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AR3500 no audio on fm tx

D

duldog

Member
Sep 26, 2011
19
0
11
hi everyone!

a friend of mine gave a me an old ar3500 in a sorry state,,, i had it repaired its amp and now it tx nicely on AM/SSB mode except that on FM it simply has no audio (FM/AM/SSB rx is ok)

i haven't started investigating whats wrong on FM tx and i before go any further i would love to have some ideas where to start searching,,,i already had the service manual and circuits but i was just wondering if someone out there had already encountered this before

thanks and more power!
 

I hope you aren't looking at a modulation/watt meter to see modulation, because you won't see any LOL
I couldn't resist replying because I have had numerous local knuckleheads bring exports to me to fix with that same complaint.
"yeah he hears me good on fm but it ain't modulating or swinging on my meters, i think i blew a final" :blink:
 
i'd be checking the wiper output of vr4, then scoping for audio both sides of c149 and r170, then i'd be looking around the fm dev varactor circuit d53 i believe, hard to read the schematic on this netbook screen.

chances are highly probably the fault lies after the 4558d dual op amp or you would loose audio on all modes.
 
thanks guys!!!!

really appreciate the ideas, BTW i don't have a mod meter just a plain pwr/swr meter(diamond sx-200) and another cb rig capable w FM mode for FM tx audio testing,,,

tnx and 73!
 
just updat my friends...

it must have been a bad VR4 (FM deviation adjustment), after i adjusted it fm tx audio is already ok,, just to be sure i just had it replaced and now its working fine all mode,,,

next problem (minor one) is the key beeps,,the unit has no beep so i replaced IC9 (CD4001) and the beep returned however, when the unit is cold the beep is harsh, but after the set warms up, the beep is already ok,,,, but that's just fine,,, thanks for all the help, 73!
 
Glad to hear you've got it going mate, the fm setup for tx on a multimode is generally very simple after the dual op amp, not much to go wrong, couple of diodes,varactor, variable preset or capacitors/resistors.

little history of why fm is an afterthought on multimode cb's and ham radios.

you have to understand most export radios are nothing more than US style AM/SSB radios with FM added as an afterthought to make them more attractive propositions in European and similar smaller than the US standard markets where FM in legal forms predominate (big bucks in new emerging markets with soaring popularity), back here in the 80's cybernet/uniden/maxon and probably others who imported their radios realised they could bypass many european import laws by selling full channel AM/FM/SSB radios with only the FM mode active and the band selector hardwired for FCC mid band, or even before we had that here they did it with uk 27/81 FM active (stalker st9fdx, nato 2000 , superstar 360 fm and probably others). it was always easy to open them up for full coverage and full mode selection,

much the same way as back at the same time yaesu/and their distributors learned slipping 11m crystals in the 4 10m segments could give you 26-28 mhz (a huge selling point in days of mega overcrowded frequencies), that has also gone full circle with the major amateur makers all including general coverage tx/rx with the tx side limited to ham bands but very very easily (in most cases easier than export style cb's) opened up to cover both the very popular 11m pirate band and even to a lesser extent the highly illegal 45m band, they knew exactly what the consequences of releasing those radios would be and exactly who was buying them.

Bottom line is both cb and amateur manufacturers only satisfied the demand of which they knew the public worldwide was craving (freedom of speech), good luck to them and long may it continue.
 

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