W2XC Hum Elimination Modification
- An easy
$20 fix !
- No holes
to drill !
- Under an
hour job !
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- I picked up this amplifier
a few years ago....
- I used it for awhile with
my "hybrid" rigs but after talking to a loud local station
on 75 Meters he reported a hum on my signal...
- To my dismay - I confirmed
the report by looking at the output on my scope...
- I simply "assumed"
old filter caps and shelved the amplifier for a rainy day repair...
Comes On - Thanks In Part To The Web:
- While surfing for some filter
board info. on the web I came across a forum...
- In the forum a fellow mentioned
how weird it was that Dentron decided to feed the finals in this amp.
on one side and was wondering what resistors he should use to "balance"
- WELL...."The light
came on" - I knew this from both audio & RF circuits - directly
heated cathodes have to be balanced fed OR you WILL have hum induced
by the AC filament voltage...
- I could not believe it -
so I pulled out the schematic and sure enough - Dentron pushed the cost
cutting envelope too far. The filament winding of the power transformer
is NOT center tapped...
- At this point I kicked myself...as
sure enough when I hooked the amp. back up - the hum observed on the
scope was in fact 60 Hz.!
- I never did like the two
resistor trick (ie...low value resistors from the winding to ground
to form a mock center tap) and besides it eats power...
- By paralleling a one (1)
amp. 6.3 VAC center tapped transformer AND using the center tap as the
ground return I can duplicate the required filament feed...
- A Hammond
166J6 (see below) was a perfect solution - the vernier is open to
1" in the photo. It easily fit on an existing bolt on the power
transformer (ie...NO drilling required) see below...
click to enlarge
- Opening the Clipperton-L
- I noticed there is plenty of room to fit the transformer (un-modified
photo to the left).
- As luck would have
it the transformer bolts are NOT varnished on.
- The brown wire (above
the filament RF choke) needs to be disconnected off the yellow
filament wire and attached to the center tap of the Hammond 166J6.
- An open hole is avalable
next to the filament RF choke for a terminal strip (it JUST doesn't
get any better than this - NO DRILLING REQUIRED).
- Remove the power transformer
bolt and mount the 166J6 with one hole. I also added a second
nut on top to lock it.
- Terminal strip added
and installed on existing chassis hole. Brown wire is lifted off
and connected to the center tap of the 166J6 (the green wire with
the yellow tracer) on the new terminal strip. I also bypassed
the connection to ground with a .01 mfd. ceramic capacitor.
- The two green wires
of the 166J6 are connected in parallel with the filament winding
(the two yellow wires of the original power transformer). The
primary of the 166J6 (the black wires) is left "open"
by simply cutting off and insulating both wires from each other.
- The hum is completely eliminated
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