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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