[AMRadio] RCA BTA1S 80M conversion?

[Donald Chester]

> Has anyone ever added a keying circuit to a ham-converted BC 
> transmitter and used it for Morse or FSK?
>

> *Charles Ring W3NU*


I converted a Gates BC1-T to 160m and added CW capability.

I cathode key the exciter stage, using a TV sweep transistor as a 
switching transistor inserted in the cathode circuit.  It keys the 200 
MA of cathode current of the pair of 807s very nicely.  I also key the 
12BY7 stage that drives the 807s to eliminate backwave. I also added a 
LC filter to round off the sharp edges of the waveform that were causing 
key clicks.

I had to make two additional modifications.  The 833As in that 
transmitter have no protective bias, relying on the overload relay to 
kick out in case of loss of rf excitation.  I added protective bias 
using a 6AS7G tube as bias regulator.

The other problem, probably something to be encountered with most BC 
transmitters, is that since they were designed to operate at full 
carrier 100% of  the time, the power supply filter choke does not have 
enough inductance to maintain critical inductance key-up and with the 
modulator load removed, and therefore the power supply filter tries to 
act like a  capacitor input circuit, causing the +HV to soar dangerously 
high. This also generates a horrendous looking keyed waveform.  I cured 
that problem by incorporating in the AM/CW switching circuit a scheme to 
switch the filter cap and bleeder resistor from the bottom end of the 
modulation reactor over to the top end, in CW mode.  That makes the mod 
reactor serve as an additional filter reactor wired in series with the 
main power supply choke.  In my case, that brings the total inductance 
up to about 65 Hy in cw mode.  I also replaced the 100K bleeder with one 
on the order of 50K IIRC.  With those mods, the power supply voltage 
rises only about 100v, from 2600v to 2700v, key-down to key-up.

I had  to increase the filter capacitance from the stock 8 mfd. to 25 
mfd, in order to get decent dynamic regulation.  Even though the meter 
indicated only about 100v change, the waveform on the scope was 
atrocious, due to the plate voltage bouncing all over the place with the 
varying load key-up to key-down.  I verified the voltage bounce by 
monitoring the plate voltage with an oscilloscope, and observed dips 
down to as low as 1400v and peaks as high as 4 KV as the transmitter was 
keyed.  25 mfd of filtering doesn't make it perfect, but I used about as 
much capacitance as I could get by with without having to go to 
step-start in the power supply, and the waveform of the keyed signal 
became satisfactory.

I kill the +HV to the modulator using the same switch (a large 3PDT 
ceramic HV switch) that switches the reactor/filter/bleeder as described 
above.  One set of contacts opens the line from +HV to the midtap of the 
primary winding of the modulation transformer.


Don k4kyv