[AMRadio] B&W 5100B

Jim candela jcandela at prodigy.net
Mon Oct 11 08:02:10 EDT 2004


     I suggest that you look at some old radio handbooks for suggestions on
how to view your transmitter with a oscilloscope using both "envelope
pattern" and "trapezoid pattern" configurations. There is a term, "carrier
shift" where the carrier shifts up or down with modulation. Your PA plate
meter should be pretty constant with modulation. Keep in mind that for 100%
sine wave modulation, your power output as seen on a watt meter should
increase 50%. With "carrier shift" the walls of the trapezoid will either
turn up or turn inward as 100% modulation is approached.
     For your 5100 you will need to make a voltage divider to divide your
audio to the final RF stage (AC coupled) as part of say a 100:1 voltage
divider. This can run into one channel of an oscilloscope, and then use a
sample of the modulated RF into the other channel. Set the sweep to X:Y
mode, and whola, trapezoid!

     At the following link I compare envelope to trapezoid pattern. Notice
that at 100% modulation the peak RF voltage is not quite twice the carrier
amplitude. It is not very apparent there , but on the trapezoid you really
see some curvature at the top (and to the right) where the final PA tube is
going into compression. I recently fixed this by changing the bias to the PA
tube (class AB1 linear amp) where the static cathode current was changed
from 25 ma to 40 ma. The curvature in the form of an 'S' curve has been
largely elininated. From zero to 50% modulation this photo shows (+) carrier
shift, and from 50 to 100% modulation, this photo shows (-) carrier shift.
The curve is much straighter now. Now I need to take more pictures...


     As for your plate modulated 5100, I presume that the RF and modulator
share the same power supply. So as you modulate the 5100, the B+ feeding
both the RF and Modulator stages drops somewhat. This will sometimes show up
as a negative carrier shift. On a wattmeter the negative carrier shift can
be offset by the added total power, and either show no change, or a slightly
+ or - change. Different watt-meters also behave somewhat differently since
the time-constant of the RF to DC rectification circuit can vary. Some have
a peak response, others average. I am not familiar with yours.

     If it is indeed your 5100 with negative carrier shift, I'd first ask
around to see if that is normal for a 5100. It is to some extent with a
Johnson Viking I or II where the dual 5R4 rectifiers drop a lot of voltage
that varies with load. If this is normal on a 5100 then you must decide
whether to keep stock, or customize. The choice is yours.

     If your 5100 is broke in some way, then look at the PA tube bias,
screen voltage, and neutralization (if B&W did that). The tubes (6146) could
also be flat. The HV rectifier tubes (5R4's ??) could also be flat. With my
Viking I the biggest improvement here was to solid state the HV rectifiers.
This will however raise the B+ to over 700 volts loaded. Increasing the
power supply capacitance also helps especially if you have a deep bassy
voice, and altered speech amplifier that passes the lower audio frequencies
more than a stock 5100.

	This is sure fun stuff!

Any 5100 experts out there?

Jim Candela

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Tom Elmore
Sent: Sunday, October 10, 2004 11:37 PM
To: AMRadio at mailman.qth.net
Subject: [AMRadio] B&W 5100B

    I am in the process of restoring a B&W 5100B transmitter and am using a
Coaxial Dynamics
wattmeter to measure power. I normally use this meter for CW & SSB and
noticed that when monitoring an AM carrier the meter deflects downward with
modulation. Is this a characteristic of  this meter or maybe an issue with
the transmitter itself.

Thank You
Thomas Elmore  KA1NVZ
Anchorage, Alaska

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