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cobra 29 mosfet problem

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BIONIC_TEXAN88

Active Member
Aug 1, 2005
479
15
28
TEXAS PINEYWOODS
I installed the mosfet mod in my 29 yesterday per cb tricks instructions. Tried it in my mobile:

Now I have a weird power problem. channels 30-40 I have good power (45w)
1-10 I only get 20w pep

10-20 I get 25pep

20-30 about 30-35 pep.

My swing improves the higher in the freqency I get. I dont understand. My swr's are good and low on all channels. I tried it on my base (different meter and antenna), same result. Maybe I have to get her realigned but I dont know why. what do yall think??
 

Sounds like an alignment/tuneup needed...try to tune/peak on channel 20. I have done this mod on several Cobras, Unidens, Galaxies, etc. You can play with cap values to get more out also ;)
 
update: yesterday I added a 4.7k resistor to the gate of the erf 2030 and to the pos side of c116. it brought my dk up to 6w. I also jumpered out r58 and r123 (the way I understood that was to provide more voltage to the driver and final).

well, she only lasted a couple keys then I lost tx/rx. I changed out the driver from a 1957 to a 2314 I had laying around. I took out the jumpers as well. I hooked it back up and still had nothing.

I went to rat shack today and picked up an irf510 ($1.99) to put in place of the erf2030. I have read of a couple people using this trans. and figured it couldnt hurt.

I dropped the irf510 in place of the erf2030. I also took out the 4.7k resistor. Low and behold my radio is back to transmitting!!!

Now I have a dk of 8w on channels 1-40. My swing is now a little better than before but I am still getting better swing on channels 30-40.

1-10 I am seeing about 25w pep
10-20 about 30w
20-30 about 35
30-40 about 45-50.
 
Thats the problem with peaking the power on a cb. The average guy will always see a variance of power from one side of the band to another.
The whole trick to peaking a radio is to get the same output power across the band. Technically you have 2 components to adjust... the coil and the cap... this sets the impediance across a set bandwidth... adjusting either of them MAY yeild a better impediance match on say channel 1 for example, BUT on channel 40 the impediance from driver to final transistor (for example)will be off and show a drop in output power. In a cb normally guys will go for adjusting the coil, which may show you an increase in output power on the channel your tuning on.... but at the oposet end of the band there will be a decrease in output power since the circuit is now tuned for a smaller bandwidth.

In the case of your radio.. you are getting a better impediance match in the rf amp section on the upper end of the band. The tuning is off and this is actually really common when someone tries to get as much as they can from their radio.
What you can do is retune your tank coils as best you can to try and even out the power across the band. This problem is very common.
Put a tone into the mic or going into the audio amp while in tx mode. Retune all the coils you adjusted for power like L20 L17 first.... if you spread the coils on L16 or L17 leave them till last. If you adjusted L21 coil that has 2 slugs in it, retuning those will help. You can put the radio on channel 1 and bring up the power while checking the power on channel 40... ch.40 will drop in power and ch. 1 should increase.Switch back and forth between 40 and 1 and try to even it out.

Basically your rf amp section is no longer adjusted for an even impediance accoss half a meg (cb band)... you might have to sacrifice a bit of power on one end of the band to get the power up on the other end .

If you want, another thing you can try is to tune for max output power at the center of the band. Right now it seems like the output power has been peaked up while you were on channel 35. Try retuning for max power on channel 20 so the loss at either end of the band isnt so band and noticable. Tuning at the higher end of the band will definatly show more power up there and alot less as you down toward channel 1.

The whole idea is to try to get even output power across the band. The tuning on your radio is off.
Put a tone into the radio and retune on channel 20...from time to time check what the power like on channel 1 and channel 40. Either you can tune for max power on channel 20 and have it taper off as you move higher or lower in frequency.... or you can sacrifice some power on channel 20 and have the radio tuned evenly accross the band.

You just have the radio tuned too tight on the higher freqs. IT looks as though you where on channel 35 or in that area when you did the mod and tuned the power up.
Tuning on channel 20 might be the easiest and the loss at either end not as drastic.

Good luck and take care
Hope it helped.
 
thanks bushwacker, that helped a bunch. I took it to my tech and he realigned it for even power across the band. Like you said, my swing did drop but I am even now, only getting 27w pep now.

think I outta try the cap resistor mod with it to get my dk down for an amp??
 
BIONIC_TEXAN88 said:
thanks bushwacker, that helped a bunch. I took it to my tech and he realigned it for even power across the band. Like you said, my swing did drop but I am even now, only getting 27w pep now.

think I outta try the cap resistor mod with it to get my dk down for an amp??
Click to expand...

Yea you can try it out... its up to you. What that cap resistor mod is doing is lowering the voltage to the driver but letting all the audio through. The resistor part drops the voltage which lower the carrier.. the cap is there so the modulation can still make it into the driver... in a cobra 29 style radio and most am radios, all the modulation comes in with the voltage to the driver and final... in a linear rf amp you have the modulation going into the base of the transistor (input) and just voltage on the collector (output) but with a cobra 29 the modulation and voltage is mixed... so if you want to lower the carrier you will have to drop the supply voltage but this will also drop the modulation... but if you add in a resistor to drop the voltage AND add in a cap it will let the modulation flow through... this is the job of the cap...all this keeps all the swing (with the cap) but lower the carrier ( with the resistor).

This little swing kit gimik is put in a convient jumper... if you want to see what part of the rf amp this swing kit effects all you have to do is follow the jumper... the jumper runs from the 12 volts modulation/voltage supply line to the driver.... right beside that jumper is an rf choke that runns off that 12 volt/modulation supply line... this is what supplies the voltage and modulation to the final.. the choke is there to prevent any rf from getting back into the audio amp.

So... all the work you did changing the final to a fet is in final rf amp .... by putting in a resistor cap you are not messing with anything running directly to the final since choke L15 is what supplies the voltage/modulation to that part of the circuit. Putting the resistor cap in the usual place will do its normal job of adjusting the voltage and modulation to the driver. There should be no worry about it affecting any of your work on the final directly.... but you are changing the drive to the final by driving it with a lower carrier.
What I'm saying is you can put a swing kit in without it messing up the fet.... but I'm not sure how the fet will respond to a much lower drive level and if it will work how you want it too.
Since you did alot of work on the final section , I'm not 100% sure the swing kit will work the way you want it to.

Basically what I'm rambling on about (lol) is for you to go ahead and try it out. The swing kit modification controls the voltage and modulation to the driver and therefore the drive level into the final. The big worry is if it will mess up all your work you did in the final rf section......the only thing this mod does to the final rf amp is change the drive level into it/..... and if the rf amp does like it.... take the swing kit out..... no harm... its very similar to changing the drive level into an amp.

I say try it if you want.... but I would not drop a huge amount of voltage at the start. In other words don't use a large value resistor to start....

Hopefully that gives you an idea of whats going on. You know what your doing when it comes to doing this mod, so the decisions yours.If you don't like the rersult just remove the swing kit

Take care and no thanks needed..... you know what your doing.
HootyAL also said the same thing I did... I just said it the LOOONNNNGGG way LOL. So it wasn't just me alone.

Take care and sorry about the long winded rambling... sometimes I just go on and on. :)
 
I appreciate the postings bushwacker. you arent too long winded for me, I enjoy the reading, thats what this hobby is about.

I will try the cap/resistor mod on it maybe tomorrow and I will post up the outcome. wont hurt to try it.

I am kinda seeing your point now on the fets, I have gone through a bit of work messing with the fets and havent had a great outcome so far. the stock final and driver was giving me 30w pep, thats about all I am getting right now.
 
well, I had time to fool with it last night. I couldnt get the thing to swing consistently on 40 and 1.

I also tried out the cap and resistor mod along with the mosfet, very, very low dk (like 1/5w) and swing was only about 20w.

I got tired of fooling with it so I put everything back to stock, except the cap and resistor. Now I am back to a 1w dk and a 30-32w swing on all channels. I had to retune l17, l20 and l21 but she's back in business!!

I think I am giving up this mosfet thing, at least on the 29's. maybe I will have better luck with an export.
 

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