Re: Cobra 142GTL cap changes

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Goldfinger
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Re: Cobra 142GTL cap changes

Post by Goldfinger »

Receive:

Change C91,C28, C108 and C110 to 2.2uF-4.7uF non polarized
Change C 118 to a 1000uF, 25 volt Electrolytic, observe polarity
More highs? Make C25 lower in value



Transmit:

Change C121, C83 to a 4.7uF, 16V Electrolytic or Tantalum, observe polarity
Change C86 and C82 to 2.2uF-4.7uF non polarized
Change C119 to a 10uF, 25 volt electrolytic. Observe polarity
More highs on transmit? Make C111 lower in value. (also affects receive)

* non polarized means Mylar, Polypropylene or non-polar Electrolytic
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677KY
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Re: Cobra 142GTL cap changes

Post by 677KY »

Thank you very much Goldfinger!!!
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Re: Cobra 142GTL cap changes

Post by 677KY »

can polarized caps be used for the non polarized changes you posted?
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Re: Cobra 142GTL cap changes

Post by Goldfinger »

If you measure which hole is more positive you can but I've found it still doesn't work much of the time as both sides of the circuit have some random + voltage on them and doesn't want to work right. It's easier just to get the right caps. I always use "new" caps when I modify a radio so I just order what I need/use.

You can make non-polarized caps: here
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Re: Cobra 142GTL cap changes

Post by fernan_01 »

I’m planning on doing this mod to a Uniden Washington with pll mb8719, what Would be a good value for C25 ? And c111?
Thanks. 73’s
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Re: Cobra 142GTL cap changes

Post by Goldfinger »

I would just do the main mods and see how it works. Every time I modded a Washington/142 I liked the receive highs how they were. C111 would be the one to experiment with as different voices or mic combinations could sound a little muffled. If you want to play with C25, try making it roughly half of the original value. If you only change it a little, sometimes you won't hear much difference.
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Re: Cobra 142GTL cap changes

Post by fernan_01 »

Thanks!! Another question c121 to 4.7uf will increase tx bass, right? Would 10uf be too much?
Thanks once again 73’s
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Re: Cobra 142GTL cap changes

Post by Goldfinger »

Not necessarily. Usually once a cap is large enough to pass 20-30 hertz, going larger is usually just diminishing returns. "Sometimes" depending on the impedance or layout of that part of the circuit, it might get a little boomy but I've found it to be rare. I'd say just try it and see. :)
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Re: Cobra 142GTL cap changes

Post by Goldfinger »

Direct injection:
After making C82 larger in value, inject through a 4.7uF into the negative hole of C83.

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Re: Cobra 142GTL cap changes

Post by DoubleR0788 »

What about ssb on Washington uniden?
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Re: Cobra 142GTL cap changes

Post by Goldfinger »

Once you inject at C83, SSB will already be wide from the mic amplifier standpoint. The crystal filter has limited bandwidth anyway but of it seems like there still isn't much low end on SSB, change C48 to a .1uF
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Mr.Lightning
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Re: Cobra 142GTL cap changes

Post by Mr.Lightning »

where the heck is C91?? Also C110 AND 108 are polarized caps, are we changing them to non-polarized?
Thanks for all you do!
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Re: Cobra 142GTL cap changes

Post by Goldfinger »

Mr.Lightning wrote:where the heck is C91?? Also C110 AND 108 are polarized caps, are we changing them to non-polarized?
Thanks for all you do!
On the schematic they don't show as polarized. If they are indeed polarized in the radio, using a non-polar should work.

C91 is there somewhere. I don't have a 142 here, I'd have to dig out the SAMS to find it's location.
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Re: Cobra 142GTL cap changes

Post by 391redneckradio »

I just bought a nice 142 GTL and wondered how they turn. I have read that they take power to drive the modulator. How do they sound through the mic jack vs DI? I saw the DI mod in this thread. Just want some insight please. Wanted to try something different besides my usual Galaxy/RCI radios. Thanks.
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Re: Cobra 142GTL cap changes

Post by Goldfinger »

Yes, if you DI at C119, you have to use a small audio amp to have enough drive to fully modulate. (10-15 watts)
The reason is because the modulator circuit on a 2000/148/142 has a 2 transistor Darlington array (less gain). The Galaxy type radios have a triple Darlington array which has enough gain that standard line level is enough drive.
I've done one 142 (DI at C83) and three 2000's (DI at C99) using the radios own audio chip and it comes out sounding very close to driving the modulator directly using an external audio amp. Line level is plenty when direct injecting here.
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Re: Cobra 142GTL cap changes

Post by 391redneckradio »

Ok. Thanks. I found the C83 DI mod on here. Im going to play with that and the mic jack approach. I bought it have something new to play with. :D
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Re: Cobra 142GTL cap changes

Post by Goldfinger »

As long as you widen the whole mic chain, the mic jack approach works OK but injecting at C83 is cleaner as there are much less amp stages in the chain. I've done both 142's and 2000's for mic jack use (customers that wanted bass without having to use pro-tools/rack gear) and you can get some pretty deep audio with an Astatic 575 mic or a D-104 with the amp widened.
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