hello again to all. another question concerning mosfet mod. mod is complete and radio is working fine. i now need to tune it a bit. i have a meter but thats it as far as equipment. pretty nice one. the last step in this mod says spread coils for best pep when modulated, adjust L17 and L20 for best pep when modulated and remove L14. ok, i am pretty sure L14 is for dead key. im going to leave that in for now. some people say leave the coils alone which is my plan for now. i dont have a scope so how will i know when i have adjusted each of thesr for best pep. will i be able to see something on the external meter that will tell me.. i assume these are 2 seperate adjustments but not certain what each one does. any help would be appreciater
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adjustments on a 29 ltd
- Thread starter zebbman
- Start date
Cobra 29LTD ST, 29WX ST
Service manual page 1:
http://www.cbtricks.com/radios/cobr..._st/graphics/29ltd-st_29wx-st_sm_pg1_pg11.pdf
Service manual page 2:
http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf
Service manual page 1:
http://www.cbtricks.com/radios/cobr..._st/graphics/29ltd-st_29wx-st_sm_pg1_pg11.pdf
Service manual page 2:
http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf
set the radio to channel 20.
mic gain at max.
best to use a dummy load, but if your antenna is well tuned it should work ok.
connect wattmeter and antenna to radio.
make sure you use a plastic or wooden tuning tool on the tuning slugs.
using a metal tool such as a jewelers screwdriver will give you false readings and can EASILY break the ferrite core, which will really screw things up.
make sure the tool fits snugly in the slug.
tune in this order for max modulated wattage: L20, L21(both slugs), L17, and L14.
to do this, key the mic, and in a steady voice, say "ahhhh" into the mic while tuning the coil for max reading on your wattmeter.
tune slowly.
you will find a spot that "peaks" the reading and once you go past it, the wattage will start to fall off again.
go back and forth until you find the sweet spot, then leave it there and move on to the next coil.
try not to leave the mic keyed for 5 minutes straight while you do this. LOL
you might find that you get more wattage with the slug from L14 removed, so try tuning it and then try removing it to see which is better.
DO NOT spread any of the coils back there.
they will not increase your power, even though your meter may say they do.
they are harmonic filters and messing with them can make your radio bleed on every clock and toaster in the neighborhood.
there is much more to that explanation, but that is the short version.
after doing all this, while saying "ahhh" into the mic, turn VR4 counter clockwise until you see that the wattage is not increasing any more.
leave it there.
no need to turn it all the way up, just turn it up until it does no more good and leave it.
good luck,
LC
mic gain at max.
best to use a dummy load, but if your antenna is well tuned it should work ok.
connect wattmeter and antenna to radio.
make sure you use a plastic or wooden tuning tool on the tuning slugs.
using a metal tool such as a jewelers screwdriver will give you false readings and can EASILY break the ferrite core, which will really screw things up.
make sure the tool fits snugly in the slug.
tune in this order for max modulated wattage: L20, L21(both slugs), L17, and L14.
to do this, key the mic, and in a steady voice, say "ahhhh" into the mic while tuning the coil for max reading on your wattmeter.
tune slowly.
you will find a spot that "peaks" the reading and once you go past it, the wattage will start to fall off again.
go back and forth until you find the sweet spot, then leave it there and move on to the next coil.
try not to leave the mic keyed for 5 minutes straight while you do this. LOL
you might find that you get more wattage with the slug from L14 removed, so try tuning it and then try removing it to see which is better.
DO NOT spread any of the coils back there.
they will not increase your power, even though your meter may say they do.
they are harmonic filters and messing with them can make your radio bleed on every clock and toaster in the neighborhood.
there is much more to that explanation, but that is the short version.
after doing all this, while saying "ahhh" into the mic, turn VR4 counter clockwise until you see that the wattage is not increasing any more.
leave it there.
no need to turn it all the way up, just turn it up until it does no more good and leave it.
good luck,
LC
Just how loud do you expect it to be with a 7 watt deadkey and 12 watts of peak envelope power?
I'd turn it down to 3 or 4 watts dead key and then peak the modulation at 100% at the mod pot...
I'd turn it down to 3 or 4 watts dead key and then peak the modulation at 100% at the mod pot...
please explain. this is exactly what i did to the letter. again im new to this and just trying to learn. please dont take offense to my lack of knowledge. just trying o learn.
Replace stock final with IRF520 MOSFET transistor (pin-for-pin match when installing). Remove R56 and in its place install a 470-ohm resistor in series with a common switching diode (1n914 or similar), with the banded end of the diode towards the front of the radio. Remove C53 and solder it across C59. In C53's place solder a 3.3k resistor. Remove C 61 and solder it across C62. Remove C126. Remove R55 amd solder a 22pf cap in its place. Remove the slug from L14. Replace C71 with a 33-47uf cap. Turn VR4 (AMC) counterclockwise fully. Gently spread L13 and L12 and tweak the spreads for best PEP when modulated. Tweak L20 and L17 for best PEP when modulated. After tuning is finished, set VR4 to half-way.
Replace stock final with IRF520 MOSFET transistor (pin-for-pin match when installing). Remove R56 and in its place install a 470-ohm resistor in series with a common switching diode (1n914 or similar), with the banded end of the diode towards the front of the radio. Remove C53 and solder it across C59. In C53's place solder a 3.3k resistor. Remove C 61 and solder it across C62. Remove C126. Remove R55 amd solder a 22pf cap in its place. Remove the slug from L14. Replace C71 with a 33-47uf cap. Turn VR4 (AMC) counterclockwise fully. Gently spread L13 and L12 and tweak the spreads for best PEP when modulated. Tweak L20 and L17 for best PEP when modulated. After tuning is finished, set VR4 to half-way.
Don't spread the coils for more power. LooseCannon already pointed out that doing this will throw off 2nd harmonics being amplified instead of a pure 27mhz freq. That is the output filter circuit, so you might want to put it back to stock - I would too...
Sounds like the bias was already adjusted when replacing the resistor. Could be wrong. Maybe someone else can figure out what he did here - I'm stumped...
Sounds like the bias was already adjusted when replacing the resistor. Could be wrong. Maybe someone else can figure out what he did here - I'm stumped...
ordered a en1230 to put in instead. im cool with it this way. just as good as stock so no sense in putting it back to stock. gonna put L14 back in and drop the dead key to 3 or 4 like you said and just go with it. if someone has any more info let me know though.
- No one is chatting at the moment.
