Date: Mon, 04 Sep 2023 01:22:56 -0600
From: John Lyles <jtml@losalamos.com>
To: amps@contesting.com
Subject: [Amps] Subject: Henry hum on transmitted signal
Henry Radio designed their single phase HV power supplies for the 2K and 3K
families of amplifiers using a single section L-C filter. But there is a
special trick in that the 8Hy series choke is resonated with a shunt
capacitor. This cap is critical and it works under stress being in a
resonant circuit at 120 hz (100 Hz for 50 Hz powerline version). It creates
a notch in the 120 Hz ripple voltage and makes that L look like a high
impedance at some load current. All power inductors have some swinging of L
as DC current is varying so this resonance is usually chosen to work at the
lowest operating current, like idling current without modulation. Or key up
for CW.
When in resonance or just on the high side as DC current through the choke
is low (L is highest), the oil/paper capacitor has large circulating
current and high peak to peak 120 Hz voltage across it.
As DC current goes up on voice peaks or key down, the critical Lmin is not
as important, as the RF load is in parallel with the bleeder. You can read
about critical inductance and bleeder resistor selection in HV power
supplies having L-input choke in old ARRL or West Coast Radio handbooks,
and old Terman textbooks.
Anyone, my point being that is this capacitor has failed or drifted off
value, then the 120 Hz ripple will not be attenuated well, at low
modulation it may sound like hum, which it is. Replacement of the 16-20 uF
filter cap will not affect or suppress this low freq hum if the resonant
section is not working. It's job is to attenuate the 240 Hz and higher
harmonics, provided stored energy for the peak RF demands, and help
regulation of Vloaded/Vunloaded. To really knock down the residual ripple
it would need to be much larger.
Mark, i would measure that resonating capacitor first. Be sure to short it
out carefully for residual charge. During operation it could also be
shorting out the choke if it has failed;? disconnect it for the measurement
of both resistance and capacitance. Report back here so we know what you
found.
The cap must be carefully selected, so don't just grab any cap as a
replacement. Henry was remiss in not explaining the importance of this
circuit in their manuals. Resonant choke input filters were clever circuits
but failure can be hidden in them, no fireworks and smoke.
Henry, Harris, Heathkit, and Broadcast Electronics all used them in certain
amplifier models. I designed the one for Broadcast Electronics in the
1980s, and it was a fun challenge for this young EE at the time, building
power electronics. I had luck of finding one of the original choke
designers at SNC transformer in Oshkosh, and we spent weeks perfecting the
L vs current response to work with microwave oven polypropylene dielectric
capacitors as they had around 4kV of AC across them at resonance. Being an
FM transmitter, the only requirement was to hold the voltage variation down
when the excitation was removed on the class C 4CX3500A PA. I never had a
chance to do it for a SSB amplifier though, a more complex design as you
don't want to create an unstable wobbly voltage at some audio modulation
frequency or keying rate.
73
John
K5PRO
### It's the resonating cap that has either gone open..or shorted. With
a 8 H choke in there, to resonate it at 120 hz, a 0.219881041 µF cap will
work. To resonate it at slightly less than 120 hz, a slightly larger cap
is used, like .24 uf...for 115 hz.
Henry radio still sells the resonating caps. You could also have a
shorted /open choke. Those chokes are failure prone. Still, a 16-20 uf oil
cap should attenuate the 120 hz ripple, and multiples of 120 hz. IE: 25 uf
used in a lot of GG amps, like heath / drake etc. With a simple C filter,
ripple reduction is proportional to uf.
Disconnect the resonating cap and measure it. And also measure the choke's
inductance..and also DC resistance. The choke may have shorted, or it may
have gone open. Like John says.... if the resonating cap has shorted, it
will also short out the choke.
The problem with the chokes in the Henry amps is they not only have the
normal pass through DC current flowing through them, but they also have the
additional DC loop current that flows through the choke + resonating cap.
Resonant choke setups really need to be rated for higher current.
Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|