Paul -
Worth a shot! :-)
Transformers that have the copper band 'shorting
turn' around them typically have a band about 1.5
- 2" wide. I dunno if the larger cross section
would help significantly or not. Thinking of a
band the width of the core, with tabs to fit under
the transformer mounting bolts. ??
Have you tried a piece of steel, (tin can, etc.,)
as a shield just behind the PTO? Solidly attached
to the chassis. IF sufficient improvement to
warrant, try a piece of mu-metal in the same spot.
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:
Stayed up last night and tried Garey's excellent
suggestion. I could hear a difference, but not
enough to claim victory over the problem. Rob
Sherwood wrote back and his notes confirm my
findings. In fact, he addressed this issue in a
PowerPoint presentation he gave at last year's
Dayton Hamvestion. Rob sent me a copy of the
presentation by E-mail attachment. I don't see
it on his website but he had documented the
magnetic flux problem with a spectrum analyzer.
Just to be clear. this isn't 60 Hz ground loop
hum. Rather, the 60 Hz transformer flux is
modulating the PTO, creating many sidebands
above and below the PTO carrier frequency. When
he placed the PTO on the bench, the PTO carrier
is clean to about -100 dBc and completely absent
of any 60 Hz sidebands. With the PTO placed in
the R-4C, the first 60 Hz sideband is down only
-26 dBc. I don't know if he made the latter
measurements at the 500 kHz VFO markings.
However, he indicated that the locking spade
pins on the PTO shield make intermittent contact
to the mounting frame and by adding a ground lug
to the PTO shield, he achieved a 10 dB reduction
in sidebands -- but this is hardly enough.
I strongly suspect the significance of power
transformer orientation had become lost by Drake
engineers in the years after early R4
development. The R-4B's transformer is oriented
properly so there's no flux induction. There's a
significant difference in PTO modulation based
on the orientation of the PTO to the
transformer. Flux induction to the PTO is
highest when it's in-line with the grain of the
transformer laminates (as it is in the stock
R-4C). But perpendicular (like the R-4B), the
problem disappears to the point that I can even
push the PTO right up to the transformer case
will no ill effects, although a spectrum
analyzer would likely show more detail. Rob's
spectrum analyzer photos reminded me again why I
need an SA here for this kind of investigative
work. There's only so much one can do to
isolate problems in the time domain.
Paul, W9AC
----- Original Message ----- From: "Garey
Barrell" <[email protected]>
To: "Paul Christensen" <[email protected]>
Cc: "Drakelist group" <[email protected]>
Sent: Thursday, June 30, 2011 9:53 PM
Subject: Re: [Drakelist] R-4C PTO Hum
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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