Cobra 148gtl no transmit

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perrym1962
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Cobra 148gtl no transmit

Post by perrym1962 »

I have a 1992 Cobra 148gtl made in Philippines radio. I accidently keyed the radio with the antenna unhooked. Now I do not have any transmit, receive worked fine needless to say. I replaced 2sc2166 and 2sc1969 with 2 out of another radio that I know are good just to see if that is the problem, but still nothing. So I am sure it is something else I am unaware of. Any help would be appreciated. Thanks, Perry in Ky.
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Post by perrym1962 »

Help... :(
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Rick
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Re: Cobra 148gtl no transmit

Post by Rick »

What is the voltage at pin 8 of IC4 in TX mode?
What is the voltage at the Base of the Driver and Final in the TX mode?
Last edited by Rick on Friday 16th May 2008, 19:31, edited 1 time in total.
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Post by perrym1962 »

Base of driver is 1.22v, base of final is .79v. I am not positive which is IC4, but with component side up with front of radio facing me there are 2 ic's on the right which maybe one of these is ic4. Cannot find a marking. Assuming pin 1 is toward the front of the radio pin 8 of the one toward the back by the audio transformer "?' is showing 6.7v and the one toward the front is showing 8.32v. As you can tell I am not technically inclined, there is really no one close to me to trust I can take it to, so here I go. Thanks, Perry..
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Re: Cobra 148gtl no transmit

Post by Rick »

The IC you are looking to test only have 8 pins and it's numbre is MB3756, pin 1 should have about 8 volts all the time with the radio switched on, but pin 8 only has volts in the TX mode.

Can you also check the voltage at the Collectors of the Driver and Final in AM and LSB TX Mode?

How long did you have the radio in the TX mode with no antenna for?
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perrym1962
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Post by perrym1962 »

Pin 8 of IC4 shows 8.36 volts in TX mode. Driver voltage on collector in TX mode on am is 3.83 volts and on LSB is 13.2 volts. Final voltage on collector in TX mode is 3.93 volts on am and 13.32 on LSB. Radio was keyed maybe 5 seconds when I noticed my error.
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Re: Cobra 148gtl No TX

Post by Rick »

The voltage at pin 8 is OK.

Being keyed 5 seconds should have not caused the fault.

Try adjusting the voltage at the collector of the final with VR10 in the TX mode on AM, for 5.6 volts.

Do you have any output power in SSB?

Are you sure the Driver and the Final are OK?
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Post by perrym1962 »

I just messed with it again I started touching components with a small plastic tip tuning stick. I touched L8 and moved it a bit and it started working. Does this sound crazy? I don't know? Guess I will try it for a while and see what happens. I appreciate your time and hope I haven't wasted it. Crazy huh?
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Post by perrym1962 »

Sorry I meant to say L37, not L8. Anyway it wasn't that either because went out again and I pushed gently on the pc board and it is what I am fairly sure is a cold solder joint. So without knowing which joint it actually was I started reheating. Slow process but so far it is still working. I guess time will tell. Thank you again for your responses. Perry..
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