cobra 29 xlr mosfet mod
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Hawkeye138
- Newbie
- Posts: 8
- Joined: Tuesday 27th May 2008, 23:55
- Location: vermont
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cobra 29 xlr mosfet mod
i just wanna know if these mosfet mods will work on 29 xlr? I've never one before and buddy of mine wants 1 in radio im working on for him. Im not a tech but capable of performing work just dont want him to waste money on kit if it wont work, also is there any way to boost recieve internally like on 29 ltd classic?
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speedracer6g
- Light User
- Posts: 165
- Joined: Saturday 23rd Feb 2008, 13:35
- First Name: Michael
- Location: East Freedom, PA
- Contact:
It should be the relatively the same as a 29ltd. Here is how i do them.
-Remover R55, ,56, and C61
-Remove TR14 and Install IRF520N
use a mica insulator if you have one and dont forget the heat sink compound.
-Install EN1230 AtR56 with positive mark facing twoard R123 or the front of the radio.
-Remove C62 and install a 390 to 480pf cap in its place. (see wich one works better for you)
-Add 68pf cap at C59 you will have to test this i dont always get better results from adding it.
-Remove L14 Slug.
-The final thing you must do is set your bias voltage with a 10k or 100k pot.
To do this solder one wire to the gate of the mosfet final. this will be the leftmost leg looking at the front of the fet. Then install the other end to the center pin on the potentiomer. Lastly take another wire and solder it to one of the remaining pot pins. You can remover the leftover leg if you want to. then take that wire and solder it to the positive side of C116. After doing that take a VOM and put the negative on ground and put the positve on the gate of the fet. have you mic gain turned all the way down and have you radio on an antenna or dummy load. then key up the radio. and turn the pot until you see 2.7 -3.2 volts. the higher the voltage the higher deadkey and swing you will get. But you will also get more heat. I usually set mine to 3.0 volts. ok phew almost done.
-lastly i would remove C71 and put in a 33uf 16 volt cap. this will clean up the amc circuit and make it work well with the mosfet.
-If you have a scope or know someone who has one you can set vr4 to where you get the best power output with no clipping.
I can do the mod for you if you need just let me know. i think that is everything. I also have all the parts in stock for the mod I can make you a deal on them so you wont have to go digging for them.
MIKE
-Remover R55, ,56, and C61
-Remove TR14 and Install IRF520N
use a mica insulator if you have one and dont forget the heat sink compound.
-Install EN1230 AtR56 with positive mark facing twoard R123 or the front of the radio.
-Remove C62 and install a 390 to 480pf cap in its place. (see wich one works better for you)
-Add 68pf cap at C59 you will have to test this i dont always get better results from adding it.
-Remove L14 Slug.
-The final thing you must do is set your bias voltage with a 10k or 100k pot.
To do this solder one wire to the gate of the mosfet final. this will be the leftmost leg looking at the front of the fet. Then install the other end to the center pin on the potentiomer. Lastly take another wire and solder it to one of the remaining pot pins. You can remover the leftover leg if you want to. then take that wire and solder it to the positive side of C116. After doing that take a VOM and put the negative on ground and put the positve on the gate of the fet. have you mic gain turned all the way down and have you radio on an antenna or dummy load. then key up the radio. and turn the pot until you see 2.7 -3.2 volts. the higher the voltage the higher deadkey and swing you will get. But you will also get more heat. I usually set mine to 3.0 volts. ok phew almost done.
-lastly i would remove C71 and put in a 33uf 16 volt cap. this will clean up the amc circuit and make it work well with the mosfet.
-If you have a scope or know someone who has one you can set vr4 to where you get the best power output with no clipping.
I can do the mod for you if you need just let me know. i think that is everything. I also have all the parts in stock for the mod I can make you a deal on them so you wont have to go digging for them.
MIKE
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speedracer6g
- Light User
- Posts: 165
- Joined: Saturday 23rd Feb 2008, 13:35
- First Name: Michael
- Location: East Freedom, PA
- Contact:
Sorry forgot about the recieve kit that is just a 2sc2999 and some different diodes i forget the number off hand. works well just throws off the rx meter a bit. I havent had time into looking into that issue yet. for this you do need a signal generator and a ac meter to do an allignment or you will waste your money. I have all those parts as well.
MIKE
MIKE