[AMRadio] B&W 5100B Modulation Update


Tom Elmore tom at telmore.com
Wed Oct 13 14:48:02 EDT 2004


Yes there is a plate current position and it shows a definite rise in
current with modulation. There are dual 5U4's in the supply feeding the
finals and modulator and I swapped them both out with the same results. I am
leaning towards Gary suggestion that there is an impedance mismatch causing
excessive current draw. Does anyone have a 5100B or know what the impedance
of the primary and secondary should be. I will try another set of caps this
evening when I get home there are 2 , 40uf in series on the B+ line.
----- Original Message ----- 
From: "Gary Schafer" <garyschafer at comcast.net>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Wednesday, October 13, 2004 10:29 AM
Subject: Re: [AMRadio] B&W 5100B Modulation Update


> Tom,
>
> Try a new rectifier tube if you have one and see what you get.
> Easy way to check capacitors is to tack a new one in parallel with the
> hv filter cap and see if you get any improvement.
>
> Does the 5100 have a meter position for modulator plate current? If so
> what is it doing.
> If you have the wrong impedance taps on the mod transformer, the
> modulators may be drawing much more current than they should. Too high
> an impedance tap into a low impedance final load will make the
> modulators draw very heavy current.
>
> 73
> Gary  K4FMX
>
>
> Tom Elmore wrote:
> > The caps are the first thing I should have checked and there is a good
> > possibility they are dried up except I don't hear any hum in the audio
when
> > monitoring on the receiver. Also I can make at least 125 watts carrier
and
> > don't see much of a voltage drop on B+ when I run carrier only.
> >
> > -Tom
> > ----- Original Message ----- 
> > From: "George Pritchard" <gpritchard at comtechpst.com>
> > To: "'Discussion of AM Radio'" <amradio at mailman.qth.net>
> > Sent: Wednesday, October 13, 2004 8:46 AM
> > Subject: RE: [AMRadio] B&W 5100B Modulation Update
> >
> >
> > Tom,
> > It seems to me that the capacitor bank for the 600 volts is not working.
> > Look at the B+ with a scope, and you may see the B+ drop at the audio
rate.
> > A "stiff" capacitor bank will show no audio waveform on the B+. If I'm
> > right, the caps are died out. Is there any trace of Hum on the "dead
> > carrier"?
> > George AB2KC
> >
> > -----Original Message-----
> > From: amradio-bounces at mailman.qth.net
> > [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Tom Elmore
> > Sent: Wednesday, October 13, 2004 12:35 PM
> > To: amradio at mailman.qth.net
> > Subject: [AMRadio] B&W 5100B Modulation Update
> >
> >
> > Now that I have had some time to troubleshoot the B&W 5100B modulation
here
> > is what I found. The B+ is around 600 volts with no modulation and I as
> > start to modulate and watch the scope when it reaches about 50%
modulation
> > the B+ drops from 600 to around 400 volts.  I changed the final and
> > modulator 6146 tubes and see the same results. The original modulation
> > transformer has been replaced with a Chicago Standard Transformer
> > Corporation # A-3891.   When I remove B+ to the final's and terminate
the
> > modulation secondary into a resistive load of about 6k ohms I still see
a
> > significant drop in  B+ as I start to bring modulation up. I don't have
the
> > original mod transformer and the manual I have doesn't say what the
> > impedance should be.  I have tried various other tap settings on the
> > transformer secondary and don't see much of an improvement in reducing
the
> > amount of voltage drop. I wonder if it is possible that this mod
transformer
> > just will not work with this unit. Anyone have experience with mod
> > transformers using 6146's as finals and modulators?
> >
> > Thank You
> > Tom Elmore KA1NVZ
> > Anchorage  Alaska
> >
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