Cobra 2000 GTL VCO expansion

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cb4ever104
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Cobra 2000 GTL VCO expansion

Post by cb4ever104 »

Hi All .

My 2000 GTL will only handle about 100 channels (of the 120ch module that's fitted) before unlocking . Does anyone have any info on how to expand the VCO on this model ?

Thanks

Sammy
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Re: Cobra 2000 GTL VCO expansion

Post by PsiDOC »

Info attached matey. ;)
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Re: Cobra 2000 GTL VCO expansion

Post by cb4ever104 »

Many thanks PsiDOC . I'll get on it asap .

Thx !

Sammy
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Re: Cobra 2000 GTL VCO expansion

Post by zephyr »

Is this any better for you Sammy.
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Re: Cobra 2000 GTL VCO expansion

Post by melaza »

Saludos,

Remove R95. There is a modification for the VCO that consist on installing a 10k VR from pin 5 (VCO) to one of the modification switches. Also install 1k resistor from pin 10 to ground (SO42P).

Saludos.
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Re: Cobra 2000 GTL VCO expansion

Post by cb4ever104 »

That looks good . Can the varactor be any type/value ?

Sammy
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Re: Cobra 2000 GTL VCO expansion

Post by PsiDOC »

In theory yes, however I'd try one from the VCO circuit of a scrap rig first and that should have the most capacitance rate of change.

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Re: Cobra 2000 GTL VCO expansion

Post by cb4ever104 »

Thx . I've got an old Jackson that here that I can use for parts so I'll get one out of it . Cheers .

Sammy
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Re: Cobra 2000 GTL VCO expansion

Post by melaza »

Saludos,

Remove R95 near PLL.
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Re: Cobra 2000 GTL VCO expansion

Post by cb4ever104 »

Weird . Cut the track from L19 and put a varactor in series but the VCO is now unlocked on all channels and a constant 8.5V at TP9 . I've double checked the varactor is in the right way with anode at L19 ?

Just a point . The Jackson has the same varactor (D31) in parallel with the VCO coil (L12) . Is it worth trying the same configuration with mine ?

Sammy
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Re: Cobra 2000 GTL VCO expansion

Post by cb4ever104 »

I should add that I can get down to about channel 6 or 7 on the low band before the VCO unlocks . It's strange though as I still get a good 3V just before it goes , which I would have thought was enough . Also we have to rememeber that I now have a eprom module in place of my MB8734 . R95 and R96 were removed as per the module fitting instructions . Worst case scenario is that I just don't use that band . I never do anyway . Although it kind of defeats the purpose of having the module there . I think Mr Maurer is off on holiday for a few days so I'll maybe speak to him again when he gets back .

Sammy
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Re: Cobra 2000 GTL VCO expansion

Post by RD250 »

Obviously I don't know for sure in what way your radio has been converted to 120 channels but if I assume it has an alternative PLL chip and some means (EPROM?) to provide the 120 divider codes to support 120 channels and if I also assume your radio still has the 11.325MHz (tripled) loop crystal then I would expect it to have a problem with respect to PLL stability on the low block.

This is because (on this radio) the division ratio in the loop will become very low as you move down the low block channels towards 26.515MHz. I'd expect the PLL to show signs of poor phase margin towards the bottom channels and the PLL would probably lose lock in the lower few channels.

If so then this isn't directly a VCO issue, it's an issue with trying to expect the loop filter in the PLL to work over a large ratio of division codes. This is why it can still lose lock despite having 3V tuning volts at the lowest channel it can still lock on.

i.e. there will be a large ratio (of nearly five!) between the code for channel 1 low block (34?) and for channel 40 high block(168?). In the absence of any other mods this will cause the PLL to behave very differently in terms of loop bandwidth and phase margin across this range. So it's no surprise that the radio misbehaves in that it loses lock on the lowest channels. i.e. the loop will head towards instability as you select lower and lower channels on the low block until finally the phase margin becomes zero and the PLL loses lock on the lowest channels.

Maybe your rig doctor missed off a mod to compensate for this?

However, as I said, I don't know what has been done to your radio in terms of its new synthesiser system so the above basic explanation might not be relevant to your modified radio.

To prevent the above issue I would have expected the 11.325MHz crystal to have also been changed (lower?) to prevent low division ratio codes in the loop on low band. Obviously this would mean a new set of divider codes for all three bands but this would be easy to achieve if there is an EPROM in the modified radio and the new PLL chip can support the higher divider codes.

But maybe your radio has a completely different synthesiser system so I can really only guess at your mods...
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Re: Cobra 2000 GTL VCO expansion

Post by cb4ever104 »

Hi RD250 . As usual , very interesting reading . Unfortunately I don't have any indepth information on the new module that was fitted . I'm presuming that it's an eprom . I've sent Mr Maurer in Austria an email and I'll post back once I hear from him will be probably be at the end of the week as I'm on vacation too . Just to mention that R95 and R96 were also removed as part of the eprom instalation . More to follow ....

Sammy
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Ex GM0 , now EA5 .
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Re: Cobra 2000 GTL VCO expansion

Post by cb4ever104 »

Hi RD250 .

I've been looking at this for a couple of hours now . Now , I've never really dug very deeply into eproms being used as PLLs but I think I've figured out a few things regarding my radio . Firstly my Xtal oscillator is 11.325Mhz , which is tripled to 33.975Mhz . I only have 6 usable program select pins on my MB8734 . The 7th pin (Pin 10) is continually held at "1" . I have no external presetable adders and as such I only have the 6 wires coming from my channel switch to the eprom . The channel selector generates the program select codes 79 to 123 . The eprom will have to generate new additional N codes internally with new downmix input frequencies . These will be activated by whatever external wires are connected from the new band switch . So basically on the low band my internal N codes will be 34 to 78 and 124 to 168 on the Hi band . With the downmix frequencies being 0.34Mhz to 0.78Mhz and 1.24Mhz to 1.68Mhz respectively .

Am I right in what I've said so far , and are you basically saying that the radio isn't happy dividing by a code so low as 34 , 35 , 36 etc ?

Thanks

Sammy
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Re: Cobra 2000 GTL VCO expansion

Post by RD250 »

Hi Sammy, you've got the right idea with your description but there is a problem.

The schematic for this radio shows an MB8734 PLL chip and I think this chip has quite a restricted range of N code support.
i.e. I think it can only support about 64 N codes starting at 64.

On a stock radio N would be 79 for channel 1 on the (USA) mid block 26.965MHz.

From memory this would require a code of 15 from the channel selector and the 64 gets added inside the MB8734. 64+15 = 79

So the problem is that the MB8734 can't go below N = 64.
So can you look again at your radio to see if there is a new PLL chip somewhere? Maybe the 8734 is now redundant?
Am I right in what I've said so far , and are you basically saying that the radio isn't happy dividing by a code so low as 34 , 35 , 36 etc ?
I can explain why it's not good to try for very low N codes but we don't yet know if your radio is doing this so maybe this can wait in case my explanation isn't relevant to your radio.

TBH the best person to advise you is the guy who modded it for you because I can only guess...
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