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Cobra 2000 gtl with No transmit limited receive.

It still almost acts like you don't have voltage to certain areas of the radio. Like the MB3756 voltage regulator has a bad pin or something on a particular rail coming off one of the pins has a short. Let me sleep on it, and I'll help some more tomorrow evening, ok? :)


~Cheers~
 
Ok sounds good, I ordered new TA7222P for it and should be here tomorrow, I also Oreded the MB3756 but it wont be here till later this week. The original order they have not sent it or can't find it, I ordered another one tonight for it.
 
Also the VR5 has not bearing what so ever on adjustment it is a definate voltage problem, just not sure of where. covers a lot of ground for sure.
 
I pulled some voltages off of the IC4 MB3756 and the TA7222P IC6.

IC4 MB3756 as follows front of radio to back.

1. .12v 2. 0v 3. 3.07v 4. 5.00v 5. .37v 6. 2.90v 7. 5.17v 8. 2.75v

TA7222P as follows front of radio to back.

1. 5.20v 2. 2.04v 3. .38v 4. .32v 5. .24v 6. .25v 7. .26v 8. .21 9. 2.56 10. 4.76

I would really say there might be a problem in the MB3756 with voltages. This was a used part I had laying here, I have a new one on the way should be here later this week, I'm going to drop the TA 7222P in it this morning. What I can't find is a 13.08v supply to the MB3756, I think the could be a problem in the power supply as well this could explain many things.
 
Will do. Dropped the TA7222P in this morning, didn't even plug it in, will wait for the MB3756 to show and get it in and see what she got. I had my hints that that chip was bad when I pulled it out of the other old parts radio I have here. I got to much of this junk laying around and got to get rid of it. I got a bad habit of setting used parts on my bench and they get thrown into the bin with good stuff, this is a result of it. LOL. That one went to the can in a hurry.
 
Ok I put the TA7222p in and the MB3756 into the board with new IC chips..

Voltages are as follows for MB3756, front of radio to back on pins

1. .47v

2. 0v

3. 8.38v

4. 13.53v

5. 0v

6. 8.37v

7. 13.53v

8. 8.34v

TA7222P

1. 13.53v

2. 6.52v

3. 0v

4. 0v

5. 1.18v

6. 1.18v

7. 0v

8. 0v

9. 6.80v

10. 12.80v

Still no transmit and does not try to switch to transmit.
 
I plug into the PA and it is giving me static now, but when I key mic nothing period but static. keying up has no effect on it what so ever. Tried another microphone same issue. Also regular speaker is putting out static. Put a tone thru generator and it will receive down to -50 dbm at 400HZ, 30% modulation with a 50 micro volt output, this is a lot better than it was, so it corrected 1 problem some what. I have a receive kit to drop into it once it is up and transmitting.
 
I have no voltages to IC5 of 8.0 volts, no voltages to vr5 and to the clarifier, There are voltages missing all over the board, I can't understand what is going on here, makes no sence.

pin 8 on the IC1 has 6.08 volt it is a bit low but the PLL must be good.
 
It sounds like you either have a shorted capacitor on one of the +8V rails of the regulator, or you have a bad diode somewhere. Also, wouldn't hurt to recheck all the wiring to make sure everything's in place, since you did a full board transplant! lol

BTW, the voltages on the regulator look MUCH better.



~Cheers~
 
Pin 1 of the voltage regulator should be 8V or so. Check D60 and see if that diode has gone short. If it has, replace it. If not, check C66 and make sure that has not gone short. Wouldn't hurt to replace it with a 16-25V cap instead, as C66 is a 10V cap.


~Cheers~
 
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Reactions: dutchman100 and Robb
I also did notice on pins 7&8 are reading 0v on the ta7222p, on my other Cobra they are reading about 14v. I'm thinking the problem might be right in that general spot off the regulator...I hope.

On the mb8735 ic 1 the 8v feed on the schematic coming out of the PLL comes from pin 6 at 8.09vdown to a junction of 248 and goes straight to the mic plug pin, but no power at pin. 3. TP13 does have 10.240mhz. and the voltages seem to be alright on that end.
 
Is the PLL in lock? Does TR37 operate correctly? Does TR35 switch properly? Pin 5 on the Voltage Regulator is the control pin - when pin 5 is high pin 6 supplies rx only voltage and when low pin 8 supplies the TX only voltage.

#248 - If this goes low there will be no TX source voltage. Also TR47 will conduct which supplies base bias to TR53. TR53 is tied directly across the audio input to IC4 (TA7222) and control of AMC/ALC amps - TR34, TR25, TR24.
A bad channel selector can cause #248 to go low as well.

It almost seems as if you have no 8v TX only source. which may explain the lack of voltages around the rig.


Hope you get it fixed!!
 
One thing I did find on the TA7222p on the voltage from the purple wire and the white wire to the chip is there is no voltage to it there at all. The purple leads out to the mic plug.
 

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