[SEDXC] FW: [Drakelist] R-4C PTO Hum

[Ed Tanton]

I didn't know about the copper transform strap/band on some transformers
being-effectively-a shorted turn for RFI/hum reduction, did you?

-----Original Message-----
From: drakelist-bounces at zerobeat.net [mailto:drakelist-bounces at zerobeat.net]
On Behalf Of Garey Barrell
Sent: Friday, July 01, 2011 9:36 AM
To: Drakelist group
Subject: Re: [Drakelist] R-4C PTO Hum

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" 
> <k4oah at mindspring.com>
> To: "Paul Christensen" <w9ac at arrl.net>
> Cc: "Drakelist group" <drakelist at zerobeat.net>
> 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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