Cobra GTL-150 no receive

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Blackolive
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Cobra GTL-150 no receive

Post by Blackolive » 09 Sep 2019, 09:16

Picked one up off ebay, there was a youtube video of it working on transmit and receive. Came without a mic, but Ive got a cobra mic. Ive had to rewire it slightly, but the receive side still appears to be dead. Screwdriver to the volume pot wiper proves the audio amp is working, but theres no radio hiss, changing the mode switch doesnt do anything, and its not actually receiving anything.

Its transmitting fine.

What does plugging the mic in actually do, Im wondering if power isnt being routed to the receiver side ?

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Re: Cobra GTL-150 no receive

Post by Otter » 09 Sep 2019, 09:26

The mike usually enables the receive audio path on Cybernet radios. It's probably wired incorrectly.

Put a dummy load on the radio. Determine which pin is ground with a multimeter between the mike socket pins and the -ve power lead with the radio switched off. Power up the radio and put it in FM mode (so you'll get a nice hiss when it works) then link between the ground pins and other pins in turn. One pin should put it in Tx, one should give you a nice FM hiss, the remaining one is the audio pin.
Wire the mike accordingly.
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Re: Cobra GTL-150 no receive

Post by Blackolive » 09 Sep 2019, 10:25

Thanks, Ill try that.

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Re: Cobra GTL-150 no receive

Post by astradyne » 09 Sep 2019, 12:02

These are the same radio as the Laffayette 1200. Built by Cybernet so the mic wiring would be different. Simply swapping over pins 1 & 2 with each other and the same for pins 3 & 4 is all that's required to switch Uniden to Cybernet wiring.
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Re: Cobra GTL-150 no receive

Post by dt307 » 09 Sep 2019, 19:04

short the 2 pins with a small bit of wire and rx should work
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Re: Cobra GTL-150 no receive

Post by Blackolive » 09 Sep 2019, 20:25

Right here’s where Ive got tonight. Pin 3 is receive and theres now a very faint hiss from the speaker and a buzz when i touch the volume pot. External speaker causes oscillation so Im suspecting a dry cap around the audio amp. Main voltage regulator is ok, and theres voltage on the collector of the first rf amp stage. No increase in noise when antenna plugged in, no change in hiss when mode selector operated, and unresponsive to signal at either 27 or 10.695 mhz. Has anyone got a component layout to make life easier to go round and check voltages please ?
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Re: Cobra GTL-150 no receive

Post by Blackolive » 09 Sep 2019, 20:27

And as its txing ok Im not suspecting an out of lock on receive vco yet but could do with test point identification

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Re: Cobra GTL-150 no receive

Post by 14CS06 » 09 Sep 2019, 20:37

Hello

Schematic

Image

@+
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Re: Cobra GTL-150 no receive

Post by Blackolive » 09 Sep 2019, 20:40

Thanks Claude, Ive got the scematic.

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Re: Cobra GTL-150 no receive

Post by 14CS06 » 09 Sep 2019, 20:41

Photo Collector CB RADIO

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Re: Cobra GTL-150 no receive

Post by Blackolive » 10 Sep 2019, 09:10

Found this useful article. I was already suspecting the dc switching circuit http://www.m0ysu.com/121d4x.html

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Re: Cobra GTL-150 no receive

Post by astradyne » 10 Sep 2019, 10:44

It is odd though as the radio was sold as working and there was a YouTube video supporting this. Then it arrives with a major fault - no RX.

Are you certain they sent you the actual radio in the YouTube video?
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Re: Cobra GTL-150 no receive

Post by Blackolive » 11 Sep 2019, 06:24

Fixed. It was Q40 which switches supply to the receive side, I may have popped it trying to work out which pins to link on the mic socket or maybe just a random failure. Using the antenna analyser as a sig gen

https://youtu.be/Tb9Cy4k1Qkg

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Re: Cobra GTL-150 no receive

Post by Black Spirit » 11 Sep 2019, 16:51

Blackolive wrote:
11 Sep 2019, 06:24
Fixed. It was Q40 which switches supply to the receive side, I may have popped it trying to work out which pins to link on the mic socket or maybe just a random failure. Using the antenna analyser as a sig gen

https://youtu.be/Tb9Cy4k1Qkg
The most common fault for the 1318 to blow is because a bezel screw was placed in bottom middle, If it goes down to far it shorts to the mode switch blowing the 1318 RX switch.

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Re: Cobra GTL-150 no receive

Post by Blackolive » 12 Sep 2019, 09:10

Ill have a look at that thanks

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