142GTL

[Marc]

I know that TR32 controls the amc on this radio and has no VR to ajust it. But is there a fixed resistor that can be changed to open it just a bit???

[2600]

TR31, 32 and 33 all contribute to the AM modulation limiter. TR31 and 32 are also needed for sideband ALC. Never remove them if you ever plan to use sideband.

TR33 is only used to detect the AM modulation percentage. Removing it will disable ONLY the AM modulation limit, without screwing up sideband transmit.

If you want to leave TR33 where it is and adjust it, remove R105, a 1.5k 1/4W resistor and replace it with a 2k or 5k trimpot. This is the point in the circuit where you find the trimpot in a radio that has adjustable AM modulation limiting. A 1k trimpot will probably work well enough if that's all you have handy.

R105 should be close to TR33, and is wired from the base of TR33 to ground.

73

[Marc]

Thanks 2600 That is just what I was looking for!!! Would that be more resistance gives you more audio or less

[2600]

You will get more modulation the lower the resistance. Higher resistance will reduce the max modulation level.

73

[Marc]

Thanks Chris. And that DX-300 still going strong!!

[Marc]

Hay 2600 by the way mine had D70 insted of R105 why did they switch from a resistor to a diode. What did the diode do that the resistor did not??

[2600]

Ummm. Okay.

Transistor TR33 is turned off when you key the mike. The steady DC voltage feeding the final and driver in AM mode comes out of the modulator transistor TR34 by way of the center pole of the mode selector switch. Resistors R124 and R125 divide this voltage down to about 2 Volts DC and feed it to the emitter of NPN transistor TR33. This transistor stays turned off since the voltage divider R105/R104 feeds about 0.8 Volts to the base of TR33. So long as the emitter voltage stays higher than about 0.2 Volts (to ground), TR33 stays turned off. TR33 won't turn on until the base voltage is 6/10 of a Volt higher than the emitter voltage. It's the difference between the base and emitter voltages that controls when TR33 will be on, pulling current from its collector to ground, or turned off like it is for a dead unmodulated carrier. When you modulate the DC power to the final/driver, your negative modulation peaks will appear on TR33's emitter. When your negative modulation peaks push the emitter of TR33 to a low-enough voltage, it switches on and pulls base current to ground from PNP transistor TR31. Of course, TR31 is also turned off UNTIL this happens. When TR33 turns on TR31,, its collector will feed base current into TR32, which causes TR32's collector to act like a resistor to ground in parallel with your mike audio, reducing the input level to the mike amp. The resistors and capacitors wrapped around these three transistors serve to slow down the response of this circuit so that only brief pulses of your negative modulation peaks charge C90, causing the mike gain to stay reduced for a few hundredths of a second until the next negative peak comes along, and repeats this sequence. If you look at the modulation on a 'scope, you'll see that this circuit takes a tiny fraction of a second to "kick in", allowing a brief pulse of over modulation when you first speak up.

To increase the max audio level it will let you have, you can reduce the steady DC voltage on the base of TR33. That causes the "trigger" level that activates this sequence to be a negative modulation peak that goes nearer to zero Volts feeding into R124/R125. This translates to a higher negative modulation percentage.

The AMC pays no attention to your positive modulation peaks, only the negative ones. A lower "peak" negative modulation waveform just means that the limiter kicks in at a higher modulation percentage, and you get a louder "upper limit".

I suspect that using a diode instead of R105 just allows a higher modulation level while keeping the radio more or less legal. The diode in that spot will hold the base of TR33 to a fairly constant 0.6 Volts. Since you will now have to pull the emitter of TR33 down to about zero volts to turn it on, you'll get a higher AM modulation level that you would with the resistor in that spot. Since the turn-on voltage for TR33 is the same 6/10 of a Volt that the diode will show in that circuit, it may provide some temperature compensation to make the circuit more stable over a wider range of temperatures.

Maybe? Since that base-to-emitter turn-on voltage changes with temperature, the voltage on the diode should drift up and down more or less in step with the transistor's temperature drift and make the modulation setting more stable over a wider temperature range.

Maybe, And then again maybe the designer of that circuit had something completely different in mind.

73

[Marc]

Thanks 2600 That is way more than I excpected but I am not going to ignore it lol. But I do see what is going on in that circuit now!! Again thanks

[BBB]

Very informative, thank you 2600

[mark4]

There is a guy wide banding these radios. Not sure what he is doing with the audio circuit?