Cobra 25 LTD Classic Plug Problem?

DT · Post by DT » Thursday 26th Apr 2007, 10:32

I was taking the internal mike plug out when i crack the pcb board that
was on the plug. so, my question is can you still wire the plug without
the pcb board on it? and if you can, can some tell me the wiring
diagram for it? any help would be appreciated. thank`s.

kc8mob · Post by **kc8mob** » Thursday 26th Apr 2007, 11:52

Just reverse engineer it. Draw your own digram. You know which way it's suppoed to be. the little notch in the plug. Then label where the wires go on the board. So just make the wires attach to the right spot with out the board.

Too easy

nomadradio · Post by **nomadradio** » Thursday 26th Apr 2007, 17:55

The small pc board has filtering components on it. They prevent RF from inside the radio from "leaking" out and radiating from the mike cord. It's a legal requirement the radio's design must meet before it can be certified as a legal USA CB radio.

In practice, those parts also keep stray RF around the outside of the radio from leaking back into the mike-audio circuits in the transmitter. A marginal antenna ground, or an antenna placed near the mike can cause feedback-squeal problems if the mike cord picks up your transmitted signal and carries it back into the radio's mike circuits.

The diagram for the "25GTL" model on CBTricks.com shows four disc capacitors: two) .01 uf, and two) .001uf. Also has two RF chokes, but it shows no inductance value for them. Pretty sure any value between 6.8 and 22 uH will work for those. The later "LTD" diagrams are too blurred and indistinct to make out this detail. Pretty sure the mike-socket pc board is the same on your radio as what's shown for the "GTL" radio.

The wiring on the mike-socket board reflects one other reality of this radio. It has two separate "grounds" in it. The wire that goes to pin 1 on the mike socket is the "circuit ground" that leads back to the negative power-supply wire. The other one is the "chassis ground". The mike socket has three of the four disc capcitors grounded to the frame metal. The other one is connected across pin 1 and pin 2 of the mike socket.

It may work okay with all that stuff left off, or it may squeal like a pig on transmit the first time you use it with an amplifier, or with a bad coax jumper or such.

Or a salvaged mike socket from a five-buck yard-sale radio could be a cheaper way to go.

73

DT · Post by DT » Friday 27th Apr 2007, 11:25

thanks kc8mob, and nomadradio for the help. it sounds like i will
go after the 5 buck replacement then trying to hook back all
those components. the 5 bucks sounds much easyer. thank`s again guy`s.

DT · Post by DT » Saturday 28th Apr 2007, 15:10

i found my other 25 ltd classic with the pcb board intact on plug.
it had the same wiring as the one i put this plug into. hook up
everything and soldered the wire`s. i have key up but nothing
else. no transmit or receive. i hope someone can give me a hand
with this problem? thank`s.

nomadradio · Post by **nomadradio** » Saturday 28th Apr 2007, 19:53

Does the receive/transmit LED on the front panel light up green?

I'll guess that it doesn't change to red when you key the mike.

Makes me suspect that they used different wire colors in the two radios. If they came from different production runs, or from different factories you may not be able to just match the wires color for color.

73

DT · Post by DT » Saturday 28th Apr 2007, 23:37

nomadradio, the receive/transmit light does light up green.
and when i key up it does change to red. hope this help`s?

DT · Post by DT » Wednesday 2nd May 2007, 9:35

thank`s for all your help.