Cobra 139 LXR VERY POOR MODULATION

[long night]

HI. MY COBRA 139 LXR HAS A GOOD CARRIER BUT VERY LOW OR POOR MODULATION. i AM JUST STARTING TO WORK ON CB EQUIPMENT AND DON'T A LOT OF TEST EQUIPMENT. COULD IT BE THE MODULATION POWER TRANSISTORS OR? ANY ADVICE WOULD BE HELPFUL!

THANKS IN ADVANCE, LONG NIGHT

 

[Crawdad]

Until the real experts come along.....have you checked the simple stuff: working mic? Dynamic setting? What's telling you that modulation is low and is it reliable? Tried SSB? Listened with monitor radio? or a station nearby if possible?
Sounds simple and stupid but sometimes............

73

 

[nomadradio]

If you have only one mike, Murphy's Law puts it onto the list of suspects.

AM and SSB transmit modes use totally separate modulator circuits. A problem that only shows up on AM or just SSB points to parts of the radio that are not shared by both transmit modes.

And if the audio level is poor for BOTH AM and SSB, this points to audio circuits that are shared by both.

Maybe.

This radio is 43 years old. That alone means that many of the electrolytic capacitors have gone way past their "use-by" dates.

This is where mileage counts. But there's no odometer reading to tell you how many thousands of hours of running time this radio has seen.

If it's 50 original miles, fresh out of the factory carton all those parts could be okay.

And if it was used heavily there will be multiple failures. Low mileage works in your favor.

Do you have another mike? Or another radio that uses this type mike? Seeing that mike work on another radio would remove it from the list of suspects.

Troubleshooting is a logical process of elimination. Finding what's broke by first finding out what's still working, step by step.

73

 

[long night]

Until the real experts come along.....have you checked the simple stuff: working mic? Dynamic setting? What's telling you that modulation is low and is it reliable? Tried SSB? Listened with monitor radio? or a station nearby if possible?
Sounds simple and stupid but sometimes............

73

Tried different mics that work on other radios. When talking with cb friends they tell me I have a good carrier but poor modulation.

 

[Crawdad]

Tried different mics that work on other radios. When talking with cb friends they tell me I have a good carrier but poor modulation.

Nomad's post is the way to go then, he's one of the very best on this forum. Especially the advice on electrolytic caps in aged radios.

Best of luck & 73

 

[long night]

If you have only one mike, Murphy's Law puts it onto the list of suspects.

AM and SSB transmit modes use totally separate modulator circuits. A problem that only shows up on AM or just SSB points to parts of the radio that are not shared by both transmit modes.

And if the audio level is poor for BOTH AM and SSB, this points to audio circuits that are shared by both.

Maybe.

This radio is 43 years old. That alone means that many of the electrolytic capacitors have gone way past their "use-by" dates.

This is where mileage counts. But there's no odometer reading to tell you how many thousands of hours of running time this radio has seen.

If it's 50 original miles, fresh out of the factory carton all those parts could be okay.

And if it was used heavily there will be multiple failures. Low mileage works in your favor.

Do you have another mike? Or another radio that uses this type mike? Seeing that mike work on another radio would remove it from the list of suspects.

Troubleshooting is a logical process of elimination. Finding what's broke by first finding out what's still working, step by step.

73

Thank you Nomad. I can get a recap kit on line I think. I am pretty sure it uses the same kit as some unidens like the 458? What would be very helpful to me would be a list of the VR pots and what function each of these are made to adjust. Any ideas where that information can be found?

 

[Crawdad]

I can get a recap kit on line I think.

There is a forum member here who sells recap kits........umm.......something Mike. Someone here will fill in the name for me.........I can't think of it

 

[Dmans]

long night,
See the attachment below. It is the same chassis as your 139XLR.
The only difference that comes to mind right off is the 139 has a front panel mike gain adjustment where the 457/458 does not. All the other variable resistors should be the same.

KlondikeMike is THE Capacitor King!

73
David

 

[Klondike Mike]

Here is the capacitor kit you need for that radio;

https://klondikemikescapkits.com/pr...-138-139xlr-electrolytic-capacitor-kit-deluxe

 

[long night]

There is a forum member here who sells recap kits........umm.......something Mike. Someone here will fill in the name for me.........I can't think of it

Thank you Crawdad. The help is appreciated.

 

[long night]

long night,
See the attachment below. It is the same chassis as your 139XLR.
The only difference that comes to mind right off is the 139 has a front panel mike gain adjustment where the 457/458 does not. All the other variable resistors should be the same.

KlondikeMike is THE Capacitor King!

73
David

Thank you Dmans. You folks out here are very helpful and good to talk to.

 

[long night]

Here is the capacitor kit you need for that radio;

https://klondikemikescapkits.com/pr...-138-139xlr-electrolytic-capacitor-kit-deluxe

Hi Mike. How much are your cap kits for a cobra 139 lxr? Do you have any info or diagram that shows what each VR pot is used for which function?

 

[Crawdad]

Thank you Crawdad. The help is appreciated.

My pleasure Long Night and welcome. Hope you get your 139 working soon.

73