Cobra 2000 GTL VCO expansion

[George EI7KO]

^^^^^Sound advice. I think the limiting factor is the pll that's in there too. When the mod boards get fitted to the src 2k's the 8719 pll has to go in there to get the right bandspread even though the modboards have their own on-board pll chips also. Be best at this point to have a chat with Maurer when he gets back Sammy....

[cb4ever104]

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?

Hi RD250 . Yeah , the PLL is gone and has been replaced with the eprom module .
Regardless of what band I select (LO/MID/HI) I always have the same codes from the channel selector (79 to 123) , so the eprom module has to modify these codes , depending on the band selected , and gererate new internal /N codes and appropriate downmix frequencies as these must change . It seems that the module has problems working with low downmix frequencies , @0.34Mhz etc . Anyway , I'll contact Mr Maurer and pass on my findings and see what he says .

Thx again , and to you George .

Sammy

[cb4ever104]

Reply from Gerhard .

Hi !

I have read now the statements in the forum, and found a real good idea there.
Maybe the low pass filter doesn't match now to the new, lower frequencies. You can try to increase C88 and C89, both are 47pF, try with 100pF each.
Usually there is no problem when modifying such radios, but usually the crystal is 11.285 - here it's 11.325. So the N of the selector must be also different and to meet the real channels again, pin10 is open and the module (same way as the pll) works with lower frequency (640kHz lower) on pin17. Therefore it will be worth a try to change the low pass filter after the mixer, you can try with an oscilloscope on pin17.
Hope this helps.

Gerhard

More to follow once I get a chance to buy the new caps .

Sammy

[RD250]

I don't think it's the fact the frequency fed into the PLL is low, or that there is a LPF issue after the downmix. It's the fact the division ratio in the loop is low.

If you like I can do a short tutorial on why having a huge variation in N code causes issues with the loop filter design.

However, one way to reduce the N ratio is to have a tripler xtal with a lower frequency (150kHz lower?) as N then needs to start off 45 codes higher on channel 1 low band.

So you get a lower overall N ratio from Ch1 Low to Ch40 high but you would also need to reprogram the EPROM to offset the codes by 1 block to get your wanted L M H blocks. If you did this I would expect the radio would work on all 120 channels 26.515-27.855MHz.

There are other fudgey mods you can do to improve the PLL stability on the low block as N gets really low. But maybe that can wait for now...

I can show you how to design a basic PLL loop filter for the radio too but the key thing here is you design the loop filter for a given N code (mid range N value?) So if you have a radio where N varies a LOT then the loop characteristics change a lot too as you go from Low to High band.

If N changes by a big ratio then the loop can go unstable on lower N codes. This is a known problem on the mk2 Cobra 148GTL-DX if you try and add super low using diodes on the binary adders and do no other PLL mods to improve the PLL stability on the bottom end of super low.

What you usually find is the mk2 148 will start to generate funny 'owl' hooting noises on FM mode as you tune down your newly added super low block and this noise is the onset of PLL loop instability. Depending on how you adjust the VCO slug you can alter where it goes unstable but generally the radio won't work well over the full 40 channels of super low unless you do more mods to the radio. This behaviour is predictable if you look at the design equations for a typical PLL loop filter.

I suspect your radio has a mild version of the same problem on the lower few channels?

[cb4ever104]

Hi RD250 . Thanks again for the reply/info .

So it looks like there's 2 posibilities .

1) It's the LPF , in which case it could be a cheap 2 capacitor change fix .

2) It's the low division ratio in the loop , in which case I'll need to hunt down a new Xtal and send the module off to Austria for reprogramming .

I think I might try fix 1) first as it's easier and cheaper . If that doesn't work then we'll go with option 2 .

More to follow .

Thx

Sammy

[cb4ever104]

Hi All .


Changed C88 and C89 for 100pF and working like a dream . Looks like it was a LPF issue after all . Anyway , thanks again to all for their help .

Sammy

[RD250]

That's good news

Asking the chap who modded it was always going to be the best route as he knows how the radio has been modded.
I could really only guess as I don't know what new PLL arrangement you have on your mod board.

In the old days here in the UK the lower channels of low band used to fill up with french CBers using FM if there was any skip.
However, there seems to be very little activity there at any time these days. It really does live up to its old 'graveyard' name now!

How does it compare in Spain?

[cb4ever104]

That's good news
Asking the chap who modded it was always going to be the best route as he knows how the radio has been modded.

I think he was relieved too ... "Wow! It was just a guess after looking at the schematic of this radio (and seen there is something different with the crystals...)."

How does it compare in Spain?

There's no-one on anywhere here Well , I hear a few now and again when the skip comes in but all in all it's pretty quiet .

[Ashtec]

Good to see a good ending

[George EI7KO]

Nice one Sammy. Glad to see you have it sorted. The old 2k is real fun to use on 38 lsb....

[dt307]

nothing like digging a dead post back from the past ......

just did my own 1 low/mid/hi/uk40/10m with a module

used the module to do the low mid hi and uk 40 and then for 10m switched pin 10 low while hi band enabled

26.515-28.495 normaly on most 8719 they go easy without a fight but i had to replace the 2 caps mensioned to get the vco to lock

this thread save me a lot of head scratching sammy thanks

[cb4ever104]

It's funny , but I remember nothing about this thread I know I wrote it , but it looks like gibberish now . Especially my "swapping out a PLL for an EPROM" ... ??? Anyway . I'd like to think that I've learned a bit since then .

S.

[dt307]

I know some times it feels like I'm going backwards lol

[cb4ever104]

I know some times it feels like I'm going backwards lol

Indeed Chris , although I do still miss RD250 . Great source of information he was .

[dt307]

I know some times it feels like I'm going backwards lol

Indeed Chris , although I do still miss RD250 . Great source of information he was .

indeed m8 i was waiting for the day when RD250 found a radio he thought was good and explained in only the way he could as to why lol