Cobra 2000 butchered clarifier mod, boy do I need some help!!!

[Lkaskel]

Hey Everyone,
So, while I am waiting on some parts for other repairs I decided to look at a Cobra 2000 for a local op. He asked to have the extra channel mod taken out but would like the clarifier mod working. Removing the channel mod was pretty straight forward as I found a mod online that seemed to match the one done to the radio so reversing that seemed easy. The clarifier mod, that does not work correctly is a different story. None of the original wires to the clarifier are the same and the wire colors do not match what is on the schematic. I can't seem to find any details online that even closely resemble how this radio is done. No worries, I'll just find one that makes sense and redo the mod to match it. The problem is, there are many versions of this mod (I think because of the fine and course controls). Can someone point me to a mod that is the easiest or best version? In the end, the op wants a straight 40 channel radio with a modded clarifier only. Any help would be greatly appeciated.

Thanks as always!!!

 

[nomadradio]

Seems to me I posted this some time in the last ten or twelve years, but here it is again. This is just faster than sifting through a forum search.


N O M A D R A D I O

1615 Bardstown Road Established 1975
Louisville KY 40205
(502) 459-5554 Chris Reifsteck, Proprietor


FILENAME: C2KCLAR.DOC 8/17/12 CR
Clarifier mod for the Cobra 2000GTL

For normal coverage:

Pull D75,D32.

Follow the red wire on the Fine clarifier CW lug to the lever-switch pc board, take that end loose. Solder it to the positive pc pad under C84, the 1000uf cap next to the 8734/8719 chip.

Unhook the brown wire from the CCW lug of the Fine control.

To travel full coverage:

As above, but CUT the blue wire from the wiper of the fine pot, so the jumper from Fine wiper to Coarse CW lug remains.

Short across R175 at the far end of the yellow wire from the CCW lug of the Coarse pot. May be easier to just push the long end of R175 deeper into the hole, and bend it over to lap-solder to the ground-foil side.

Short across R174 12k, remove R44, 8.2k.

Sometimes this hookup reduces the range of the Fine control to less than a 1/2 kHz. Adding a 5.6k from the CCW lug (now empty) of the Fine control to ground will increase its range to 1 kHz or more.




To disable the between-channel lockout:
No particular reason, until it malfunctions and locks out legit channels.

Gray wire at the front edge of the pcb, just next to the screen-print "PC497xx" ID.




This is what we use for a hacked-up radio.

73

 

[Lkaskel]

Seems to me I posted this some time in the last ten or twelve years, but here it is again. This is just faster than sifting through a forum search.


N O M A D R A D I O

1615 Bardstown Road Established 1975
Louisville KY 40205
(502) 459-5554 Chris Reifsteck, Proprietor


FILENAME: C2KCLAR.DOC 8/17/12 CR
Clarifier mod for the Cobra 2000GTL

For normal coverage:

Pull D75,D32.

Follow the red wire on the Fine clarifier CW lug to the lever-switch pc board, take that end loose. Solder it to the positive pc pad under C84, the 1000uf cap next to the 8734/8719 chip.

Unhook the brown wire from the CCW lug of the Fine control.

To travel full coverage:

As above, but CUT the blue wire from the wiper of the fine pot, so the jumper from Fine wiper to Coarse CW lug remains.

Short across R175 at the far end of the yellow wire from the CCW lug of the Coarse pot. May be easier to just push the long end of R175 deeper into the hole, and bend it over to lap-solder to the ground-foil side.

Short across R174 12k, remove R44, 8.2k.

Sometimes this hookup reduces the range of the Fine control to less than a 1/2 kHz. Adding a 5.6k from the CCW lug (now empty) of the Fine control to ground will increase its range to 1 kHz or more.




To disable the between-channel lockout:
No particular reason, until it malfunctions and locks out legit channels.

Gray wire at the front edge of the pcb, just next to the screen-print "PC497xx" ID.




This is what we use for a hacked-up radio.

73

Thank you for this Nomad!!! I have 2 questions for you.

1) In the instructions it says to remove D32. That Diode is coming off of L22. Is that supposed to be D52?
2) In some of the mods online they call for removing D51 and sometimes jumping it out. I am unsure what that would accomplish?

Thanks again!!!

 

[nomadradio]

Holy Moley! A typo.

In a file decades old. Oops. Yeah, D52, not 32. Been doing that job mostly from memory long enough to totally miss that error. And once we show the procedure to a new tech, they can find the components by sight without squinting at the screen-print callouts.

Thanks for the proofread. A jumper across D51 will increase the max voltage to the varactor diode by about 2/3 of a Volt. Not a lot of added tuning range, and not worth doing. Pretty sure D51 provides some temperature compensation. Jumping it might or might not increase frequency drift from temperature rise. No real benefit that I see.

With any luck there's no hidden sabotage in that radio.

73,
Chris

 

[Lkaskel]

Ok, so the radio is back in service. Thanks again Nomad for your help on this one!!! Check the attachment out for the final result.