As promised, here's my update concerning the R-4C power transformer and PTO
modulation. First a special thanks to Bill, W6FF for providing me with a
NOS R-4C transformer. Very much appreciated. However, a rare occurrence of
good fortune came to me. It turns out that the existing R-4C transformer
can be rotated 90 degrees and still have enough lead length on the
transformer wiring to terminate the end points -- at least on my R-4C.
Since transformer installation and wire termination was a manual process at
Drake, exact lead lengths will differ. That said, I had more than enough
wire length to accomplish the task without having to try the NOS
transformer. In the links to images below, you can see the transformer
after rotation as well as a ground lug added the PTO shield. The addition
of the lug was suggested by Rob Sherwood in his PowerPoint presentation. He
reasoned that the spade mounting forks do not always make good metal contact
with the PTO frame to adequately allow for good PTO shielding -- although
aluminum is a relatively ineffective shield against low frequency flux. In
his case, he was able to attain a 10 dB reduction in flux induction with the
addition of the ground lug.
http://72.52.250.47/images/R4C.jpg
http://72.52.250.47/images/R4C-1.jpg
The rotation worked. At no point of VFO rotation can I detect PTO carrier
modulation. This is not to say that it is 100% eliminated. A spectrum
analyzer is needed to ensure absolute PTO oscillator purity. However, any
remaining level of induced flux has now dropped down to the point of being
inaudible. I'm saving my pennies for an HP 8591E spectrum analyzer w/
tracking generator. Every time I save enough, temptation comes over me like
it did last Thursday when I purchased a mint S/Line. When I do obtain the
SA, I plan on making more tests, including effects with Mu-metal shielding.
I'm still looking for a source of authentic Mu-metal sheeting.
I was also concerned about potential mechanical interference to the crystal
calibrator board with the transformer rotated. Good news again. There's
still ample room to insert/extract the calibrator board to/from its socket.
No issues here.
Transformer re-mounting: Even more good news. Unlike the R-4B transformer,
the R-4C transformer mounting centers form a perfect square and the existing
chassis holes are fine for mounting after rotation. However, new chassis
access holes ARE required for the transformer wiring. I used a Greenlee
3/4" chassis punch to form two new holes. The chassis is still plenty
strong after inclusion of the new holes, especially since the transformer
mounting frame helps to ensure that the chassis remains rigid after bolt
tightening.
For anyone contemplating transformer rotation, it's a good idea to first
pave the way for a chassis punch. I temporarily removed the chassis fuse
holder and AC line select switch. This gave me all the room I needed
without causing damage (and I'm good at causing damage to my gear in the
shop). Before removal of these two items, I drew a sketch to help remind me
of wiring placement. A quick snapshot with a digital camera is also a good
idea.
Paul, W9AC
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