Hi Ed, Some scopes don't sync well, either because of bad design from day one, or because of old age. You may do better to just let the horizontal sweep free-run, or lock it to the power mains. Other than that, you could experiment with the trigger settings. You want something around 2 milliseconds per division sweep rate.
A compressor definitely helps, but if there is a lot of phase distortion in the modulator, it will only do so much. The better you get the transmit audio chain and the modulated stage, the more a processor will prevent overmodulation. There are also some multi-diode high-level clipper circuits that help by preventing the overmodulation in the first place. If this is done right, it helps, but it does introduce some harmonic distortion and therefore some splatter. The more gradual the "clipping," the less distortion and splatter. It also helps to get the modulation to the screens set up right. The original circuit was not ideal. It takes modulated B+ and sends it through a resistor to the screens, but this attempts to apply 100% mod to the screens, and they work better with less than 100% mod applied to them. Too much mod to the screens tends to cause a sudden pinch-off of the RF envelope at around 90% negative modulation. That results in distortion, splatter and sudden overmodulation, which is just what you don't want. The best approach is to replace the original single screen resistor with two higher-value resistors, one going to unmodulated B+ and the other going to modulated B+. These two resistors should be proportioned to give the right amount of modulation, and the same amount of screen current as before. As I recall, the ratio was about 60% of the screen current from modulated B+, and 40% of the screen current from unmodulated B+. Look for an article Dino WA1KNX on this subject, probably on this website, and also in back issues of the AM Press Exchange. By the way, I recall that there was a fairly obvious connection error in the original hand-drawn schematic from the first edition of this article, so be careful if you find an old edition - inspect the circuit. Bacon, WA3WDR ----- Original Message ----- From: "Edward B Richards" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Wednesday, April 14, 2004 8:03 PM Subject: [AMRadio] Modulation Question > Hi Guys; > > I am new to this list so please forgive me if my questions are dumb or > redundant. I just rebuilt a much abused Heathkit DX-100B. As far as I can > tell it is operating OK into a dummy load. I have not put it on the air > yet. For one thing, I don't have a 75 meter antenna up yet. I have a > mobile ant I will install as soon as I have time. I do have a MFJ G5RV jr > that is good for 40 meters and higher. I have not heard any AM on > anything but 75 meters. Is there activity on 40 and / or 20 meters? > > I modified a scope to serve as a modulation monitor. I noticed 2 things > that puzzle me. > > 1. It is hard to tell modulation percent with voice as the waveform keeps > changing and unless I use a sine wave I can't sync it to get a good > envelope pattern. Any trick to this? > > 2. Using the mod mon I see that it is very easy to over-modulate. If I > keep the modulation down so I don't see short bright lines in the center > of the screen (carrier disappearing, I think) , the average modulation is > around 50% except for occasional peaks. Is this normal? would a > compression amplifier help? Thank you. > > 73, Ed Richards K6UUZ > > _______________________________________________ > AMRadio mailing list > [email protected] > http://mailman.qth.net/mailman/listinfo/amradio >
