app.goo.gl/GHHkoJznE9S5zEEV8

The largest coil on the left of this view is
the PA Tank Coil.  It is variometer-tuned.
https://photos.app.goo.gl/waZmDUVhXihq2fNq6

When I received the tuning unit, one of the
phenolic members which mounts part of
the channel-switching mechanism was broken
in two.  I used glue and some scrap perf board
to repair and strengthen it.  NOTE:  The little
piece of perf-board on the outside interfered
with the channel-changer, so I had to remove
that piece.  Still appears strong enough without it.
https://photos.app.goo.gl/1W19hj4112ejBQnu9

While cleaning the unit and inspecting it, I saw
that the litz wire at the bottom of the PA coil
was no longer connected to its terminal.
A mouse had chewed it.  Had to peel-back
part of the last turn and extend it
to the proper contact.

Inserted the tuning unit and tested.  Got
Plate current, but no dip or power out.
Pulled the unit and looked closer at the PA
circuit.   Mr. Mouse wasn't done it seems.
Here's the internals of the PA variometer
from the 500-1500 KC tuning unit:
https://photos.app.goo.gl/tak7FqJSMgHAjZK57

The variometer coil's axel is hollow.  Flexible,
cloth-covered wires go through it and provides
a solder-point for the coil's litz wire turns.
The cloth-covered wire then exits the axel,
is wound a couple of turns- one side
clock-wise, the other counter-clockwise,
so it "loosens" when you tune the thing
from 0 up to 100.

This was the inside of the 200-500 KC
PA variometer:
https://photos.app.goo.gl/6sshHDfWRFW1fWmc7
The little mousy creep had eaten the variometer
connection wires like candy-canes, and even ate-
off the cloth-covered wire solder connections right
down to the axel.   Between the copper fibers and
the lead solder he ate, I hope he shortly found
himself in Mouse Hades, to be forever
batted-about by Tom-cats demons.

Removing the coil is fairly straight-forward,
with one "oddity."
https://photos.app.goo.gl/kkRqNQ39T1saHL7t7

Set the control to "0" and carefully note the
mechanism that projects the tuning shaft
through the cover.  It has to go back in the
same configuration for the "stops" to be in
the correct places.  There is a friction plate
on the inside that is used to friction the
tuning knob and to lock the control
in place.  If you remove just one of the
cylindrical nuts, the plate will flip left out
of the way and you can pull the coil out.

The PA coil has selectable taps. Marked which
side was "up" when the variometer coil is "0",
then set to work disconnecting the taps from
the PA range switch.

The oddity:
If you number the coil taps from the top
to the bottom, it's 1-2-3-4-5.
If you label where the taps connect to
the switch from the bottom up to
the top, than taps are connected
1-2-4-3-5.
https://photos.app.goo.gl/RxDA6hBHS9Mg7sQL8
Wonder if that was intentional
or some sort of error.  Be sure to note
closely how everything goes back together.
There's a little sheet of insulating material
under the coil which is easy to forget.
"Voice of experience."

With the coil out, I removed  the cloth-covered
wires in the axel, noting which end was wound
clock-wise and which counter-clockwise
so that, when the coil is rotated to "100,"
it loosens the turns and when rotated to "0"
it tightens them again.

Striped about 2mm back from the chewed ends
and tinned them.  Straightened and "rounded"
the end of the wire that makes contact on
the outside of the coil, gave that end a slight
bend and fed the wire back through the axel
from the inside to the outside.   A little tricky,
but "your patience will be rewarded."
There is just enough wire to work- not a bit more.

One side of the coil litz winding had enough of
a pigtail to clean and solder, so a jumper there
was easy.  The other side- the little vermin
ate right down to the wire hole, so I had to
peel back the end of the winding, scrape
and curse and clean that litz wire and feed
a jumper through the cleaned-out hole.
The hole wasn't large enough for the
green jumper wire and I wasn't going
to enlarge it, so stripped the green jacket
from the jumper and that fit just right.
https://photos.app.goo.gl/Jvp5jZxA56KdWWFL9

Put everything back together, and the
tuning unit now works as it should on 473 KC.

Hope this is helpful to someone.
GL OM ES 73 DE Dave AB5S



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