Copper Talk » Ask The Tech » Microphone Wiring » Cobra 148 gtl will not transmit « Previous Next »

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Big_bear
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Username: Big_bear

Post Number: 1
Registered: 2-2013
Posted on Tuesday, February 05, 2013 - 4:57 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

ijust need someone who knows what transistor switches the radio from rx to tx i am sure this will fix the problem
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Tech237
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Username: Tech237

Post Number: 1650
Registered: 4-2004


Posted on Tuesday, February 05, 2013 - 5:43 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Before looking at transistors, try working out which two pins on the mic socket control PTT and short them together. if it switches to TX the issue is the mic or mic cable. if it doesn't then 6yes it may be the switching transistor, which I recall correctly is TR35. Also check to make sure the CB/PA switch is actually working too.
Tech237
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God made me an athiest, who are you to question his wisdom?
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Big_bear
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Posted on Sunday, February 10, 2013 - 2:48 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

thank you for the info. i will check these and give an answer when i know.
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Big_bear
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Posted on Sunday, February 10, 2013 - 3:59 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

it must be another transistor, tr35 did not work.
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Tech237
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Post Number: 1651
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Posted on Monday, February 11, 2013 - 9:07 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Did you check the other options I listed first??

I dug out my manual and per the schematic and block diagrams TR35 is the TX/RX switching transistor.

What do you mean by "TR35 did not work"?
Tech237
N7AUS

God made me an athiest, who are you to question his wisdom?
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Big_bear
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Posted on Monday, February 11, 2013 - 12:27 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

unless i put in another bad tr , witch may be possible.the rx/tx light is not changing color and is not transmiting. i believe you are right about tr35, just jumped the gun when saying it must be another. again thank you so much for the help!
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Big_bear
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Post Number: 5
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Posted on Monday, February 11, 2013 - 1:19 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

i tried another c945 but no luck, anymore suggestions would be highly appriciated!
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Big_bear
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Posted on Monday, February 11, 2013 - 8:53 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

also the mic., cord, and mic. socket are all good.
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Tech237
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Post Number: 1652
Registered: 4-2004


Posted on Tuesday, February 12, 2013 - 9:04 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Did you try shorting the pins at the mic socket??
Did you try jumpering teh CB/PA switch so that is is shorted out in the CB position??
Tech237
N7AUS

God made me an athiest, who are you to question his wisdom?
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Tech237
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Username: Tech237

Post Number: 1653
Registered: 4-2004


Posted on Tuesday, February 12, 2013 - 9:05 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

PS: Let me go over the schematic Wednesday when I am off work, and I'll see what else may be a possibility.
Tech237
N7AUS

God made me an athiest, who are you to question his wisdom?
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Big_bear
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Posted on Tuesday, February 12, 2013 - 7:58 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

the p.a. is working and the rx is working when switched back to cb it did not key up when shorted.it still had rx when shorting the cb/pa switch.i agree this deos not make sense,thought tr35 would work myself.
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Tech237
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Username: Tech237

Post Number: 1654
Registered: 4-2004


Posted on Friday, February 15, 2013 - 8:57 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

OK, thyen you have a resistor or cap that has failed. Most likely a resistor. Didn't get to pull the manual over the last couple of days. I will tonight and take a second look.
Tech237
N7AUS

God made me an athiest, who are you to question his wisdom?
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Big_bear
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Posted on Friday, February 15, 2013 - 1:14 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

thanks so much tech237 , it will be great to get it workin again !
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Tech237
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Post Number: 1656
Registered: 4-2004


Posted on Saturday, February 16, 2013 - 9:04 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Not a problem. I'll even see if I can con work into making a copy of it for you, but no promises.
Tech237
N7AUS

God made me an athiest, who are you to question his wisdom?
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Sonoma
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Username: Sonoma

Post Number: 242
Registered: 11-2001


Posted on Saturday, February 16, 2013 - 10:55 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

here is a link to a schematic for this radio.

http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_redraw.pdf
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Big_bear
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Username: Big_bear

Post Number: 10
Registered: 2-2013
Posted on Saturday, February 16, 2013 - 7:13 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

allright thanks again for all the help but i am no tech,i have looked pretty much all the parts and see no signs of problems,i know they dont allways show.i thought q35 would do it because my dx 959 had the same problem witch i fixed with a new q33,but this one has got me baffled.i have been modding my own radios for a while and can figure some things out,but i think i may have to take this to a real tech and get it fixed.my regaurds to tech 237 and sanoma for the link info.also ,nice dog !
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Big_bear
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Username: Big_bear

Post Number: 15
Registered: 2-2013
Posted on Saturday, March 02, 2013 - 7:22 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

hey tech 237,i replaced c95,153,186,84 and c172 and did not have any results.i set the radio foil trace side up and started to touch arond tr 35.i noticed when i touched where d43 and r164 and where the two orange wires are,both on opposite traces,d43,r164 on one trace and two orange wires on another ,with mic. keyed.the radio would key up.so i replaced all the resistors and diodes around that area and still nothing. then i thought i should trie a resistor frm d43,r164 trace to the trace wiyh two orange wires and now i have a radio that works both on rx and tx. i geuss i bypassed something.but what i dont know.
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Tech237
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Username: Tech237

Post Number: 1658
Registered: 4-2004


Posted on Thursday, March 07, 2013 - 3:59 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Good work on that. Glad to see it finally works for you.
Tech237
N7AUS

God made me an athiest, who are you to question his wisdom?

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