On Mon, 10 Nov 2008 10:29:31 -0800 (PST), Bob Gibson wrote:
>Hi..I have built a YC156 amp and I have a problem with hum in the
audio..I key the radio in SSB with the amp in line but not keyed
and I get this hum and its coming from the filament transformer..I
opened it up and disconnected the filament transformer then again
keyed and no hum..I have tried grounding all kinds of ways and
still hum..Would some beads help out? Thanks Bob W5RG
The hum is caused by magnetic coupling between that transformer
and the audio circuitry (maybe even within the radio). A VERY
common cause of hum pickup in audio wiring is an un-shielded audio
transformer. The keys to reducing that hum are:
1) Rotating the transformer(s) and/or the wiring so that the angle
between their fields is 90 degrees.
2) Move the power transformer further from the audio circuitry.
3) If the pickup is in the audio wiring itself, reduce the loop
area of the audio wiring. Because the equipment ground (green
wire) of AC outlets is usually part of the signal return path, it
helps to short out this path by running a low impedance bond
between the chassis of all interconnected equipment (that is,
computer, rig, SO2R boxes, etc.) To reduce the loop area (and hum
pickup) of audio wiring, it should be run very close to these
bonding conductors.
This is an AUDIO FREQUENCY problem, so ferrite beads will do
NOTHING.
One other thing to check that could also be causing the problem.
Is the primary of that transformer connected between the two hots
(phases) of a 240V circuit (or between hot and neutral of a 120V
circuit? It should be. If the primary is connected between hot
(phase) and GROUND, a large leakage magnetic field will be formed
by the filament transformer's primary current, and that field is
the cause of the problem. To fix it, you must remove ground from
that current path.
73,
Jim Brown K9YC
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