Paul -
Interesting.
Something to try. A piece of copper wire, as big
as will fit, looped around the outside of the
transformer in the same plane as the transformer
winding. Short the ends together, forming a
complete 'shorted turn' around the transformer.
I suspect a sheet of mu-metal stuck vertically
behind the PTO, (don't forget the extension of the
guide pin!,) might also help.
Hopefully a combination of small 'fixes' can take
care of the problem, rather than go through the
pain of re-orienting the power transformer!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Paul Christensen wrote:
I spent a part of the afternoon isolating the
PTO audio fuzziness I described when the VFO
dial is moved from "0" then upwards to "500"
when listening to a CW carrier. I hear a pure,
clean CW carrier at the "0" VFO position but
then progressively gets worse, ending at "500."
Based on how the fuzziness behaved, I guessed
that it was more than likely related to a
magnetic field -- either the power transformer
or filament AC currents in the chassis. So, I
completely disassembled the entire PTO and dial
assembly and placed it on the bench, about a
foot away from the R-4C. I ran RG-174 from the
PTO to the R-4C.
Sure enough, the problem is completely gone when
the PTO is external to the receiver. After
about an hour of testing, here's what going on:
As the dial changes from 0 to 500, the PTO's
ferrite core moves toward the back of the
receiver. As the core moves rearward, it is
also coming nearer to the AC power transformer.
PTO cores are especially susceptible to magnetic
fields. To prove to myself the AC power
transformer is the culprit, I took the PTO
assembly in my hands and moved to and from the
transformer. When the PTO is within about three
inches of the transformer, the CW note starts to
become fuzzy. The degree of magnetic coupling
is highly dependent on the orientation of the
PTO to the flux field of the power transformer.
The current orientation of the PTO with the
transformer core (i.e., PTO core in-line with
the laminate core direction, produces the worst
results. By contrast, there's no fuziness
whatsoever if I take the PTO and run it right up
to the transformer when the PTO core is 90
degrees perpendicular to the orientation of the
transformer core.
If a were a "bettin" man, I would say a great
number of R-4Cs (but NOT R-4Bs - see below) are
affected by this -- some to a greater degree
than others. I noticed this mainly because I've
upgraded the entire path from the product
detector to the final AF amp. Audio fuzziness
on both CW and SSB that were previously masked
by mediocre audio performance are now clearly
audible -- but only when the PTO is on the top
of the band -- like when I'm listening to 75m
around 3950 kHz. Down on the CW band edges, all
if fine.
Some ideas on where to go from here and I would
like input from others:
1) Investigate a real Mu-metal shield for a PTO
cover. The stock PTO shield is aluminum and is
fine for RF shielding -- but wholly useless for
low frequency magnetic fields. This is where
Mu-metal shines. Mu-metal is composed of 75%
nickel, 15% iron, 15% copper and/or molybdenum;
2) Turn the power transformer 90 degrees.
Easier said than done because there are no
service loops in the transformer wiring. Would
take extending the leads -- or purchasing a NOS
transformer, subject to availability. I'm also
unsure if the Xtal Cal board interferes with the
transformer in that orientation. More
measurements needed;
3) Bolt the AC power transformer to the back of
the R-4C and get the flux field out of harm's way.
Here's the kicker. When I replaced the power
transformer in my R-4B with an R-4C transformer
to gain some advantages previously discussed
here, I oriented the transformer 90 degrees from
the R-4C mounting scheme as the R-4B's
transformer is 90 degrees turned from the R-4C
orientation. I'll bet R-4B PTOs are probably
cleaner than that of the stock R-4C when the
VFOs are tuned up to "500." My R-4B with all
the audio mods and new R-4C transformer does not
exhibit this problem.
I'm glad I conclusively found the root cause but
I'm a bit bummed-out over what this next level
of refinement is going to take to fix. There, I
said it. This is a fix to a design problem.
Sure, the R4 receivers are meant for
communications and not audiophile use -- but the
problem could have been managed better in
design. It seems more proximity testing of
PTOs to the magnetic field of the power
transformer was in order. Or, perhaps Drake did
run these tests with the older A and B series
but became complacent when they changed the
transformer orientation in the C series.
Pointless to guess, I suppose.
Paul, W9AC
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