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Cobra 2000GTL, clock but no power on.

R

robinz

Member
May 7, 2010
11
0
11
Canada
Hello,
I dusted off my old Cobra 2000 GTL and plugged it into the wall and it will not power up. The clock is displayed but the main power button will not power everything on. It was last working before I boxed it up a few years back.

I've done some additional troubleshooting and the results are as follows;

1. hooked up 12v instead of 120v and results same, power to clock but no main power on.
2. disassembled and hooked up voltmeter to power supply. I get 16 vac on the output of main transformer. I get 0 v from the output of the bridge rectifier (D403), therefore I conclude that the bridge rectifier is bad. Can anyone confirm or deny my findings? I have not had to troubleshoot a power supply for some time and am rusty.

Robin
 

that wont stop it from powering off of 12v. try using you meter and checking the power switch for continuity. dirty contacts are common, more than likely there just needs a good cleaning.
 
i was under the impression that those old school radios did best when kept below 14 1/5 volts . but thats just what i herd/read .
 
the 16 volts he measured was out of the transformer, which is always higher than the regulator output in CB style power supplies.

first thing to do is trace the power from the output of the power supply (AC or DC) to the on off switch, and from there to the board.


if you find the 14 or so volts all the way to the PC board, i would suspect the voltage regulator. (MB3756)
it might have shorted.
LC
 
Isn't there also an AC/DC switch on the back of the radio? If it doesn't operate off of AC or DC I would suspect the power on/off switch to start with. There were also some issues with the alarm and snooze options that can cause this.
 
Hello,
thanks for all the feedback. I was able to resolve the issue.

As I was probing around the power supply section, checking voltages, I moved around a stiff wire connected at the output of the bridge rectifier and all of a sudden things lit up. Some kind of cold solder joint thing I guess!

While I'm in the neighborhood of the power supply, is there any other considerations I should be looking into for the 2000?

Robin
 
go to yahoo groups and join a groub called the mod. they deal with 8719pll radios. they have a list of all the stock capasitors and some that can be upgraded. i would order every cap on the list and change them out. be good to go for another 20+ years.
 
UPDATE: its only been less than 24hrs since I brought this system out of retirement and now another issue has cropped up. The frequency counter is displaying 92.200 on all channels.
From my extensive research on yesterdays problem, I came across an number of things to follow up on:

1) Continuation of my initial problem, a power supply issue.
2) Cobra service bulletin that describes the the cap/resistor fix on IC152.
3) VCO out of lock.

If any one can add to my list, please do!

Robin
 
Cobra 2000 Frequency Counter Issue

I had the very same issue. Even after the doing the Cobra fix it did not change one bit. Some will require replacing the whole frequency counter module. I was very lucky to locate one from a fellow who bought two Cobra 2000's. One he kept and other sold for parts. I bought the frequecy counter module and LED display. With the replacement module there will be some sort of alignment done so the frequency display matches exactly to the frequency being TX/RX.
Look for My world Famous Videos on You tube, I love taking videos of myself
 
that 92mhz problem with the 2000 counter is VERY common.

as a matter of fact, i believe it will happen to all of them sooner or later.

i have not heard of one particular way to fix this that will work in all radios.
some do the fix you posted, some replace a couple of FETs, some re-cap the whole counter module.

as far as i remember, all these methods have worked for some and not for others.

the fact that the counter board is double sided and plated through makes any repairs more difficult and ruining the board is a real possibility.

i do remember that NOMADRADIO over at the cbtricks forum has some great info on this phenomenon, and that is where i would suggest anyone start looking for more info on this.

the easiest fix?
find another counter board assembly from a working radio, and switch them out.
LC
 
one more thing to add.

i think what you are experiencing with your 2000gtl is that the caps have dried out during its long shelf existence.
electrolytic caps have a way of dying when they have been out of service for a while.
they also dry out with age.

this is happening to many others, and it has made me consider opening up a "re-capping" service more than once.

of course the problem is that it takes a fair amount of time to re-cap a radio and im not sure anyone would want to pay for that.

oh, and i would charge extra for re-capping the 2000gtl counter module LOL.
LC
 
Hello,
thanks for the feedback.

>>as far as i remember, all these methods have worked for some and not for others.

>>of course the problem is that it takes a fair amount of time to re-cap a radio.

Yes, I'm beginning to see that bringing my unit out of long term storage is going to take some time and effort. Oh well, there's absolutely no activity in the band around here and appears to be no skip happening so I'm in no rush to get back on air. I'll have to start drawing up the shopping list and get started on this recovery.

Robin
 
Status update:

I'm half-way through the re-capping process and things are looking good. The frequency counter section has been done and I did an alignment of the PLL. I was getting 92.200 displayed at the bottom range and this has now gone away.

I'm just replacing all the electrolytics. I have seen elsewhere of people also replacing the tantalums. Should this be a consideration as well?
 
Last edited:
you dont need to replace the tantalum caps unless they are bad.

i asked this question before and was told by someone i trust that tantalum caps last a very long time and really need not be worried about.

some upgrades to consider when you are replacing all the electrolytics:
C172 to a 2200uF 16 volt cap
C18 to a 470uF 16 volt cap
C181 to a 1000uF cap

you can change the main filter cap on the power supply, C410 board to a 5600-6800uF 35 volt cap.

dont forget to change TR41 on the main PC board, and TR301 on the power supply board to a 2N6487 or NTE152.

good luck with it!
LC
 
loosecannon, thanks for your input. I went to the wiki site and read the same about tantalums and shouldn't worry about.

There is negligible deference in cost between 16v,25v,35v capacitors so I have been using values of 25v and above for all my replacements.

I have collected information from around the net regarding improved values for certain caps but you have provided some new information. Thanks again.
 

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