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cobra 2000 gtl headache

B

Big Dummy 123

Member
Aug 29, 2009
63
1
16
Hi all.I havea 2000 that was given to me looks new.Was pretty well hacked.Power supply dead repaired it 13.55v to board .removed hacks restord voice lock to stock, repaired freq counter,remove resitors and diodes in front of pll area that was covered with hot glue.Repaired cliped doides d75 ,52 ,d51 and relaced r174.Replaced L19 core loose. It will tx and rx. Rx 27.1296 and tx 27.2011 same as external. counter.vco wont lock changes Freq.Recaped with Klondikes Mikes Kit. The Mb3456 regulator voltage ok p1 8.39v p2 13.55 p3 8.38 p4 0 ground p5 13.35 p6 8.39 P7 0 p8 tx 8.43 rx .50.Changed Vco with knowen good one no change Ic 2.p5 1.87.not right. Pll8734 p1 1.87 p2 4.86 p3 5.90 P4 8.22 p5 8.22 p6 5.44 p7 5.44 p8.7.30 p9 8.34 p10 0v p11 though 16 8.28 and 0 program pins volage from channal select look ok change as rotate knob. p17 3.74 p18 0v. I have a good working 2000 that i follow the repairs for voice lock and removing other hacks. also L59,L21,L22,L23 do not respond. I do have the scamatic and alingnment manule parts list. I have a B&k 1040 and 2040 no scope.err i do but only a 15 meg, Thanks Ben :confused: :bdh:
 

wow thats quite a post! LOL

you have to understand that giving that much info all in one paragraph is going to send a reader running for another thread.

you also have some inconsistencies in your post that tell me that you are grasping at straws for answers without taking the time to figure out what the problem really is.

for example, you say "it will rx and tx" and you also say "the vco wont lock". these two things cannot both be true.

so, lets back WAYYY up and see if we cant find your real problem.

the best way to do this is to do a full alignment, starting with the PLL section.

we will do this in sections. first, using your freq counter probe, touch the probe to pin 8 of the PLL chip (MB8719 or MB8734).
do you read 10.240mhz?

if not, stop there and we'll check that.

if so, put the radio on channel 40, AM mode, and using a DC voltmeter on TP9, adjust L19 for 3.5 volts.
check channel 1 for about 2.2 volts.

ok, lets stop there for now.
let me know what you find.
oh, and go slow. LOL
LC
 
There ya go someones going to take the time and the service manual and walk through this with you!! You cant ask for a better forum with better help ;)
 
Hi All I need to get a decent counter any sugestions? Thanks LC for reading my mess was pretty agavated with it the voltages stay around 2.52 to 2.55 at tp9 when changing channel selector. I pulled pll and put a socket in. It will transmit with no pll in it. Thank Ben
 
as for the freq counter, something like this will do just fine:
NEW HQ MASTECH MS6100 FREQUENCY COUNTER 10Hz - 1.3GHz - eBay (item 140473257088 end time Nov-11-10 18:15:29 PST)

yes, its a cheaper model, but i think its better to get a cheap new one than a better brand used.
you have a better chance of it being accurate. this is just my opinion.

you do need a freq counter to really troubleshoot this problem, but there are a couple of things you could try.

as far as the radio transmitting without the PLL chip in there, i dont know what to think about that one. LOL never tried it myself. i would think that its not possible, but who knows.

you said you removed the PLL chip and put in a socket. any chance you put the PLL chip back in the wrong way?

dont just turn it around and try it, because im not sure what the effect of the chip being backwards is. wouldnt want you to blow the chip.

just confirm that its in there the right way.


MB8719

use that link and the pic ive attached to this post to confirm it.

looking down at the PC board with the radio knobs facing you, radio upright, pin 10 should be in your front left corner.


it also sounds like your VCO is free running and not locked in.

you said that you replaced the slug in L19 which is the VCO coil.
did you just move the slug that was in there or replace it with one from a different coil?
the slugs in these coils are not necessarily interchangeable.

try adjusting L19 a bit and see if you can get it to read 3.5 volts on channel 40.
check the PLL, and we'll go from there.
if you can borrow a freq counter for a few days that would help alot.
LC
 

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$36???
Grreat deal. I bought a new Protek for $100.

A 2000GTL is worth the time to fix.

Look at the freq counter purchase as a plus for the next time you have to tune up a radio - I do . . .
 
Here is my work station that consists of a Tektronics 100 MHZ Scope, IFR 1500 service monitor, Leader LDC 822A frequency counter, Fluke meter and a bird 43.
 

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Hi the L19 i put in is from another 2000 same number on the side of it. move the L19 it will change freq.It will not go to 3.5volts stays around 2.5 volts. I do get the proper voltage to the data pind on the pll from channel selecter. The pll is installed right Pin 1 is on back side facing to the right pin9 toword power supply same as the other 2000 i just finshed up. Its new had 10 volt blues. thanks for your time LC some times im hard headed. i have Lou Franklens Books my reading comprhention isnt to good lol. Ben
 
I fogot to say. When you turn channel select Data pins are eather 8.28v or 0v and coraspond to the truth chart. do the same at tp 9 the volage is 2.3 to 2.5 or so when u tunt the channel select fluctuate between those voltages.Ben
 
sorry it wasnt something simple like the chip being in wrong.

you will need a freq counter to go any further. we need to know if the signal freqs are getting generated at all, then find out where we lose them, or if we do.

if you have lou's book called "understanding and repairing CB radios", he goes through the MB8719 PLL in detail explaining how it works.
try reading that section and see if it gives you any ideas.

also read anything on VCO's in that same book.

this will keep you busy till your freq counter gets here.

im hesitant to suggest certain things until we know more about whats going on.
for example, i could tell you just to try the PLL from your other 2000 and see if it works, but if the problem is not the chip itself, you might blow the other one as soon as you plug it in.

seems like you have done lots of work to that radio, and im guessing that you didnt check it at each stage to make sure it was still working. this means you need to go back through all the things you already did to the radio and make sure you didnt put a cap in backwards or something like that.

this too should keep you busy. LOL

sorry i cant be of more help, but if you PM me when you get a freq counter, and link me back to this thread, we can start again.

for now, my gut is telling me that something went wrong when you put the clarifier back to stock.
look for a backwards diode or something like that. make absolutely sure that everything is where it should be.
you might just be missing one part or have a bad solder joint.

good luck,
LC
 
Hi all im looking at a Hickok model 380 80 mhz on eekbay $34 atm 7 day money back warenty is this any good? Ben i dont know how to link stuff sorry
 
its not about whether or not its a "good" freq counter or a "bad" freq counter, they all do the same thing.

the difference is accuracy. the entire point of having a freq counter is that you need that accuracy.

all test equipment needs re-calibration every so often, and its not cheap, so people sell their old counters on ebay instead to help finance the purchase of a new counter.

my advice would be to stay away from the used/old counters and buy a new one, even if you have to wait.
LC

oh BTW, here is now to post links to the forum. (well, one way anyway)

go to the page you want to link to and move your cursor into the address bar.
right click. you will see the whole address get highlighted and an option box will appear.
left click on "COPY".

now, in your post to the forum, move your cursor to where you want the link to appear in your post and right click.
an option box will appear.
left click on "paste".
the address you just "copied" and "pasted" will appear.
when your post appears on the forum, your link will be in it.
 
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Hi All order the new counter today.Also have new Mb8734 I forgot i had will not put it in till we figure out what is wrong rechecke cap all in right Ben
 

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