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Oldtimer · #1 10-17-2009, 08:53 AM

I tried the mosfet mod on my cobra 25 but no work.
I'am gonna put it back stock.
My question is what does C-45 do.
It had to be removed for the mod and I cracked it while removing it.
Is it necessary.
I am not going to rat shack anytime soon.
Thanks

moleculo · #2 10-19-2009, 10:41 PM

First a disclaimer: I am lousy at interpreting a schematic.

I looked at the schematic for the radio and here's my take: It looks to me like it does two things: First it has to filter some output harmonics produced by the final transistor (probably just a little bit). Secondly, since that capacitor is just in front of the low pass filter, it also must affect the impedance matching between the transistor output and the filter.

I'm not sure why it needs to be removed for the mosfet mod unless the impedance that is presented from the mosfet doesn't need the matching before the filter.

Radio Tech · #3 10-20-2009, 04:14 AM

C45 is a 33pf cap. It needs to be in the circuit if you put it back stock.

But if you do not mind, can we see why the mod did not work?
What did it do?

It is a simple mod for the 25 and 29 and I have not had one yet that did not work. If you would like we can look at it step by step and see what is happening.
That is if the radio was working properly to start with.

Just let us know.

Oldtimer · #4 10-20-2009, 06:47 AM

I put the radio back to stock and it works fine without the c-45 in place.
Don't ask me how cause I dont know.
Thanks for the response.

Radio Tech · #5 10-20-2009, 07:22 AM

It will work with out the cap. But it will not be as clean. Although you may not notice the difference.