[ARC5] Fall Project: Navy ATD Transmitter Part 3 (last)

[David Stinson]

Re:  Fall Project: Navy ATD Transmitter Part 3
(last)

Once we're satisfied we've found the "baddie"
parts, it's time for some testing.

If you're using the dynamotor, remove the top
cover so the interlock prevents it from starting.
Connect primary 28V to the transmitter (no high
voltage yet).  Repeatedly key the transmitter and
check for proper relay operation.

Here's a "watch out for this:"  
The "propellar-looking" end of  that weird,
rotating keying relay has a small metal tab as a
rotation "stop."  This tab has to be carefully
flexed so that when the relay activates, it
compresses the tab about a millimeter.  The
rebound of the tab helps push the relay back into
the "off" position.  Here's a video:

https://photos.app.goo.gl/7JVVYS4CwJTqS7pq2

If this tab doesn't push back on the contact, the
relay will tend to stick (it will also do so it
you forget to lubricate the relay armature).  If
it sticks, you'll have the PA on with no drive,
the PA Current meter will stay up-scale when
un-keyed and the 814 plate will quickly go red.
Not good.  Be very careful and gentle with this
adjustment.  The metal of that tab is old and you
don't want to break it.

If all the relays are working, it's time to do
what I call a "stepped smoke test."  Since one of
the goals of this revival is to do only the
minimum required to get the transmitter playing
and avoiding unnecessary damage, it is easy to
miss something during the bad-parts hunt.  One way
to avoid smoking something is to apply B+
gradually up to the 300V LV buss level, watching
for any problems before they reach the "smoke
stage." If there is a problem on the 1000V buss,
it will be evident by 300V.  

Note:  On the right-hand side of the "D" socket is
a switch to select Carbon or Magnetic microphone.
Use a coin to select.
https://photos.app.goo.gl/h7N0eArfZF6aVfs92

The following is written from the perspective of
using the dynamotor, but of course you can do
essentially the same thing with a "home brew"
supply by using a variac in the B+ primary.
You'll need a means to monitor the current on the
B+ buss.  I use a metered Heathkit variable B+
supply here.  We're going to bring the B+ up
slowly so any shorts on the B+ busses become
obvious before doing damage.

Remove the cover of the dynamotor so the interlock
prevents it from starting.  Remove the HV and LV
fuses (carefully pry them up from the end; don't
pull from the middle).  Jumper the ends of the
fuse holders nearest the transmitter power
connector and hook them to the 28VDC Primary buss:
https://photos.app.goo.gl/tXkgHbXtiTkG7NbG3
Select "PHONE" mode so the Modulator and Mic
stages are active. Connect the transmitter to a
50-ohm dummy load (we'll be testing on 7160 KC
because that requires no external matching parts).
Use a 3-9MC tuning unit.  Set the active tuning
unit thus:

A 15
B 15
C 55
D 42
E 6
F 45  
There is a "K" contact inside the tuning unit that
places 40pFd in series with the Antenna connection
or shorts it out.  You should turn the switch to
include the capacitor as this will allow the
transmitter to match your 50-Ohm load on 7 MC.

Key the transmitter and observe the meters.  They
should move up-scale a tiny bit- about the width
of a hair.  If all is well, we can begin
"stepping" the B+ busses to look for trouble.  The
idea is that with each "step," we watch the
current on the buss for erractic readings or if it
"creeps up" after being keyed for awhile.  If you
see that, you've got another bad cap in there
somewhere that's either shunting the B+ to ground
or leaking to a grid, causing the tube current to
rise. 

Remove the jumper to 28V and connect to the
variable B+ supply, set for 100V.  Key the
transmitter, tune PA Plate for max output and
observe the meters and buss current.  Remember the
buss current is also reading that supplied to the
Mic amp and Modulator, so will not match the PA
current meter.

Here are the steps I used:
100 Volts-
PA Current indicated: 10 mA
Buss current: 40 mA
Power out: 0.5W

200 Volts-
PA Current indicated: 30 mA
Buss current:  80 mA
Power out: 2W

300 Volts-
PA Current indicated: 45 mA
Buss current: 140 mA
Power out:  5W

In each case, I did several keying cycles of about
30-seconds on.  All currents remained stable.  So
it was time to put it all together, cross my
heart, say a "Hail Mary" and start the dynamotor.
SAFETY FIRST:  Your transmitter is probably
sitting there with no cover so you can find any
"gremlins" before things catch fire.  Please
remember there are lethal 
voltages in here.  Slow down- think twice- stay
alive.  We kinda like having you around.

Tuning Unit Settings-
The tuning units must be internally set for the
desired band within their range.  The settings are
self-explanitory.  Like most all WWII Aircraft
transmitters, we'll need some help to feed a
50-Ohm non-reactive load.  The solutions and
settings presented here are certainly not "the
last word;"  everyone has their way of doing it.
Fixed caps in series with the Antenna lead works
for me. YMMV.  I got away with mounting the
matching cap internally in the 160 mtr tuning
unit, but that didn't work with the 80 mtr unit-
too much interaction between the tank circuit and
the High Voltage cap.  I haven't found the "right"
value to use internally for 80 meters, so just
hung it in the Antenna lead externally as I do
with other rigs.  Power output measured with a
Tex. 453 scope.

1.5-3MC Tuning unit, 1825 KC
300 pFd High-Voltage matching cap, mounted
internally.
A 13
B 11
C 53
D 50
E 8
F 80
K Shorted.
Power Out: 43W

3-9 MC Tuning Unit, 3890 KC
100 pFd High Voltage cap mounted externally.
A 09
B 07
C 40
D 50
E 5
F 62
K Shorted.
Power out: 50W

3-9 MC Tuning Unit, 7160 KC
No additional matching parts required.
A 15
B 15
C 55
D 42
E 6
F 45
K Included.
Power out: 45W

CAUTION:  This rig has a Mic amplifier and
push-pull 6L6s.  It can make a lot of audio power.
Unlike the ARC-5 and ART-13 rigs, there's not
protection for the Modulation transformer from
too-large voltage excursions.  Be gentle.

630 Meters, or Why "I hates Meesses to Pieces!"

I was so happy to have the 200-500 KC tuning unit.

https://photos.app.goo.gl/bVXhYtEL9t7r0x963
That original marking for 472 KC is a nice
coincidence. Don't remember where I got it, but it
was dirty and obviously a "barn find."  Cleaned it
up a bit, gave it the De-Ox-It treatment, put it
in slot "A" and.... dang.   The MO is working fine
and peaks on the bottom meter just right.  But the
PA Plate won't dip.  Lots of PA current, because
it's a Shunt Feed amp, but no dip.  So I pulled
the tuning unit out and found the Litz-wire tank
coil, which is a variometer and tunes the tank
using a series, rotatable internal link, was not
connected at the bottom of the coil.  It looked
chewed.  Spent an aggravating time getting every
one of those hair-fine wires tinned and
reconnected to the terminal.  God, I hate that
stuff.  Plugged it back in and... no dip.  Pulled
it back out and looked closer.  Inside the tuning
link was the answer.  This is a photo of the
variometer tuning coil:
https://photos.app.goo.gl/Xf3gXXrEsIxpA2m43
There should be two wires running from the coil
winding to the coil axle, thence through holes in
the axle out to the main coil.  A mouse had gotten
in there and eaten the wires like Christmas candy
canes.  I hope he got the belly-ache.  "Ohhhh, I
hates Meesses to Pieces!"  I'll see if I can
rewind it later.

So what to do.  Enter the "useless" Broadcast Band
tuning unit.  If you set the internal switches for
the lowest band, the MO will tune down to 472 KC
no sweat, but the PA Tank bottoms at 520 KC.  The
answer is to pad the big PA tuning capacitors with
doorknob caps totalling 150 pFd.  Now it will tune
630 mtrs easily.  To match a 50-ohm non-reactive
load, used two high-voltage caps:  1000 pFd from
ANT connector to Ground and 850 pFd between the
ANT connector and the dummy load.  Settings:

"Padded" 540-1500 KC Tuning Unit, 472 KC
A 4.5
B 03
C 15
D 45
E 8
F 75
K Shorted.
Power out: 40W

So now we can put the covers on and "go to town?"
Well, there's an old saying in the AM community:
"It will work until you put the covers back on. "
Yep... Did that twice.  That's how I found the
"sticky keying relay" and "Burnish K101 more
carefully" issues.  Oh well- if it was easy, it
wouldn't be cool.
https://youtu.be/rIbN35B4x44
(the chatter in the background is the SCR-274N VHF
listening to DFW Departure)

So there it is.  I hope this will help others to
get this interesting transmitter "up and at'em."
Please write about your experiences and
"adventures" with it.

GL OM ES 73 DE Dave AB5S