Cobra 29 LTD Classic NO RX/TX HELP!!!!!!!!!!!!!

[Handy Andy]

You mentioned...

i have no output of L20 at R63... pin 9 of IC3 is showing 7.5V

and its is in TX mode

i played with L20 a bit but no change...

Pin 9 or Pin 8? Pin 9 is Amp Out

IC 3 looks like this...

​

If you see Pin 8 having the voltage - Pin 9 should be signal with perhaps some DC bias of that 7.5V mixed in with it. The output of L20 thru R63 39 ohm ORANGE - WHITE - BLACK - is RF.

So recheck IC 3 make sure your pin count is right...

Pin 8 should be that SAME 7.5 to 8 Volts while Pin 9 DOES show 7.5 volts but sends RF to L20.

• - then from the L20 out to and thru R63 - C63 - which at TR16 that C63 cap 103 or 0.01uF DISC - BLOCKS DC so TR16 will have it's own bias voltages
• - C63 ISOLATES the Bias of TR16 from the GROUND of the opposite side of L20's Coil that sends that RF power CAPTURED from the other wind of L20 that goes to Pin 9
• - Pin 9 Retrieves power for the internal amplifier from a Resistor that SENDs power thru L20's Wind that goes to Pin 9 .
• The RF and power present on Pin 9 is also present at L20 - BUT ONLY ONE SIDE OF THE WINDS supply power to Pin 9 - BUT WOULD ALSO HAVE RF POWER ON THAT WIND.
• - L20 then DDECOUPLES the RF from the DC bias Side of the WINDS and sends it to TR16 thru a Series Filer C63 and R63

Reflow your solder in there, make sure the output of Pin 9 goes thru a DC blocking cap and resistor before it goes to L20 to be PEAKED - you should have 0.7 to 1.1 V DC power on BASE of TR16..

 

[troyota]

Andy, I just want to say that I appreciate the time you have taken to spell out all of this. You have inspired me to get out a stack of problem cobra 29s, 25s, and a uniden pc78xl I have. This thread is crammed full of great info. Thank you for taking the time to explain how the rx and tx circuits work, plus showing the components and their locations. Giving the voltage that they should be. I am very grateful. Thank you Sir!

 

[Handy Andy]