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Cobra 142 gtl Transmit problem.

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Old 11-09-2010, 04:25 PM
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Default Cobra 142 gtl Transmit problem.


I used the radio before I went to work, came home, and now it dosn't transmit at all. The Meters don't move, the light dosn't turn red, alsmost like it was a microphone problem, but its not, I tried about 5 mics, and nothing,

there isn't a relay in the rig, so what else can I check or replace,


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Old 11-09-2010, 07:22 PM
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is it in PA mode?
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Old 11-09-2010, 08:22 PM
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no, but i did flick the switch and put it on pa mode,, and when you key up the mic the green light goes out (rx light)

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Old 11-09-2010, 08:24 PM
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Did anyone else have a chance to mess with it while you were out?
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Old 11-09-2010, 10:29 PM
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First you need to rule out if it's a PLL issue. Do this by checking to see if the radio is actually able to receive signals on the channel you are tuned to. If it does this, the PLL is likely to be locked. Then we have to suspect it may be a voltage switching issue. If you ruled out mic connections and checked the internal connector also, then look at the 8 pin MB3756 voltage regulator mounted next to the audio chip on the heatsink to the right. It's been a while but at least one pin should go from 0 to 8 volts on TX. I think it's pin 8. If that's not happening, you may have a shorted capacitor or this chip could be bad.

To determine if the chip is working check pin 5 to see if the control voltage changes from RX to TX. If not you probably have a shorted cap on the control line. If the control line pin 5 changes and pin 8 doesn't have 8 volts on it in TX, lift the pin from the board and check the pin again. If you still have no 8 volts on the pin in TX the chip is probably dead. If there is 8 volts, you may have a short on the load side of the chip. Good chance it's a cap. They are getting old.

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Old 11-10-2010, 03:36 AM
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Quote:
Originally Posted by gogo_gadgit View Post
I used the radio before I went to work, came home, and now it dosn't transmit at all. The Meters don't move, the light dosn't turn red, alsmost like it was a microphone problem, but its not, I tried about 5 mics, and nothing,

there isn't a relay in the rig, so what else can I check or replace,
Ok,
I take it the receiver is working? Just want transmit....

Lets take the easy route first. Check pin 8 of IC 5. This is your 8 volt regulator. You should have 8 volts there.

If the receiver is working then the TX supply should be ok, but we need to see if TR37 is pulling pin 2 of IC 5 low to switch it in TX mode.
The emitter of TR37 is the tx line to the mic plug.

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Old 11-10-2010, 10:11 AM
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I'm only looking at the data sheet for the MB3756 and it indicates pin 2 it the main supply voltage to the chip and not the control line. Pin 2 should have constant 12 volts on it and pin 5 should be the transmit and receive control line to the IC.

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Old 11-10-2010, 12:02 PM
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Quote:
Originally Posted by Shockwave View Post
I'm only looking at the data sheet for the MB3756 and it indicates pin 2 it the main supply voltage to the chip and not the control line. Pin 2 should have constant 12 volts on it and pin 5 should be the transmit and receive control line to the IC.
Correct,
I guess I did not word that good enough. If the circuit of IC 5 is bad, then when TR37 is switched to transmit, pin 2 will go low if there is a shorted circuit. If pin 2 (supply) goes low, probably a bad cap in the T/R switching circuit or an internal short in the IC itself.

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