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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