[AMRadio] B&W 5100B


Jim candela jcandela at prodigy.net
Tue Oct 12 08:29:26 EDT 2004


Tom,

    This new information might help figure this out. Since the modulation
transformer was swapped, maybe the modulator has a too low P-P impedance to
look into, and then loads the B+ heavily. Does a 5100 have a modulator
current meter indication? It seems that using a scope as a troubleshooting
tool is rarer than I thought. So here are a couple of alternatives. Measure
your B+ as you modulate to see how much it drops. Is your AC line voltage
stable? If you monitor yourself in a receiver that is NOT overloaded, does
it sound OK, or get progressively more distorted as the mic gain is
advanced? All sorts of things could be going on. To those on this reflector
who don't use scopes, how do you troubleshoot a problem like this?

   One last thought. The final RF stage could be breaking into a parasitic
oscillation when being modulated. A high bandwidth scope will show this as
severe distortion that comes and goes with modulation level. A poor mans
parasitic indicator is a neon bulb placed at the end of a pop-sickles stick.
With the transmitter on, place neon bulb near one of the 6146 RF plate caps.
It should glow amber in color. Then modulate the rig. The neon bulb should
vary in intensity with the modulation, and NOT change color. A change to
purple usually means a VHF parasitic is occurring. This would cause severe
distortion, and downward modulation.

Regards,
Jim Candela
"I like Trapezoids!"

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Tom Elmore
Sent: Monday, October 11, 2004 1:26 PM
To: Discussion of AM Radio
Subject: Re: [AMRadio] B&W 5100B


The B&W 5100 transmitter I recently acquired does not have the original
modulation transformer but a replacement made by Stancor.  When I modulate
the transmitter with a 400 Hz tone about 50 % into a dummy load I see a
significant decrease in power at least 25 watts. I measured this with both
my Bird and Coaxial Dynamics wattmeter.  I don't see a return to normal
power until I remove the modulation. I'm wondering if the primary and
secondary on the modulation transformer are possibly backward. The original
owner included some documentation for the mod transformer that shows it's
possible to use either the primary or secondary winding connected to the
plates of the modulator tubes in my case 6146's for different types of
transmitters and impedances.


-Tom
----- Original Message -----
From: "Donald Chester" <k4kyv at hotmail.com>
To: <amradio at mailman.qth.net>
Sent: Monday, October 11, 2004 9:32 AM
Subject: Re: [AMRadio] B&W 5100B


>
> >It's normal for power to deflect downward on AM during modulation. You
have
> >to have a peak reading wattmeter to see the peaks..... Sounds like you
have
> >good modulation.
> >
>
> With an average-reading wattmeter, theoretically the power should shift
> upwards 50% with 100% modulation with a sinewave tone.  An rf ammeter in
> line should kick upwards about 22%.  A field strength meter should show no
> change, since it reads average voltage output.
>
> With voice modulation, the upward shift in power will be much less, since
> the duty cycle of a voice signal is much less than that of a pure sine
wave.
>   The rf ammeter will show little, if any deflection with voice
modulation.
> Whistle into the mic or play some music, and the meters will show more
> positive  deflection.
>
> If the carrier level (field strength) meter, rf ammeter or wattmeter show
> any downward deflection during modulation (voice, tone or music),
something
> is wrong.  The transmitter is showing poor modulation linearity.
>
> There may be a slight downward shift in carrier strength due to less than
> perfect voltage regulation of the power supply, especially with plate
> modulation.  This should be no more than 5 percent.  The power and rf line
> current should still kick upwards.
>
> Don K4KYV
>
>
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