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148GTL high dead key, no modulation

MrExtreme

Member
Sep 27, 2013
13
1
13
Still working on this one, almost done un hacking it, now I have a DK of up to 20 watts, no modulation on AM, maybe 2 watts modulation SSB, Is this most likely that IC4 MB3756 that is listed in the Cobra service bulletin, along with the diode change? This is a 1994 model.

I found an extra channel mod and a clarifier mod which was only partially done. Got the tone switch working again, still working on the display dimmer. The power mod was half done as well, replaced the missing tr24 but none of the resistors were changed to go along with it, everything besides the missing tr24 was still stock. I have changed D1,D2,D21,D22 to the 6263 diodes.

Thanks Lee
 

Usually in the case of real high deadkey and no modulation, along with Robb's response, check TR41, the 2SC1419. If that has failed, this can also cause the problem you are having. In some radios, TR41 is labelled D1135 (2SD1135) on the transistor. Either way, replace that and TR42 (2SC945) and that fixes the problem in most units.

Good luck!


~Cheers~
 
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I will do that, thank you.

Lee

Usually in the case of real high deadkey and no modulation, along with Robb's response, check TR41, the 2SC1419. If that has failed, this can also cause the problem you are having. In some radios, TR41 is labelled D1135 (2SD1135) on the transistor. Either way, replace that and TR42 (2SC945) and that fixes the problem in most units.

Good luck!


~Cheers~
 
Usually in the case of real high deadkey and no modulation, along with Robb's response, check TR41, the 2SC1419. If that has failed, this can also cause the problem you are having. In some radios, TR41 is labelled D1135 (2SD1135) on the transistor. Either way, replace that and TR42 (2SC945) and that fixes the problem in most units.

Good luck!


~Cheers~
Ummm . . . you have been working on radios for a lot longer than I have . . . but . . . didn't you mean a NTE-152 to replace the 2SC1419? . . .was that a typo? . . .TR42 I know is a 2SC945 . . .

I'll have to look at the schematic again . . .

Yep.
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_redraw.pdf
 
Robb, you are correct.

i think his response was just worded in such a way that it could be misconstrued.

NTE152, NTE291, 2N6487 are all good replacements for TR41.
none will make the radio perform any better than the other.

and yes, when TR41 shorts, it can send the full supply voltage to the driver and final collectors.
LC
 
Oops, my bad.

Only reason I mentioned 2SD1135 is because in some 148's, this transistor is present instead of the 2SC1419. I have seen a few Cobra 148's across my bench that use the 2SD1135, including my current 148. The 2SD1135 cross-references to the NTE-291 that LC recommends for a replacement.


Robb and LC are correct about the what to use for replacement (which I didn't mention and should have). LC has listed what can go back there. Another good option is to use a 2SC1061. Sorry for the confusion, gang. :oops:


~Cheers~
 
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dx959 exhibiting same symptoms

So, new to me 959 arrives and we check it out.
DK 18W and meter light doesn't just dim, it virtually goes out.
First thought was, which bozo tuned this thing. Switch meter to mod, 20% yikes!

I figured not good, unplugged it and put it up.
Symptoms rang a bell and found this thread to reread.

Found the Q47 1969 on the dx959 schematic and worked back a bit.

Q52 is 2sc1906, Q51 is 2sc2314, Q49 2sc2166.

Q52 looks close to the mixer so I suspect it's in the buffer neck of the woods.

So, I'm guessing Q51 and/or Q49 have snuffed it.

Close???

BTW exit13: I grew up @exit292 and attended school@ exit244
 
So, new to me 959 arrives and we check it out.
DK 18W and meter light doesn't just dim, it virtually goes out.
First thought was, which bozo tuned this thing. Switch meter to mod, 20% yikes!

I figured not good, unplugged it and put it up.
Symptoms rang a bell and found this thread to reread.

Found the Q47 1969 on the dx959 schematic and worked back a bit.

Q52 is 2sc1906, Q51 is 2sc2314, Q49 2sc2166.

Q52 looks close to the mixer so I suspect it's in the buffer neck of the woods.

So, I'm guessing Q51 and/or Q49 have snuffed it.

Close???

BTW exit13: I grew up @exit292 and attended school@ exit244


look at Q54 shorted , it is a common part for the radio to short and give a high dead key. should be a 2sb827 or 817 or 754. 827 or 817 is a better choice than a 754 ( old part )
 
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Hmmm. On sideband transmit there should be NO carrier at all.

Does turning the mike gain down make this carrier go away?

Might be helpful to hear what's coming out of the radio when the meter is showing this 'carrier' on sideband. If it's some kind of audio feedback or noise, the wattmeter would show it as a 'carrier'. If you have another radio, take a listen to what this carrier sounds like.

73
 
If you can put a frequency counter on the radio's output, that would reveal the frequency of your carrier.

If there is a problem causing the driver and/or final transistors to oscillate, you'll see a frequency that is not ANYWHERE near the channel frequency selected. This points away from the balanced modulator, and tells you that the power stages are the source of this carrier.

And if the frequency of this carrier shows to be the channel that is selected, this would suggest trouble in the sideband modulator.

Or maybe in the microphone amplifier that feeds into the balanced modulator.

Have you tried to listen to this carrier on another radio yet? Just because the wattmeter shows a steady reading doesn't mean that what's coming out of the radio is a carrier. If the mike circuit develops a feedback squeal, the wattmeter would make that look like a carrier. Having a listen to it could reveal a lot.

73
 
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