Cobra 25 ERF2030 Upgrade

[fsjonsey]

Hello Everyone!
I just finished installing the ERF2030 MOSFET upgrade in a Cobra 25. It works, but the problem is, I'm only seeming to get a 1 watt dead key, and about 4 watts swing into a dummy load with the mic gain maxed. I followed the instructions on cbtricks.com to the letter. Shouldn't I be seeing more power? Is this common?
Could the Bias on the 2030 be incorrect?
I appreciate any advice. This is really leaving me scratching my head.
Thanks in Advance,
-Bryan.

 

[Flying Dutchman]

vr2 is the power on that radio if i am not mistaken, try turning that up a bit. and the modulation pot to 100%

 

[fsjonsey]

vr2 is the power on that radio if i am not mistaken, try turning that up a bit. and the modulation pot to 100%

I already adjusted the AM Power and Modulation pots. Four watts is the most I've been able to get out of this radio with the ERF2030 installed. Is there anything else I could be missing?

 

[fsjonsey]

Sorry to keep bugging you guys, but I just checked the Bias voltage on the gate of the ERF2030, and I'm only getting about 1.3v at transmit. Is there a way to adjust the bias voltage so that it is closer to the recommended 2.7v?

 

[TruckDrvr46201]

Sorry to keep bugging you guys, but I just checked the Bias voltage on the gate of the ERF2030, and I'm only getting about 1.3v at transmit. Is there a way to adjust the bias voltage so that it is closer to the recommended 2.7v?

Did u Double Check that u changed all Resistors and Caps to the proper values if i remember correctly there is something about C62 in that mod... Double Check it and make sure everything is working sounds to me like u might have missed some little detail i have only done 2 of these myself and never ran into this ...

 

[fsjonsey]

Did u Double Check that u changed all Resistors and Caps to the proper values if i remember correctly there is something about C62 in that mod... Double Check it and make sure everything is working sounds to me like u might have missed some little detail i have only done 2 of these myself and never ran into this ...

Everything is installed as it should be. I removed the final, R38, R40, C45, and the slug from L10. I installed the MOSFET, the 1230 companion part (polarity observed), and the 68pf cap across C42. Still, I'm only deadkeying about 1 watt and swinging to 3.5.

 

[Warbird]

Sorry to keep bugging you guys, but I just checked the Bias voltage on the gate of the ERF2030, and I'm only getting about 1.3v at transmit. Is there a way to adjust the bias voltage so that it is closer to the recommended 2.7v?

It sounds like you have answered your own question, you need to add more bias voltage. What I do is find a 12v source, put a 100K (because that's what I have) variable resistor from the 12v source to the gate, adjust until you get the voltage you are looking for, remove the variable and test for resistance and replace with the proper resistor... or leave the variable. I shoot for more in the range of 3.5v and you should get 4 watts (plus) dead key and good forward power on modulation. If your voltage is too high... it'll swing backward on you, if it's too low, you get low dead key.

 

[TruckDrvr46201]

something else is that an OLDER 25 or a NEWER 25 if its the OLDER 25 Remove the 68pf cap and put the slug back in L10

 

[fsjonsey]

It sounds like you have answered your own question, you need to add more bias voltage. What I do is find a 12v source, put a 100K (because that's what I have) variable resistor from the 12v source to the gate, adjust until you get the voltage you are looking for, remove the variable and test for resistance and replace with the proper resistor... or leave the variable. I shoot for more in the range of 3.5v and you should get 4 watts (plus) dead key and good forward power on modulation. If your voltage is too high... it'll swing backward on you, if it's too low, you get low dead key.

Where would I find a good 12v source on the board to jump the 100k potentiometer to?

 

[Warbird]

Probe one of the pads in the back right corner (next to the final) as you look at the solder side with the front of the radio to you. It has to be close because the resistor isn't too big.

This is a Cobra 29, but you get the idea.

 

[fsjonsey]

Well, I got the bias in line. I added a 7.2k resistor between the gate and a 12v source. The radio is now Dead keying at 10w and swinging to 35. I tried it out on a drive down to school yesterday and got a contact around 15 miles from me. Thanks for all the help. I just modded another, older 25 today and didn't need the biasing resistor. It's also dead keying at around ten watts and swinging to 25-30.

The older Cobra 25 I just finished is going to be mounted under the stock AM radio in my 62' Corvair when I get it out of the garage this spring.
I appreciate all the help!