Cobra 2000 eprom channel mod

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BigW
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Cobra 2000 eprom channel mod

Post by BigW »

I recently acquired a Cobra 2000 GTL in rather rough condition. Surprisingly it powers up transmits and receives; problem is that there's a channel mod consisting of an eprom board which is not working properly (frequency displays 25.520 no matter which channel is selected, it TX & RX on this frequency) There's two screened leads running from the board to the main PCB and the centre of one is no longer connected but I cannot see where it should go; I suspect this may be the problem. I cannot find any information online about this board, it looks like it's a kit or home built. The PLL and other components in this area of the radio have been removed, I assume the eprom board replaces these. If I can get the channels working either via the board or back to original I'd restore the radio as I think it's one of the nicest homebase CB radios I've seen even in it's present condition. I'd really appreciate any information or help on this.

Thanks.
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cb4ever104
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Re: Cobra 2000 eprom channel mod

Post by cb4ever104 »

BigW wrote: 29 Apr 2021, 15:01 The PLL and other components in this area of the radio have been removed, I assume the eprom board replaces these.
Hi . An Eprom alone won't replace the PLL . It's purpose is just to provide new N codes for the PLL .

Can you upload some photos of that area ?
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Re: Cobra 2000 eprom channel mod

Post by BigW »

Will do.
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Re: Cobra 2000 eprom channel mod

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cb4ever104
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Re: Cobra 2000 eprom channel mod

Post by cb4ever104 »

Yeah. The original MB8719 has been replaced with a 145106. You've got a decision to make. Leave it as is and fix it. Or rip it all out. Replace the original parts , and mod it again. Option 1 is probably best , assuming that it all worked in the first place. Depending on your technical ability you might need to send it to someone. Especially if that eprom is knackered. If you're stuck on 25mhz then it sounds as if the VCO isn't locked.
Last edited by cb4ever104 on 29 Apr 2021, 19:49, edited 1 time in total.
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cb4ever104
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Re: Cobra 2000 eprom channel mod

Post by cb4ever104 »

One of the screened cables looks to be the 10.240 from the crystal on your board. The other one might be the downmixer to the PLL. I've never done that particular mod , so someone else might have more information. I fitted a Maurer expansion board in mine.
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Handle "Fortune Hunter" back in "the day"
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Re: Cobra 2000 eprom channel mod

Post by BigW »

I think I'll have a go at fixing up what's there already. My technical ability is limited enough but I'll have go at it. If that fails I'll follow your advice on the second option and restore it to original and mod it again. I tried a counter on both the screened cables and there's nothing so the board could be toast. It does TX & RX on the frequency displayed, would it do this if the VCO wasn't locked,
I was looking at the Maurer expansion board and they look like a neat option, Do they give you an installation guide for the Cobra 2000?
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cb4ever104
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Re: Cobra 2000 eprom channel mod

Post by cb4ever104 »

BigW wrote: 30 Apr 2021, 12:18 Do they give you an installation guide for the Cobra 2000?
Yes . I think it's on the website . I might have it somewhere , but Gerhard will send you it if you ask him . It's really straightforward .

You should be able to check the eprom with a voltmeter . Just check the output pins that go to the new PLL and see if they change as you rotate the selector .

Download the service manual and check the VCO voltage and frequency alignment parts . The radio will still be using the same VCO and LO stages , so you should be able to check those . The PLL has been changed as the 8719 has less available N codes .

Sammy
Last edited by cb4ever104 on 30 Apr 2021, 14:25, edited 5 times in total.
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Handle "Fortune Hunter" back in "the day"
Radios ....? Anything that says "TAIWAN R.O.C" on the back.
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Otter
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Re: Cobra 2000 eprom channel mod

Post by Otter »

It looks like there ia a crystal on the new board which can only be 10.240. There will be 'Fin' from the main board to the PLL, and there will be PDout from the PLL. I'm guessing those signals are the ones in screened leads. One is likely to go to pin 1 of where the old PLL was, while the other goes to pin 5. If you trace back to the pin on he MC145106 that the broken wire connects to you'll have more of a clue.
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Re: Cobra 2000 eprom channel mod

Post by Techquest »

The disconnected wire goes to where TP10 is if you have not tried tuning the vco etc it should work fine.
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George EI7KO
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Re: Cobra 2000 eprom channel mod

Post by George EI7KO »

Techquest wrote: 30 Apr 2021, 15:58 The disconnected wire goes to where TP10 is if you have not tried tuning the vco etc it should work fine.

Good man Ray, The 'Jedi' strikes again! :idea: :idea: :idea: :idea: :idea:
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