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Uniden 78ltw project

Eagleeye

New Member
Apr 30, 2016
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This may sound like a dumb question, but here goes....
The ch.9 switch has been disconnected, which is fine (im going to use the switch for something else anyway). All the wires that were connected to the swith are loose on the switch end but go to a plug on the board. Years ago, i knew how trace out the wires so the radio was "stuck" in the normal position and remove the remaining unused wires. Thats what i want to do now. I guess im a little rusty. The plug to the board has a red, white, grey, black, and orange wires on it. I guess i need to know which ones i need to tie together to get back to the normal channel selection
 

well im afraid that without access to a schematic for that particular radio, you might be out of luck unless you can find someone who owns one and is willing to take a few pics for you.

however, this radio is nearly identical on the inside to a cobra 29LTD, so i will try to give you something you can try, but bear in mind that this is a guess and might not work.

looking at the connector that is on the PC board, find which pin leads to pins 20 and 21 of the upd2816 PLL chip. (they are tied together)

now find out which color wire goes to that pin, and make a note.

now look for the pin on the connector that goes right to the channel switch, and find the wire color that corresponds to this pin.

connecting these two wires together should give you your full channel coverage.

the only other wire i can see in the 29LTD is one that would go to the cathodes (banded ends) of D31,32,33, and 34. connecting this wire to the wire that goes to the PLL chip (pins 20 and 21) would put you in "channel 9" mode.


the problem is what to do with the other two wires you have there. my guess is that they go to power some LED indicator that shows that you are in channel 9 mode, or maybe they power something on the backlit faceplate.

that's up to you to figure out, but what i showed you should work to get the radio working.
LC
 
well im afraid that without access to a schematic for that particular radio, you might be out of luck unless you can find someone who owns one and is willing to take a few pics for you.

however, this radio is nearly identical on the inside to a cobra 29LTD, so i will try to give you something you can try, but bear in mind that this is a guess and might not work.

looking at the connector that is on the PC board, find which pin leads to pins 20 and 21 of the upd2816 PLL chip. (they are tied together)

now find out which color wire goes to that pin, and make a note.

now look for the pin on the connector that goes right to the channel switch, and find the wire color that corresponds to this pin.

connecting these two wires together should give you your full channel coverage.

the only other wire i can see in the 29LTD is one that would go to the cathodes (banded ends) of D31,32,33, and 34. connecting this wire to the wire that goes to the PLL chip (pins 20 and 21) would put you in "channel 9" mode.


the problem is what to do with the other two wires you have there. my guess is that they go to power some LED indicator that shows that you are in channel 9 mode, or maybe they power something on the backlit faceplate.

that's up to you to figure out, but what i showed you should work to get the radio working.
LC
Thanks for the info friend. I did get it straightened out. I went through and checked the voltages on each wire. I had 2 wires with power on them, so that made quick work for trial and error. One gave me channel 9. So i got the other one. Lol. I tied the 2 good wires together. Sealed off the others nice and neat out of the way. The other problems i had with no tx was from a bad relay in a roger beep board. Fixed that as well. So now I am good.
 
glad to hear you worked it out.
if you can, please post what wire went where for others who may need this info in the future.
LC
 
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