Cobra 148 GTL Receives no Transmit. All checks Good?

[Dmans]

Low_Boy,
TX voltage to pin 5 of your mixer comes from pin 8 of the (IC4) MB3756 Switch/Regulator.

73
David

 

[Low_Boy]

Low_Boy,
TX voltage to pin 5 of your mixer comes from pin 8 of the (IC4) MB3756 Switch/Regulator.

My dumb mistake. I was not keying the microphone. So now I am only missing the pin 2 output. Possibly another bad mixer. The replacement mixer came out of a parts radio that I never actually saw work. Could this be a internal mixer problem?

 

[Dmans]

If you check pins 7, 8, 11 & 13 with your oscilloscope and see the same readings as the schematic, then I would say it is a mixer problem.
You should see roughly a 7.8 MHz (L48) signal on pins 11 & 13 and roughly a 34 MHz (L47) signal on pins 7 & 8.


73
David

 

[NODENS]

Thats what i tried to mean too, see if TR32 oscillator is working (not seeing only voltages, see if freq is being delivered) . I have seen cristals not working, capacitors, colis on that area in several radios causing problems.



Regards.

 

[Low_Boy]

I will do more probing today. All crystals checked good. I tested the mentioned transistors for voltage and checked them out of the radio on a tester they tested good. I will see if they are getting freq.
Thanks for the help guys. I will keep on plugging along.

 

[Low_Boy]

Fixed....surprisingly it was a bad electrolytic capacitor. C95. Off pin 6 of the balanced modulator I cant figure where that affected anything. I done even see it in the circuit. The plastic coating was a bit shrunk so I decided for kicks I would test it. It was showing 2uf not 47uf like it was supposed to. Changed it everything worked. I tested everything else 3 or 4 times.

 

[Handy Andy]

Hmm, it can make sense - RX would not need it - but TX could.

It produces the 2nd IF mixing the TX mixer needs thru R63 and C54 - it's up there in the mess around the "Shrink Wrapped" Xtals L48 picks it up, L47 takes in the PLL side of it - the two give you your 27MHz...

Good catch -

 

[JOSE A VELEZ]

Having a similar problem a friend of mine gave me a radio as a gift the cobra 142 GTL I noticed is that the meter's don't move it came with a power a mic I'm not familiar with the radio I am a ham operator I want to get this vintage radio working I'm receiving good signal it came with a power mic requires a battery 7.5 volts TR 175 the battery is dead I'm assuming that the battery needs to have power to make the microphone work tracking stations I hooked up a dummy load keyed it up. With a 7 watt . Bulb it didn't let up of course the battery does not have any power so there's no audio I will purchase a battery to see if that's the problem that means I'm getting power but there's no audio because the radio came with a power mic I have never work with this particular radio wish I had the schematic and diagram also notice that the SWR meter don't move also notice is that the bulbs are not working on the meters can anybody tell me if they had any similar problems have never worked on CB radios have worked on ham radios want to get this vintage radio working if anybody's got any idea please let me know thank you very much sincerely yours kg4acg.73. If someone's got some information they can help me this is my email kg4acg59@gmail.com. will greatly appreciate any help thank you very much God bless sincerely yours Jose Velez 73

 

[Low_Boy]

Thanks Andy I was hoping someone would explain where that Cap. came into play. It was something that I never saw in the schematic. Ant I am not too good at reading them. I appreciate the info.

 

[Low_Boy]

Jose. The best thing to do is start your own thread on this problem. You can find a schematic at cbtricks.com radio section. the microphone will key the radio but you will get no modulation with a dead battery. Also a radio of that age that has been sitting probably has several bad electrolytic capacitors. If you are going to restore it a recap is probably a good idea.

 

[Handy Andy]

@JOSE A VELEZ - Welcome to the forum, best for you to locate the "SEARCH" section on our pages - upper right corner of any of our pages you see now, and type in Cobra 148 and look at the message threads that are listed.

That is the best starting point for you to even begin locating any information.

Also, Try this thread...
https://www.worldwidedx.com/threads/cobra-148gtl-this-issue-has-me-stumped.228935/

For CBTicks itself appears to be "dead" again, Account Suspended - so it can't even bring you any info.