[Milsurplus] ARC-5 Transmitter at Low B+ (24 volts)

[David Stinson]

Several folks have asked me about this so here goes.

Everything that you need to do to prepare the old girl 
for 600V B+, you still need to do for 24V B+.
Assuming most have just the transmitter and no accessories:

Be sure you have good tubes that haven't been popped 
with 1000 volts by some doofuss.

Lubrication and De-Ox-It where required.

Check the caps under the chassis to make sure they
still have their stator mounting spheres intact.

You will use the selector relay (under chassis) for keying.
Make sure the OSC. contact makes before the 
PA CATHODE contact, that the points are clean
and that the relay is not "sticky."

Disconnect one wire from the "ANT" relay and rotate
the contact around the contact end to keep it connected.

You'll need to transform the low output imped. to match
the average 50-ohm antenna.  Easiest way to do this is
a good quality capacitor in series with the antenna lead:

50 pFd for 40 meters.
75 pFd for 75 meters.
90 pFd for 80 meters.
240 pFd for 160 meters (1885 KC).
Anything -20%, +50% of these values will work OK.
At these low power levels, a good silver mica
or a quality NP0 ceramic disc will work fine.  
If the rig is in good shape, it's not going to drift around 
on you.  Don't introduce drift by using a cheap cap
that is thermally unstable.
You'll need caps with some "huevos" or a good variable
if you want to run the rig at full power later, 
but quality "babies" will work for 24 Voltin'.

Connect all the power pins and fil pin to 24 volts.
Key with the selector line.
Hook up to a 50-ohm load.
Start with the ANT COUPLING at about 4.
Roll the roller coil until you get max out.
Tweek the ANT COUPLING up and the roller
until you get max out.  If you're worried about it
or have reason to believe the PA tuning has been 
boogered-with, you can pull the side plugs,
loosen the lock screw and tweek the final tuning.

If you're keying the rig and it suddenly seems to de-tune
or take a big freq jump (and it's not the roller coil), 
turn the rig over and take a look at the ends of the 
stators on the three variables.  You'll see a small metal 
strip to which solder connection is made, and these are 
"pressed" onto the end of the stators.  The metal junction
between this strip and the ends of the stators have begun
to oxidize and this will cause intermittant detuning.
You'll need to use a good soldering gun to solder
one of the stator posts to the metal strip.  Be careful
not to use so much heat you warp the stator mounting 
spheres, which is a bigger problem than you want, 
believe me.  A good, well-flowed "tack" is all you need.
This is starting to show-up in the variables of some of 
the receivers and transmitters.

That's about it.  Let us all know how you do.

73 DE Dave AB5S