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
73 de CU2JL (aka N2CQR)
Sao Miguel Island,
The Azores, Portugal