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CObra 200gtl issue

redlght

Active Member
Feb 1, 2013
131
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Just got the radio i was having issues tuning people in on ssb plus the freq counter has it's 92.2000 issues but i started the alignment of the synthesizer section 1st 3 steps went well next step was:
input of freq counter to tp10 radio on ch1 am look for .790MHz I get 3.050 MHz?
Next step was input of freqc to tp3 ch19 usb adjust ct2 for 7.801 MHz mine was 29.620 and bouncing?
next step LSB look for 7.7985 mine was 24.620 bouncing around ?
next step was input of freqc to tp14 ch19 am xmt adjust L31 for 7.8000 Mhz mine was jumping form 15MHz to 16 Mhz?
Any ideas on whats up with this radio would be welcomed.
 

You must be talking about the 2000GTL; not the 200GTL to begin with.

The 92mhz reading on the freq counter is common:

http://www.worldwidedx.com/cb-band-cb-radios/56928-cobra-2000gtl-clock-but-no-power-2.html

Cobra 2000 frequency counter repair | Radioaficion Ham Radio

Have you done many alignments?
If not; then follow each step one at a time and follow through. You need a bench freq counter, an accurate DVM, a scope, and an accurate watt meter. Aligning the receiver will require a sig/gen of a decent quality.
 
Yes the 2000 sorry for the typo
I started from the first step in service manual/ the 1st 3 went perfect anything after that did not match up to spec nor could it be brought into spec by adjustment of L's or CT's as the book states. AM transmit is off ch1 is 27.040 and ch 40 is 27.490.

The check of input of freq counter to tp10 radio on ch1 am look for .790MHz where i get 3.050 MHz to 1.230 ch40 is the divider input of tp10.
 
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Yes the 2000 sorry for the typo
I started form the first step in service manual/ the 1st 3 went perfect anything after that did not match up to spec nor could it be brought into spec by adjustment of L's or CT's as the book states. AM transmit is off ch1 is 27.040 and ch 40 is 27.490.

Check for PLL hacks ('extra channel mods') and see if any of the traces are messed up. The PLL is a chain of events inside and outside of the chip that results in locked freqs. If there is no lock; then something is open in that chain. It is a common problem with older radios that have been hacked. You might do well to find out how it was modded and reverse it. If done this way at least you can be sure of the problem area. Other known problems are old, dried out electrolytic caps in the PLL/VCO circuit - as well as various other caps throughout the radios. Keep in mind; these radios are old and getting older.

I would also recommend doing the voltage regulator mod/Service Bulletin #1226 for this radio, as the MB3756 chip can be unstable and/or failing. Make sure that the power source is stable - first!

http://www.cbtricks.com/radios/cobra/140gtl/graphics/cobra_140gtl_142gtl_148gtl_tb1226.pdf
 
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No mods were done to this rigs in the pll section the clarifier is still locked I am on the process of getting all the eletro caps for the radio and replace them all and in the freqc in one shot. Just thought maybe there was a certain place i could look for the prob a known snafu of sorts. Thanks Robb
 
Did they use conductive glue in them?

I've never seen any . . .

Either there had been a mod done to that radio or the PLL is whacked if it has the freq range you've described. Do any of the channels repeat? Might be a bad PLL. Also check the first and second crystals on the board with a freq counter. Does the VCO voltage change when adjusted? If not; then the PLL loop is open if it is stuck on one voltage.
 
I didn't get around to checking the xtals yet but i tried it on DC and it won't power up I also noticed on ch 1 2 3 it makes a sound like a bad plug wire coming over a car stereo. Out to get caps for the radio today. P.S. After a good cleaning of the ac/dc switch dc now works I do how ever get a different freq on channels compared to ac power the motor boating was the dc meter to tp9 ch 40 am 3 volts aprox 2 volts on ch 1 ...AT the correct setting ch 1 to 5 will not lock the rest will setting it slightly higher to lock them all. go figure. I can transmit to another radio and it sounds really good and clean just slightly off freq still. Ok cap time thanks a ton for all your input.
 
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Doing some voltage checks I found the pin that should have 8 volts on it from D31 was varying the voltage the pin was broke right where it goes into the board ran my own pin and soldered it to the chip pin and things are looking better.
 
OK here are the numbers after the = sign is the result i got wth is up with this junk: FC= Freq counter OC= oscilloscope

FC to tp13/ch19 check 10.240= 10.2421

OC to tp10/ch19am check for max = max

Meter to tp9/ch40am adj 3.00v check for aprx 2 ch1 = ch40 3.0v ch1 2.02

OC to tp1 adjt max = max

FC to tp1/ch1 AM ck 34.7650 = 34.7655 lowest possible slug to top of can

FC to tp1/ch1 USB ck 34.7665 = 34.7671 lowest possible slug to top of can

FC to tp1/ch1 LSB ck 34.7635 = 34.7636 slug to top of can if i go any lower receive drops out no static out of speaker

FC to tp10/ch1 AM ck 0.790 = 0.79028

FC to tp3/ch19 USB ck 7.8015 = 7.80013
best can get out of ct2

FC to tp3/ch19 LSB ck 7.7985 = 7.79766 best can get slug bottomed out

FC to tp14/ch1 AM XMT ck 7.800 = 7.8001

FC to tp15/ch19 AM ck 7.3450 = 7.34503

FC to ANT./ch1 AM XMT ck 26.9650 = 26.96500
 
It all fell into place when i noticed that L22 and the 11.325 xtal had solder on them I placed a wire to them and poof the alignment went good. Last owner must of removed it for some reason.
 
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BTW Thank you Robb you got me looking in the right place from that PLL whacked statement when i found the broke off pin at board level.
 

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