Any 2000GTL Main Board Experts Here? Need Help Putting Back To Original.

[pede69]

Hi...

I would like to kindly request that someone whom knows the Cobra 2000GTL main circuit board inside and out, lend me a helping hand if you can afford the time to do so?

Lots of pics at (some pics are huge) : Index of /tools/pics

I use Picasa to load large pics and the zoom in and out is great and shows awesome detail.

I purchased a 2000 off of eBay and the listing stated that the unit was virgin except for a 40+ and 40- mod.

I received the unit and it looked great. Kicked ass on AM but had total modulation distortion on SSB. And also a pretty loud buzzing or hum when the mic was keyed on SSB with no audio into the mic. (this noise was heard on receiving end of another radio, not the 2000)

Reports said that the signal was very strong on SSB but the voice was worbley (that was the word used) or distorted to the point of not being able to understand it.

After doing some research and learning just a bit I opened her up for a peak.

First thing that I found was that R131 had been cut. R126 was removed and replaced with a jumper. R124 has also been removed. There are also some other areas on the board where I feel parts have been removed or otherwise messed with. These areas can be seen in the pics.

I took it to someone I know pretty well that works on radios and without the exact parts, he played with it and ended up placing a 300 ohm resistor at one end of where R124 goes and at one end of where R126 goes. This solved the humming/buzzing and over modulation problem. (also removed jumper wire)

Now... however... while the transmit audio on SSB is loud and clear, it still has a ton of outrageous modulation. (at about 20% Dynamic Mic volume)

You can hold a stock mic even a foot or so away from you while speaking in a normal tone of voice and it sounds like a bodacious D-104 with a KW behind it.

I would like to figure out what all has been done to this radio (besides the 80 channel mod and the unlocked clarifier mod) so that I can restore it back to original. I will leave the unlocked clarifier and the 80 channel mod, but I wish all power and modulation circuits to be original.

In the pics the two colored wires go to the channel mod and the red and black wires go to the Voicelock knob.

Can you 2000 experts here please look at all the pics and figure out what may have been done, that gives the still extra loud modulation? And what I need to do to make it original again?

I realize that this may be a bit time consuming so thank you very, very much.

John

 

[Robb]

Well, let's start to sort this mess out.

The hum might be AC hum caused by the power supply filter caps gone bad - this is a consequence of time/age. Use an external power supply and plug that into the receptacle for external power in on the back. There is an AC/DC swith right next to it that you will need to use at this time too.

Check and see if the AM regulator has been removed. This may well be the cause of the 'too much AM modulation and distortion on SSB'. Replace that transistor if it has been removed. Don't recall the TR number or the part number ATT; but that info should be easy to find on cbtricks.com.

AFTER doing those things, you will need to turn down the ALC adjustment pot on the board for side band. No more than 90% ALC! If it is set too high, it will cause that 'warble sound'.

Same goes for the AMC or 'modulation pot'. Turn it down. Too much modulation doesn't make a radio better, it can also be nasty sounding if not adjusted at the 100% mark! If it has an NPC mod in the radio, then set the modulation at no more than ~130% once the AM limiter has been replaced.

Not such a 'pristine radio' as you were led to believe.

Now, there's a fair start for ya . . .

 

[pede69]

Thanks Robb...

The over modulation and humming sound problem was resolved by removing the R126 jumper and putting in a new resistor. Radio is usable now.

Just need a 2000 expert to look at the board pics and see what should and/or should not be there.

Thank you for your help though.

John

 

[unit_399]

If the unit warbles badly in SSB, check the driver and final bias. Driver bias - 30ma, Final bias - 60ma.

 

[pede69]

Thanks guys...

Anyone know this board well enough to look at the pics and tell me what has been done/undone?

Or at least, does anyone have a virgin board that they can take some pics of for me to review?

Thank you all for your time and help,
John

 

[1iwilly]

Thanks guys...

Anyone know this board well enough to look at the pics and tell me what has been done/undone?

Or at least, does anyone have a virgin board that they can take some pics of for me to review?

Thank you all for your time and help,
John

well to start of r124 and r126 are 10k ohms resisters i would put those back in the circuit also some guys take tr-24 which disables both am/ssb which will make sound bad in ssb also some take out tr-26 which is just for the am mod
or cut r-131 the am regulator is tr-41 which can be swap for tip-3055 from radio shack for better performance vr-8 should be set to 25 to 30 milli amps and vr-9 50 milli amps when the limiters are cut the mike gain is control in the front of the radio and ususally no more than 1 oclock is needed sometimes less that radio is no different than a 148 gtl so don't be afraid to ask

 

[pede69]

Your awesome iwillie... thank you.

Few questions please...

What do you mean tr-24 "disables both am/ssb? Exactly what is it disabled?

I did see ONE T-92 style transistor that has been removed but don't remember which one.
R-131 was cut but I resoldered it.

I would like to have all power and modulation circuits back stock on this radio. Any improvements I might
wish to have, I can do externally.

Thank you again,
John