[AMRadio] B&W 5100B Modulation Update


Edward B Richards zuu6k at juno.com
Wed Oct 13 15:18:17 EDT 2004


Sounds like the power supply cannot deliver the current necessary. Could
be excessive resistance somewhere along the line, or bad filter caps, or
even the primary 120 VAC dropping . Start with the Primary AC and work
your way through the power supply, measuring voltage drop under
modulation. Don't overlook resistive ground connections. I will bet on
the caps. Good luck.

73, Ed Richards K6UUZ


On Wed, 13 Oct 2004 10:48:02 -0800 "Tom Elmore" <tom at telmore.com> writes:
> 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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