[ARC5] Looking for BC-AL-430 Transmittter Info and Related Items

[David Stinson]

Some Tips for the BC-230 / -430 Transmitters.

Disclaimer:  
I don't know everything.  
Sometimes I don't remember how to tie my shoes.
I learn as I go, so some things I wrote long ago are obsolete.
This is the best information I have, 
right up until I hit the "SEND" button.
After that, all bets are off....
Other comments (about the radio) are welcome.

Tubes:
The majority of the the transmitters sitting on shelves are 
missing the tubes, gone to feed the Audiophoolery market.
There are always "trade-offs" when trying to find a solution
to the problem of $150 tubes. 
Many 6-volt power tubes, triode-connected will work 
as replacements.
Here's a go at using 6146 in a BC-223.  Works excellently
on CW, but haven't gotten it to settle down enough for phone:

http://home.netcom.com/~arc5/BC-223/6146.jpg

This of course completely changes the MO and PA
calibration figures, but that's a minor issue compared
to spending hundreds on tubes for a 4-watt rig.
With the BC-230, I've had some luck with triode-connected 
5763 and a base adaptor as PA and Modulators.
Others tubes have worked in these stages.   
While there seems to be no problem with using subs for 
the PA and the Modulators, getting the right tube for 
the Master Oscillator is tricky.  When using the radio
as-designed with a VT-25, it is remarkably stable.
Many subs will chirp and drift to various degrees
and can be difficult to neutralize, 
though the 5763 neutralized without trouble.

While the original VT-52s and VT-25s have fast- heating 
filaments, heater/cathode tubes like the 5763 do better 
if you give them 15-20 mins or so to warm-up and 
settle down.  You need to put the top on the rig if you
have it, because it will improve stability by bringing some
"thermal balance" to the set.
Connect the cathode to the side of the heater 
closest to Ground.
 My work on subbing and stabilizing the set with 
less-expensive tubes is on-going.  I'll post any 
new progress.  So far, the 5763s look promising.

Coil Sets:
Tuning coils marked to bottom at 4 MC will generally
go at least to 3850 KC.
http://home.netcom.com/~arc5/SCR-183/TXcoil.jpg

The transmitter coil sets have two compound inductors- 
one for the OSC and one for the PA and ANT.
There is a small, separate coil form mounted within 
the PA inductor (unshielded, with the ANT loading "slider")
with a single winding of very fine wire.
This coil is used to neutralize the PA.
In about a third of the coil sets I've tested,
this coil has gone open and must be rewound.
A meter on these two pins should read 30-40 Ohms
or there-abouts if the coil is good:

http://home.netcom.com/~arc5/SCR-183/neut1.jpg

If your coil is open it is easily repaired with a little care.
Mark the PA coil mounting, the coil and the coil frame
so you can get everything back together correctly.
Remove the coil from the mounting and turn it over.
Inside, you will see the smaller neutralizing coil.
Unsolder the leads and cut-out the mounting pins,
leaving enough of each pin for you to grasp.
*Read below before attempting to remove the coil.*

http://home.netcom.com/~arc5/SCR-183/coilfix.jpg

*This is important:*
Do NOT attempt to unsolder the mounting pins
and re-use them.  This internal coil form is fragile-
the 1930s-production forms are nearly as fragile 
as egg shells.  The diameter of the mounting pins 
fits the holes in the form exactly.  
The slightest remaining solder on the pins WILL 
break the form if you try to unsolder them and 
pull them outward, intact. Cut-out the soldered "cross" 
and pull the studs gently INWARD to remove them.
Once you have the coil form out, the winding 
can be replaced with regular enamel-covered wire.

Frankly, I don't now remember what I used for
mounting pins.  You're smart- you'll find something.
I do remember mine weren't quit as tight as 
the originals.  Two tiny dabs of "Shoe Goo," the 
excellent all-purpose glue in the Shoe department
of your local WallyWorld, secured the form.

Mounting:
The transmitter must be properly grounded and,
preferably, mounted over a conductive ground plane
along with everything connected to it.  
Skimping on grounding can cause stray RF into the 
transmitter and FMing.  
This is easy with the dynamotor and control boxes but
a bit of work if the transmitter is "stand alone."
If your power supply is "remote," you may need to add
decoupling by passes to power and audio leads
at the transmitter.

Power and Connection:
The BC-230 needs 12 volts.  The -430, 24 Volts.
The PL-64 connector for the rear of the transmitter
appears regularly on Ebay for a reasonable price.

In the original design, the transmitter was keyed via
a relay in the B+ lead.  Despite protests from some
quarters, this is, IMHO, still the best way to do it.
Supply 300 V B+ via a set of keyed relay contacts
or a MOSFET switcher with RF decoupling on the Gate.

Connections for using the transmitter "stand-alone:"
http://home.netcom.com/~arc5/SCR-183/BC230TX.JPG

Antenna Matching:
As with nearly all WWII aircraft transmitters, you'll
need some form of external matching to feed a 
50-Ohm antenna.  This has been covered many times.
The simplest way, IMHO, is a 200-pFd or so air
variable or 100 to 150 pFd RF-rated fixed cap
in series with the antenna lead (thanks for those, Mike!). 
Follow the tuning procedure in the manual and tweak 
for best output.
Note the passage in the manual about keeping the 
PA current down to 25-30 mils so the modulator stage
can deliver enough power.

Neutralization:
Proper neutralization is essential to stop FMing on voice.
The procedure is easy, but touchy.
There is a small hole (usually with a swing-cover) between
the PA and MO tubes.  Inside is an adjustment with 
a quarter-inch hex head.  This is the Neutralizing cap.
A completely non-conductive tool is needed here.
There used to be grey "Philco Alignment Tools" available 
but I haven't seen them in awhile.  You can take the stiff
barrel from a Bic pen, heat it up and press the hollow end
over a 1/4 hex wrench or the head of a 1/4 drive bolt,
then trim.  You're making a plastic nut driver.

Once you have your Neutralizing tool, key the radio
and tune it up on your operating freq.  
On the side of the rig, there are two holes with jacks.
One is for measuring PA current.  Insert an INSULATED
and open 1/4 inch phone plug into the jack, which removes 
B+ from the PA.  Monitor the output of the rig with 
a scope (preferably) or a very sensitive meter or even 
a remote receiver.  Key the rig and adjust the Neutralizing
cap for minimum output.  Remove the plug.
The best audio and the least FMing will be within
a couple of degrees of this setting.
Listen to the rig on phone.  If you have it tuned and 
neutralized correctly, it should sound pretty good.

These are cool little rigs and, when properly powered
and tuned, are clean and stable.  Would be a treat to 
hear more of them on the air.

GL OM ES 73 DE Dave AB5S