142 gtl issue

[Spec2079]

148 gtl has ssb but no am power or receive also has no audio on pa mode

 

[nomadradio]

The 148GTL isn't just one radio, it's a whole product line covering 40-plus years. Much like motor vehicles the internal layout, wire colors and design change over the decades. Not every year like automotive models, but a 1979 Cobra 148GTL is very different on the inside from a 2009 model.

First question that comes to mind is how many folks have tried to fix it and failed so far?

Second question would be which 148GTL is this?

May not sound important, but it is.

73

 

[SuperLid]

The 148GTL isn't just one radio, it's a whole product line covering 40-plus years. Much like motor vehicles the internal layout, wire colors and design change over the decades. Not every year like automotive models, but a 1979 Cobra 148GTL is very different on the inside from a 2009 model.

First question that comes to mind is how many folks have tried to fix it and failed so far?

Second question would be which 148GTL is this?

May not sound important, but it is.

73

Second question....is it a 142 or 148?

 

[Spec2079]

This one is a 142 gtl has ssb out put but no am output or modulation also no pa modulation

 

[Spec2079]

It’s a 142 gtl base station

 

[BC Coyote]

It would be super helpful for the folks here to know the number printed on the PC board in the radio. It will be something like PC-412, or PB-010 or similar....
Usually printed in the middle of the component side of the board with a white border around it.
Also helpful to know the place the radio was made. ie: Phillipines, Malaysia, Taiwan etc. It's printed on a little tag on the back of the rig.

 

[Spec2079]

Board #PC-385AG
Made in Taiwan

 

[nomadradio]

So let me get this fault list straight. No receive, no AM transmit.

Does this mean that it transmits on sideband? Or is there no transmit on any mode?

Does the mike you're using work on another radio? Or do you have a mike that fits this radio that you know is working?

Mikes tend to break more often than radios, so it can cause some or all of this trouble.

Does the S-meter kick around with an antenna connected? Would be helpful to know if that part of the receiver is doing something, even if you hear no audio from the speaker.

73

 

[doffo]

I had a 142 GTL that had no mic audio at all. on AM/SSB or in PA... Stupid shorted capacitor was the culprit! This was in 2017.

If you have no carrier on AM, your voltage regulator may be toast... No receive? Also that regulator... If you replace the regulator, do the service bulletin to protect that voltage regulator.

 

[Spec2079]

I have tried several mics same result I get modulation on ssb also puts out 20 watts only on ssb I can here my self on my test radio just nothing as for am tx does activate on key up but there’s no modulation and no carrier also no receive

 

[brandon7861]

Since SSB works, everything from the mic to the balanced modulator works. If you have SSB, you have power to the audio IC and regulator, because the finals use the same source. AM and SSB audio splits off after TR27 where it then goes to the TA7222P audio IC and from there to the power regulator. I don't think it is the power regulator because the RX audio does not go through that (and RX don't work either), so IMO that likely narrows things down to the TA7222P or (more likely) its associated caps. For example, if C119 shorts, it can kil the regulator and the audio.

 

[Spec2079]

Oh k thanks

 

[Shadetree Mechanic]

Maybe the AM / SSB selector switch needs to be cleaned?

 

[Spec2079]

It’s not the audio chip as I have a working radio and pulled out that audio chip from it and installed it in 142 gtl so something else is faulty

 

[BC Coyote]

If the electrolytics or other components related to that chip are defective, it will just blow each chip you put in as soon as you power up the radio ! You need to check the correct voltage is there, and replace any electrolytic caps in the vicinity. Then replace the chip.........