Low AM Modulation On President P300

[Wire Weasel]

PM sent

 

[loosecannon]

its the same on the inside as the 146GTL.

your symptoms sound like someone tuned the radio for max deadkey instead of max modulated wattage.

go back and re-peak the TX coils while saying "ahh" into the mic, or do it on SSB with a constant tone.

there is also the possibility that VR5 is bad.

if you remove it from the radio, you can test to make sure it is working, as in the resistance changes when you turn it.

a dirty mic gain control might also cause this, and some contact cleaner might just do the trick.

after that, we get into bad electrolytic caps as the most likely cause, and that can get very tedious.
i just go through and replace them all if its a radio i plan on keeping.
you can also just bridge a new cap of the same value across the old one and check to see if there is any improvement.
that way you dont have to unsolder the old cap from the board until you know its the culprit.

the caps exit13 suggested are a good place to start.

good luck with it,
LC

 

[ExitThirteen]

LC,

I've got this radio on the way to me, and I'll post any findings right here for future reference, but at this point I'm suspecting the TX chain is out of alignment, or a faulty coupling capacitor in the mic audio circuit. I'll let everyone know when I get the radio.


~Cheers~

 

[Wire Weasel]

Good luck ! i sure doo wish i was won of them thar tekyynicians!

 

[ExitThirteen]

Update:

The problem was a capacitor that had gotten snipped near the 2SA1012 AM pass regulator transistor. Someone had replaced the protection diode at one time, and when they snipped it off the 12V connector, they must have accidentally snipped 1 leg of the capacitor. Replacing the capacitor restored AM modulation. Radio now does 100% modulation (4W swinging 16W on my B&K 1040 Servicemaster), SSB does 19W on my Bird 43A wattmeter.

~Cheers~

 

[Darkrfoverlord]

I have one and absolutely love it!

 

[HARDROCK]

WW, I would gladly help you if I knew the answer. The old P300 I possess seems equal in performance on both modes, and not a rock star on either. The internet is a virtual graveyard of info where this radio is concerned, so any search I've made for the potential of this radio is a few places repeating what you said in your first post. It's the same as . . .

Hopefully someone with experience with these radios that are alike will step up and answer your question knowing it is a kind of guess without the radio on the bench in front of them. But again, that's all you're asking for - possibilities.

Good luck on the fix.

I had a p 300 years ago a cb tech either changed the voltage regulator out or put a bigger audio transformer in it.

The radio came alive it had killer audio.
Sorry i cant remember. Good Luck