Red Wave Radio

I made my first QSO with the 17 meter DSB rig, but it was more of a
test than a QSO: after much fruitless calling I was forced to resort
to a sked. CU2BD pointed his beam at me from the other side of Ponta
Delgada (3km)...and still had trouble hearing me. He said the audio
sounded OK, but very weak.  

The problem is low output in the DSB mode.  I think I've localized the
trouble to the Balanced Modulator.  This is a simple transformer/two
diode circuit. It appears on Page 139 of W1FB's QRP Notebook.  

I am able to 'balance out' the RF.  When I unbalance the modulator
(using the balance pot), I get about 1 watt out of the final (this is
about right).  But when I once again balance out the carrier and try
to modulate, I can barely make the SWR/Output meter wiggle.  Even when
whistling, I can barely get the meter to move.  I am modulating the
signal, but just not enough.

I don't think the problem is insufficient audio. My audio amplifier (a
741 op amp) is working fine. And I tried injecting a 1 khz tone (at
about 1 volt rms) into the AF port of the balance modulator -- no joy.
I put the same signal into the input of the Op amp. Again, no luck.

So it is as if  half of the balanced modulator (the RF half) is
working fine (I can null out the carrier)  but the audio portion seems
unable to "unbalance" the modulator properly.  I think the audio
should be able to produce a level of "unbalancing" similar to what
happens when I turn the balance pot to either end, but so far it is
not even close.  

I'm using two Motorola IN5821977 Hot carrier diodes.  The both show
forward resistance of about 5 ohms. (This is what I had in the junk
box -- do you guys think this is a suitable component?)

I'm stumped.  Anyone have any ideas on what's wrong or how I should
proceed?

Thanks,    
73 de CU2JL (aka N2CQR)

Bill Meara
Sao Miguel Island,
The Azores, Portugal
http://planeta.clix.pt/n2cqr

There's nothing special about that modulator, and it's the sort of
circuit that has shown up plenty of times before.  Note that he
doesn't even bother using fancy diodes, but simply specifies
"1N914's".

Actually, just to see if the circuit was correct, I went looking
and while simple two-diode balanced modulators are common,
this one isn't quite.  You see it in reverse, ie the signal
comes off the toroid transformer and the oscillator feeds the
pot, and you see other variations of the same basic parts.
Without digging through magazines, the only similar circuit
I found was in Doug's W1FB's Design Notebook, where he shows
a similar circuit, but no pot.

It can't be the circuit.  I assume you've checked the circuit
to make sure the toroid is wired right and the diodes are
properly in place.

Try putting some voltage, current limited, at the junction
of R9 and the transformer.  That should unbalance things and
give you a strong output.  If not, the thing must not be working.
Maybe you've got a false null, that for some reason the pot
isn't balancing things, but just reducing output.

    Michael  VE2BVW

There is no substitute for looking at the circuit with an oscilloscope.
Make sure the output of the amplifier is substantial; if it's not working
the modulator won't work either.  Check the output of the amplifier and see
that it's giving you the gain you expect and that it isn't distorting the
signal.

73, Bob K6DDX

On Sun, 27 May 2001 12:07:46 -0400, Michael Black

Michael:  I put 2 volts at the junction of R9 and the transformer. I
got some small increase in output, but not much.  It was sort of
similar to the levels I get with the microphone -- I can see the
output meter moving, but it is not swinging up to anywhere near the 1
watt level I get when I unbalance the modulator.

A guy who's worked with these circuits e-mailed me saying that
modulated output from this type of balanced modularot SHOULD be about
10db below the level you get when you unbalance the circuit.  If that
is correct, my device is performing to specs!  (But I can't believe
W1FB woulldn't have mentioned this and designed for it -- I'm using
both his DSSC generator circuit and the RF amp that he designed to go
with it.  And he said it would yield 1 watt PEP.)

I'm now thinking that one more stage of RF amplification might bring
me up to the 1 watt level.  I am getting some generation of a Double
Sideband sig -- just not enough.  What do you think of an additional
stage of RF AMP?

73  de  Bill   N2CQR   CU2JL  
http;//planeta.clix.pt/n2cqr