Grant 2 deviation

[rpcomms]

OK just a little update
checking the resistor across resonator on AT-5555,Lincoln 2,CRT6900V3
R254 next too Y1 resonator check value some ive found too low typical 1K8 to 2K2 in value,I found around 3K3 to 3K8 (on resonators ,not coil based detectors) in value improved things on AF gain detected from FM chip.
to increase detect range around 1.2khz dev to about 3khz dev,full volume AF volts from radios speaker jack into 8ohm loads
ull need a fm RF gen to test it
this will bring RX FM detector gain up,and no more "you sound low to me"
CRT SS6900 has a 3K2 so around 1.5khz dev starts to get gain from fm detector,i went with 3K9
now fm detector 60% deviation system on cb is 1.2khz(not 1,5khz),ull get full volume from set around 5,5V RMS into 8ohm load

so now more compatible with legal cb fm dev system 1.2-2.00khz and upto 3khz tops

I know why this has happen,but too long a story to tell,ive tried this on CRT6900V3 now have bags of recovered audio in fm mode so stations dont sound quiet around transmitting legal fm devs typical 1.2-2.00khz as it should be!

you can apply this resistor trick with fm decrim detector coils,ull see a resistor added,increase value for more recover audio gain at your 60% dev level,so legal cb is 1.2khz 10khz spacing system
ham narrow 1.5khz 12.5khz spacing system
ham wide 3.00khz (not advised) 25khz spacing system

OK did same on AE-6110 add a 3K2 resistor in place.
You can do same if have a wide fm HF radio go from 3khz 60% dev to 1.2khz 60% dev by adding new resistor value,thats how they switch wide\Narrow on radios by different resistor values for AF gain adjustment,Plus filters and TX dev!

Ull also notice have slight better squelch action as more white noise detected now gain increased.

I havnt got a grant 2 or 2 + so cannot confirm resistor value but looking at schematic it uses a MC3361 with FM coil detector,ull see R54 33K
may need to go up slightly in value to get more AF volts from FM chip at 1.2khz dev,use a RF fm sig gen to check b4 modding
-73dBm full volume,measure across a 8ohm resistor load on ext speaker jack,need scope to measure in RMS or meter

change dev level on RF sig gen and see if AF volts drop around 1.5khz deviation or less down to 1.2khz dev cb 60% level,if it is change R54 to say 47K,that should bring fm detector level up in gain more suit for new cb specs of around 2.00khz dev max on untouch ETSI modern cb radios
reduce volume control till can see a sine wave at 1.2khz
sweep RF mod level from 1.2khz upto 3khz dev should be able window for legal cb
and check for any audio distortion and new window range is suitable for fm cb use

thats how got my CRT6900V3 optimized on RX,now no one sounds low dev to me lots of gain detected
Down sides,having to ask people to reset your devs correctly! to 2.0-2.2khz max and no more,thats the real pain!
then trying to explain why,sigh

I have used this method to convert ham HF FM boards from wide fm to cb narrow fm and no complaints of people sounding low audio
As a note the filters do not affect the fm det audio!
only that resistor either across fm detector coil or resonator does.

so please dont turn fm dev's beyond 2-2.2khz otherwise be splattering over 10khz channel spacing
worse still radios that are correct will sound distorted or worse heard squelch chopping in out from over FM
deviation,esp at distance

Dev is not mic gain!
If your set cannot produce enough peak deviation,it down to lack of microphone gain! or poor setup electronics in CB
Fm always needs to be audio driven hard to get the clipping action(if radio supports it),that how you get audio punch.
sometime you can change the op-amp gain via a resistor inside cb to drive the fm clipper harder
always inject a high audio signal at 1khz say 30-50mV RMS,when testing radio on deviation meter (ensure using 15khz LPF)
then set dev for 2.00khz (approval specs) or can lift it a little higher around 2.1-2.2 max and no more
Diode clipping is best method ive found to get Fm waveform correct,almost squre-wave + and -,thats compression helps aid the audio punch needed on fm mode.
oh should have mention do a audio sweep in transmit,often seen the roll of filters way to sharp and fm modulation dull and not bright sounding
there normally a active filter in transmit side usually a 2 stage op-amp filter affair.
ive found pushing audio out a bit wider at 3khz roll off and then setting dev at 2.00-2.1khz max has nice sound and clarity as more hi tones in modulation,some of these new cb ive seen start rolling tx audio of way too soon!
Again if your a cb tech youll know what im on about.

Hope all this info is of some help for you chaps,sorry if its a bit technical,some of the good cb techs on here can expand\Understand on this in more detail,im too busy repairing niggling bloody defects in china cb radios at mo! wish we had schematics!

rob 73's

[rpcomms]

CRT6900V3-A
all modded
TX fm sounds lush wide 3khz roll off,added a diode clipped fm
RX Fm detector @ 1.2khz and up full volume
new wider 455khz filter for fm use
Am crisp TX\RX and wider audio
compression fixed too not so aggressive
squelch close delay now 5mSecs
RSSI agc fixed debounce
Fixed Hi-Cut
thermal internal heat cut down on crystals via cover socks in plastic foam material
no more over heating issues,has 18AMP reg and internal heatsink
address a few issues well happy with set ,shame could build it this way detail detail

still waiting on Alinco DX-135cb should be able to cross mod this with same parts hopefully.
well happy with set,like to try a Ver V6 next.
really like these radios now.

rob 73's

[rpcomms]

just another update ive checked a few recent radio schematics and noticed the wrong incorrect resistor value across the 455khz discriminator resonator on the MC3361 fm chip used!
seems our friends in china have mixed up the values between wide fm and CB narrow fm,the data sheets often show the resistor set for 3khz 60% system for fm wide.
Ive done tests and found 3K9 suits fm cb system 60%=1.2khz
ull get full volume from set and stations wont sound low audio to you.
AT5555-V3.1 R254=fit 3K9 0603 0.1W
AT-5555-V6\7 R254 they fitted 4K3 value,i think this is too high and would swap to 3K9 as per V3.1 mod
Lincoln 2+ R254 2K2-change to 3K9
AE6110 found 3K2 works best

On some cb radios they use a tuned inductor coil with the MC3361 fm chip
47-68K in value was found best for 60% dev @1.2khz for max audio from speaker
but recommend checking on fm signal generators and monitor speaker audio volts,at where peak volatge kick in,if too low at 1.2khz deviation,raise value of resistor for more audio gain recovered,if fm detector too senitive ie 1khz or less and full audio gain ,reduce resistor value.
Max audio needs to kick in around 1.2khz which is 60% deviation level

it was also noted the fm squelch improved response.

TX fm deviation should be set with microphone gain max
if its a anytone AT-5555 note there is no fm speech clipping/limiter so fm will sound soft modulation
using two schottky diodes
back to back to each other at the feed point of dev pot,you can add diode audio clipping to the speech audio
resulting in highly compressed transmission on fm and audio punch,it will almost square the audio waveform and produce a much better punchy fm audio quality.
Ive successfully done this diode trick on AE-6110 and works fab in fm mode now
you may need to change the feed resistor to wiper of deviation pot lower to get enough audio drive as diodes will load TX audio.

set deviation (use 15khz LPF on FM Deviation meter-Importants! esp IFR/Marconi 2995 test sets)
2.00khz max with 1khz test one for EU CB use OR
2.20khz max and 1khz test tone more for UK40\ham use

speech peaks will be slightly higher when a real microphone plugged into radio,bear this in mind when setting the fm deviation up,i tend to use a 50mV RMS audio signal to ensure enough audio gain drives radio under test on microphone socket.

Hope this helps chaps
Rob 73's