|[AMRadio] Low modulation |
k4kyv at charter.net
Mon Feb 19 11:24:03 EST 2007
To test the modulation transformer for sure, try high voltage a.c. on it.
The insulation may be breaking down at higher voltage, while appearing
normal at 110 v.a.c.
Using some clip leads or temporary wiring, set the mod transformer on the
floor away from the transmitter, so that it cannot accidentally become
grounded. Connect a wire from one of the modulator transformer plate
terminals to a grounded point on the transmitter. Disconnect both plate
caps to the 8008 rectifier tubes. Temporarily tie one of these so that it
is suspended away from everything and there is no possibility of it touching
anything. Preferably using some high voltage wire, connect the other plate
lead to the remaining modulator plate terminal on the mod transformer.
Again, make sure the plate cap connector where you temporarily attach the
wire is suspended out of the way where it can't short out or touch anything.
Fire up the transmitter. If you have a low power position, try it in low
power first. If you have a meter capable of measuring high voltage a.c.,
see what kind of voltage appears across the secondary. Make sure you are
well away from the thing before hitting the plate switch, since this is a
death-trap setup. If the mod transformer is shorting out under high
voltage, it should blow a circuit breaker. Better still, temporarily wire a
low-amperage fuse in the primary circuit, or connect an electric hot plate
or 1.5 kw space heater in series, to act as a current limiting resistor, in
case there is a short circuit. If it appears that there is no short, try
again in the full HV position. If it still holds up, you are getting
approximately 3100 volts r.m.s. across the plate primary winding, from
plate-to-plate. That should indicate that the transformer is good.
If you don't have a high voltage a.c. meter, connect several 100K 2-watt
resistors in series, and bridge them across the secondary winding. Using
the resistors as a voltage divider, measure the a.c. voltage across one of
the resistors to indirectly measure the total a.c. across the winding. If
high voltage a.c. is appearing across the resistors, the transformer should
Try the same thing with the mod reactor. Apply the high voltage to the
reactor, just exactly as you did with the mod transformer primary. If it
shorts out with the high voltage, that's the problem.
You should now try them in combination. Run the transformer test as
described above, but with the mod reactor connected across the mod
Each time, stand clear of EVERYTHING before turning the plate switch on.
With everything wired up properly, if the mod transformer is shorted, the
833A's will turn bright orange, with adnormally high plate current, when
audio is applied. If the plate current swing is substantially lower than
expected, but each tube shows about the same plate current, something may be
disconnected, or the coupling cap could be open. CAREFUL! UNDER THOSE
CONDITIONS, THE MOD TRANSFORMER MY BE WORKING INTO NO LOAD. THAT IS ONE OF
THE SUREST WAYS POSSIBLE TO BLOW A MODULATION TRANSFORMER.
I mounted my BC1-T mod transformer on ceramic standoffs, and installed spark
gaps across the secondary.
I also made a set of metal standoffs to space the righthand panel about a
half inch away from the cabinet. In its original position, I didn't like
how close the mod transformer winding came to the panel. When I was working
with one of these transmitters at a broadcast station years ago, the plate
transfromer, which is constructed similar to the mod transformer, shorted
out through the paper that covers the winding, when the 220 v.a.c. wire
going to the primary came too close to the paper. It arced right through
the paper to the wire. One of the plastic ties that hold the primary leads
to the transformer in place had disintegrated with age and heat, allowing
the wire to come too close to the transformer. I suspect the same thing
could happen between mod transformer and side panel, if the panel gets too
close. Mine came to within about 5/16" of the panel, and I didn't feel
comfortable with that.
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