The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

I am working on a cobra 29 ltd. Checked the radios receive and found that the audio dosnt clean up until around 7-8 uV on the service monitor. Tx is fine. I checked R31 and it was fine I replaced anyway for PM. I replaced D1, D2, D4, D5, D6 and Tr7. Still no luck. I noticed when running through the allignment that L2 dosnt seem to make much of a difference. So i replaced that still the same issue. I check the voltage on fet1 and they are as follows:

D 8.3 Volts
S 1.5 Volts
G 0 Volts

From what the manual says Source votage should be 3.6 Volts. I have had little luck checking this fet for some reason I think it may be bad but I am not 100%. Any input would be great this is an older cobra 29 ltd. It was made in june of 1990.

thanks,

Mike

FET1 is the 1st mixer, check that you have 16.270 MHz on channel 1 and 16.710 MHz for channel 40, if all is ok change the FET, it is not a common for the FET to fail but it does happen.

Also check C20 and R48 known to cause low RX.

Thanks Rick your a big help I will check them later. I did finally check my fet and it is fine so i will look at the other issues you told me about.

thanks,

Mike

ok I checked and replaced both r48 and c20. Both were ok but i replaced them anyway. checked ops and no luck. checked the 1st mixer frequency and it is 16.714 on CH40 and 16.270 on CH1. I checked fet1 and it is ok. I also replced c11 and checked c9. I noticed something interesting though when i jump a wire from the antenna input to R9 my signal comes back pretty good. I cant seem to figure out what is bringing my recieve signal down between R9 and the signal input.

MIKE

Have you check D1,D2,D3 and C8?
Is the RF gain control working?
Do you have Sig gen?

Well i have found the problem. someoe thought he would put a 390ohm resistor in line with the rf gain pot. for some reason i missed that, it was hidden in the bundle of wires. Dont know what they were thinking but anyway after all that messing around that is all it was. thanks for all the help rick. if anything i am more intemently familiar with the recieve section of the cobra radios. I found this problem when i traced the problem to the rf gain circuit.

thanks,

Mike

ps: I have a Motorola R2200B communications service monitor And many other toys ha ha