W2XC - Dentron - Clipperton-L Hum Elimination Modification

Clipperton-L W2XC Hum Elimination Modification

- An easy $20 fix !

- No holes to drill !

- Under an hour job !

Some Background:

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...

The Light 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" the filaments...
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...

The Solution:

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...

Hammond 166J6

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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).

click to enlarge

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 Result: