Cobra 148 GTL DX MK1 - Mic Audio Mod

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Phuture
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Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by Phuture »

Hi,

Is there a good mod to improve the Tx audio quality of the Cobra 148 GTL DX MK1? Compared to newer radios using the same mic (Astatic teardrop), mine sounds a bit thin at at the bass end and just not really a warm sound.

Cheers
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by ch25 »

A lot can be improved.
PC879 or PB010?
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by Otter »

Mk.1 is PC879.
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by ch25 »

Wide version:
C135, C128, C217 1uF non polar
Wider than stock, but not that much use MKT 330-470nF
Last edited by ch25 on 10 Nov 2021, 18:18, edited 1 time in total.
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by Phuture »

Yes, I have the PC879 board - thanks Otter.

Thanks for replying to my post, Lemmy. So do you mean that to create a wider frequency response ("Wide version") I can replace C135, C128 and C127 with 1uF non-polar electrolytic caps? But, that using "MKT 330-470nF" caps would be better? Is MKT a manufacturer name and would these caps be ceramic or mylar?

Thanks!
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by ch25 »

MKT - mylar.
I use even 4.7uF in some radios.
Start with 470nF, difference to current ones will be huge.
For a try no need to replace, just add new, parallel to existing ones on the foil side.
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by Phuture »

So just to be crystal clear, attached is a close up of the PC-879 schematic around these caps. These are the ones you are suggesting I replace, right? (C128 circling corrected)
Cobra148GTLDX_MicAudioMod.jpg
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Last edited by Phuture on 11 Nov 2021, 10:44, edited 1 time in total.
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by ch25 »

Different numbers on my schematic.
http://www.cbtricks.com/radios/cobra/14 ... /index.htm
Replace green if you want to open SSB as well.
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by Phuture »

Sorry, I circled C127 instead of C128 in my image (corrected). So the cap is actually C217, not C127 as you previously described? Thanks for clarifying.
Last edited by Phuture on 11 Nov 2021, 11:03, edited 2 times in total.
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by ch25 »

217, fat fingers, corrected
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

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Got it - thanks, Lemmy. I will experiment, as you say, and feed back. Looks promising! :thumbup:
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by Phuture »

Hi Lemmy,

I received the 470nF Mylar caps today. It looks like there are far fewer sellers selling values of these caps in between 470nF up to and including 1uF, but I will try the 470nF caps first to see how it goes.

Was the idea that I should replace all three of the existing caps (C128, C135 and C217) all at once or one at a time and then to test the audio quality?

Thanks!
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by ch25 »

All of them.
Each one of those is a bottleneck.
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by Phuture »

Got it - thanks! Will report result tomorrow.
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Re: Cobra 148 GTL DX MK1 - Mic Audio Mod

Post by cb4ever104 »

ch25 wrote: 10 Nov 2021, 13:28 For a try no need to replace, just add new, parallel to existing ones on the foil side.
Are you saying that these new 470nF caps need to be added in parallel on the solder side , or do they replace the original part ?
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