[AMRadio] B&W 5100B Modulation Update

George Pritchard gpritchard at comtechpst.com
Wed Oct 13 12:46:52 EDT 2004

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
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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