[ARC5] Tuning-Up the GF Transmitter

[David Stinson]

Tips on tuning-up your GF transmitter.

Note: Make sure your tuning drawer is indeed for GF.
SCR-183/-283 drawers will fit, but will not work.
Also, I know most people aren't building a complete set.
If anyone is interested in using their GF "stand alone,"
I can post connection and power info.
Pull out your 837s before trying to remove the rig
from the case.  No counting how many 837 plate caps 
have been broken off by forgetting that.

After a good cleaning, lubing/De-Ox-It-ing
and replacing the bad caps, get the right-sized screwdrivers 
(the screws in this rig are soft) and loosen/retighten every single 
grounding point and mounting screw you can find.  
Even the "Ground" post in mine was loose.
As with many of these WWII rigs, the grounding points 
are going Hi-Z for RF and even for DC.  Bad grounding 
points will result in parasitics, drift, poor mod 
and other unpleasantness.

Speaking of drift-  I can't say for the later Western Electric 
sets, but this A.R.C. transmitter is yet another example 
of their superb engineering.  The transmitter is a 1932 design 
and a 1939 build.  After a decent "warm-up" period, 
it is more stable than just about any commercial "ham" 
rig from the 1950s and '60s pre-synth days.  
On 75 meters, from a cold start to 1 hour on, 
the rig drifts about 600 cycles. 
Then it sits within 100 cycles during the next 2 hours.  
Of course, I'm using the dynamotor
 and that may make a difference.  
The rig is nearly as stable on 40, though it does drift 
about 300 cycles during key-down.  
I think this has to do with the "temporary" series cap.
Will know better when the good ones get here.
The only real freq problem I have with the transmitter 
is getting it set in the first place.  
The MO adjustment is "touchy" and subject to backlash.  
This may be something I can fix with mechanical work.  
I'll look further.

Conditioning and tuning the output of the GF transmitters
to match a 50-ohm transmission line is somewhat different 
than that for SCR-274N or for the SCR-183/-283.
While you still need a cap in series with the output and
(to do it "right") a 4:1 UNUN installed in the antenna switch,
the cap values are different from those in the other sets.

You can get full output ( 10-12 W on CW ) with just a 
good series capacitor (though the tuning points will change)
but series cap plus UNUN works better and you get to see
the RF Ampmeter work.

The series cap can be installed right inside the tuning unit.
At the base of the output coil is a short buss-wire jumper
to the pin socket which connects the Antenna post.
Here is a photo of the installation point:

http://home.netcom.com/~arc5/tunecap.JPG

Note that this is a "temporary" capacitor.
I have good-quality 2 KV DC, NP0 caps on order.
Cheap caps will heat up, eat your output and
pull the oscillator as the PA loading changes,
all in preparation to liberate the smoke captive 
in the cheap cap.  Spend the 87 cents to get a good one.

Here is how to make a 4:1 UNUN, which you can install either 
in your antenna switch or "stand alone" at the Antenna post:

http://home.netcom.com/~arc5/UNUN/unun2.jpg

These are the tuning units I'm currently using on 75 and 40 meters,
with values and transmitter tuning settings.
I don't know if all GF tuning units have the 1-10 scale 
on the sliding coil tap, but all mine do:

Unit for 3675-4525 KC, tuned to 3890 KC.
Series Capacitor:  150 pFd
Coil tap: 4.5
Osc:  1239
Ant Coupling: 20
Power Out in CW mode:  14 Watts.

Unit for 7350-9050, tuned to 7160 KC.
Series Capacitor:  95 pFd
Coil tap:  5
Osc: 240
Ant. Coupling: 22
Power Out in CW mode: 12 Watts.

One of our members, who has kept me from stearing onto the rocks
more times than I can count, is sending a 4000-4900 KC unit.
I will post results for it if there is any interest.

73 DE Dave AB5S