[AMRadio] B&W 5100B Modulation Update


Gary Schafer garyschafer at comcast.net
Wed Oct 13 14:29:52 EDT 2004


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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