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Low AM Modulation On President P300

Wire Weasel

Wire Weasel

Senior Moment
Dec 13, 2008
3,112
780
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Also same as Uniden PC 244 40 ch SSB

Sideband is fine. Loud clear 15 watts

AM carrier is fine. AM modulation is present but very low, around 10-20%. VR5 mod. adj has no effect. Mods sites also point to limiter being TR29 which is adjacent to VR5. Clip for more modulation. Clipped one leg and no effect either.

Anyone know what component may be bad causing low AM modulation on this unit?

Thanks
 

So what component failure would explain low AM modulation with normal carrier present on any typical/modern CB SSB radio where the SSB side is working completely normally?
 
Well I for one am utterly appalled at the pathetic lack of competent technicians on this forum who can't diagnose and repair my cheap radio over the internet and without the benefit of being able to look at it or check components with testing instrumentation.

Y'all suck
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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.
 
Beggars cant be choosers some radios just wont operate right without a power mike. Given that ssb power output is proportional to mike gain maybe someone screwed up thd alc to mask the problem on ssb. My tristar 747 has the same issue ever since the power mike blew up and i changed to a dynamic
 
By swapping a resistor to change the modulation according to one of the cb sites. I forget what the mod was. Most people on this site are aware of the problems caused by golden screwdrivers so dont be supprised on my account.
 
What's the PCB number on this radio? If memory serves me right, it uses the D2824 PLL, tell me the PCB number if you could, I think I can be of help.

If it's what I think it is for a PCB number, change R156 (should be a 10k ohm, brown/black/orange are color codes) to a 100k ohm (brown/black/yellow). Hope this helps!



~Cheers~

(edit) Also wanted to mention that if the TX chain is slightly out of tune, it can cause low modulation on AM, even though the SSB output is decent. It wouldn't hurt to align the TX chain at some point, but get back to me on that PCB# and I try to guide you in the right direction.
 
Hey 13, Right you are, PLL is D28240

# on the board is PC-833AF

I looked an indeed there is a R156, 10K next to TR29, next to VR5, modulation adjustment. So R156 is looking right. I'll go ahead and try your changeout and report back.



Thanks buddy !!
 
Last edited:
Made the swap. Made no difference. I'm thinking that maybe whichever TR in the circuit is responsible for amplifying the audio for AM is weak. But that is a guess.

Still modulates only slightly on AM. Mic gain is full up and using a known good power mic.

Next idea - any ideas?

TNX !!
 
Have you talked to anyone on AM with this radio? Out of curiosity, has anyone given you a report on how it sounds? Does it sound good on AM with really low mod, or does it sound muffled? If it sounds good, but weak, I'd almost suspect that something is out of alignment possibly. If it sounds muffled, I'd suspect more of a faulty part... can't say I've worked on one before with low AM mod!

Could possibly be a fault with the AM pass regulator transistor, which usually is a 2SA1012 or 2SA473. It'll be located in the back of the radio on the right side, and it's green in color. Could possibly be that, or one of the electrolytic capacitors is bad in the mic audio circuit. If you don't mind experimenting, and have some parts available, try replacing C113, C128, C126, C116, C142 and an unmarked capacitor that's below R161. You could also try replacing TR25 (C945) and TR30 (C711). Good luck!


~Cheers~
 
Hey 13, Mod is clean but very low, hardly moves the Dosy meter on Modulation, maybe 10-15%

I don't really feel like doing much more to it. Will probably end up a parts radio sale. Not that important but many thanks for the ideas!
 

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