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Cobra 2000 gtl problems

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CW_357
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Cobra 2000 gtl problems

Post by CW_357 » Sunday, 18 October 2009, 9:01 AM

Can anyone here help me with some info? I got an old cobra 2000 the other day & i can't figure out whats going on with it. I've got it hooked up to an antron 99 & it has a really awesome receive. And when i talk it's got some serious swing on the modulation meter. But, it doesn't seem to have any power. The rf needle doesn't move when i key & whistle. I talked to someone local the other night, & they could barely hear me. I was thinking it may be the antenna? So, i hooked up my trusty ole' Cobra 139 xlr & it was smokin' so it can't be the antenna. Now, the guy i got it from said he had the watts turned down to 2 & 1/2 because of the amp he was using. But, it doesn't even show that on a meter. The radio isn't in that good of condition, the tuning knobs are scratchy but workable. & i think someone put the expo 100 kit in, cause the "Tune" knob only clicks once to the left & once right. Anybody got any ideas on my problem? Also, does anyone here know of any radio Dr.'s east of I-95 in NC? Thanks, CW_357


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Re: Cobra 2000 gtl problems

Post by Black Lightning » Sunday, 18 October 2009, 10:29 AM

It almost sounds like something in the RF chain. Possibly a final, driver or predriver, or one of the TP8/TP8 wires is disconnected.

Does it have output on SSB?
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Re: Cobra 2000 gtl problems

Post by CW_357 » Sunday, 18 October 2009, 11:19 AM

Nope, nothing at all. No modulation or power on either side upper or lower. Same for the extra channels, just modulation no power. Upper & lower on those "nothin".
CW_357

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Re: Cobra 2000 gtl problems

Post by TheCBDoctor » Sunday, 18 October 2009, 12:51 PM

Hi CW_357

If the radio is receiving that is a good thing. The fact that it does not transmit on AM or on SSB means the problem is in the RF circuitry. The expo kit would have no affect on the radio's ability to transmit if the radio is receiving. If it receives then it should transmit. What does the counter read on Channel 20 with the Expo selector in the center? Does it stay the same on Tx.

As long as the counter does not read 92.2000 MHz the radio is definitely worth saving. The scratchy controls can be cleaned with contact cleaner, just be careful not to get it on the meters. You may click on my link below to go to my educational website. I specialize in the Cobra 2000 GTL and other 8719 Chassis'

I may be able to talk you through fixing the radio. All you may need is a multi-meter. My direct email is on the Website

Good luck,
Best Regards,

Rick, The CB Doctor


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Re: Cobra 2000 gtl problems

Post by CW_357 » Sunday, 18 October 2009, 13:33 PM

The counter on 20 shows 27.2031.
When i key, it stays on 27.2031, but every 3rd or 4th key it'll flash just for a second to 31., 35., 30., 32., but just for a second then stays on 27.2031. CW_357


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Re: Cobra 2000 gtl problems

Post by TheCBDoctor » Sunday, 18 October 2009, 14:59 PM

Hi CW_357

The fact the counter is reading a bit off is due to the expo kit. The wires from the kit to the board act as capacitors or inductors. The frequency should read 27.2050 on channel 20. This can be compensated by adjusting the voice-lock control, or after the repair, the expo kit has trimming caps to align the frequency.

The occasional jumping to 34 or 35 MHz is due to a slight delay in processing the 7.8 MHz signal that comes from the radio's main board. Again, that is not a serious problem. If the radio transmitted on sideband, but not on AM, then the problem would be in the AM modulation circuit. The fact that it does not transmit in either mode suggests that the Final 2SC 1969 is bad.

Before replacing the final I always measure the voltage on the collector. That can be done by measuring the Tab of the transistor. Make sure you do not short it to the heatsink. On Sideband the voltage should be about 12 volts when transmitting. On AM the voltage should be half that, about 6 volts.

The driver is 2SC 2166, it sometimes fails but not as often because it does not work as hard as the final. If you put another radio close by and can hear something while transmitting on the Cobra 2000 GTL, and the voltage is what it is supposed to be, then you have a bad final. It can be replaced with another 2SC-1969, a 2SC-2312 or a 2SC-1307.

You may find many of these final transistors in a scrap sideband radio. A regular CB final will work but might not hold up on SSB. I will bet that you have a bad final. Finals go bad because the radio was keyed up into a bad antenna system. You can fix that radio if you have even the slightest mechanical ability.

Try it out and I bet it will work. I can talk you through any alignment if needed.

Good luck,
Best Regards,

Rick, The CB Doctor


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Re: Cobra 2000 gtl problems

Post by Seecar » Wednesday, 30 December 2009, 9:47 AM

HI

You can easily determine whether the driver or the final is defective if you have a monitor receiver to
listen to while testing the 2000 driver and final.

Put the monitor receiver on the same channel on a matching sideband if using sideband; Either sideband if the unit is an AM rig but we're working with a 2K. You may need a mic on the broken radio if AM does not work on the 2K.
You will need to adjust the RG Gain on the receiver.

With a metal probe in your hand touch the collector [the mounting tab] of the driver with the probe while transmitting.
If on SSBand you need to whistle into the mic to get output.

If there is any RF there it will be received by the monitor receiver transmitted thru your hand. If there is a lot of RF you need to test the final next.

Do the same thing with the final. There will be a marked difference between the 2 regardless of which one is bad.

There is 1 gotcha here. The 2K has an AM power regulator. Checking on SSBand is reccomended for the AM TX mode may not have power.

If the driver has a much greater signal on the monitor receiver then the final needs to be replaced and vice versa.
There will be a feedthru of the signal thru the driver so that the final will get a bit of signal even if the driver is bad.
There will be a significant difference in the received signal.

That's what I used for 35 years in this 'bidness. :icon_e_smile:

Carl /N9EFJ

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Re: Cobra 2000 gtl problems

Post by CW_357 » Wednesday, 30 December 2009, 16:30 PM

Hey guys, I really appreciate all the info from ya'll. But i really have no clue when it comes to all that tech stuff.... the only thing i know about a collector is that he wears a black suit & when he comes to your house he either wants money or your soul... lol hehehe i wish you guys lived a little closer to my location so's i could get this old base workin properly! But hopefully i can find someone local to fix it for me. Thanks again for the help! & ya'll have a happy new year this Friday! 73's Cold_Whiskey_357

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Re: Cobra 2000 gtl problems

Post by Sporty Mike » Wednesday, 30 December 2009, 19:54 PM

Hey CW, I've got a Navaho TRC-457 with the exact same problem, Let me know when you get your's fixed what it was, and if I get mine fixed before I hear from you I'll do the same!!


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Mike
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