Cobra 148GTL, only 2 channels functioning

[frogman296]

Radio Make and Model: COBRA 148GTL
PCB Number: PC416AA
PLL Number: MB8719


Test gear available: bird meter, BK2040 sig gen, multimeter, freq counter

Symptoms: radio has rx and tx on channel 1 and 20

What is working: only channel 1 and 20 work on the receive and transmit. radio is showing the watts it should.


What has been tried: the radio on the external freq counter is showing that it is receiving on frequency. went through 1-40am and found that only 2 channels work

Did it suddenly happen ?: yes it happened all of a sudden

Anything else that may be of help?: this radio was just professionally aligned, got a clean bill of health.

any help is much appreciated, i am sure somebody here must have had this problem before

[The Defpom]

You need to check the binary code going into the PLL, pins 16 through to pin 10, it is likely that a solder blob is shorting a couple of pins out, causing the PLL to jump to the wrong frequency, or there is some other fault causing an incorrect binary code.

So what you need to do is tell me what the binary code is for every channel, measure the voltage, anything below 1.5V is "0", above that is "1", you should see a distinct voltage difference between a "low" and a "high".

Example:
Ch 1: 1 1 1 1 0 0 1
Ch 2: 0 0 0 0 1 0 1

[frogman296]

went through the radio.... reversed the channel mod, and clarifier is now locked.

replaced the crystal with the stock crystal....changes mike plug to 4 pin

powered the radio up... worked perfect... 5 minutes later..... no power output in any mode... no mike audio through talkback, and no "tx light"

weird?

[frogman296]

You need to check the binary code going into the PLL, pins 16 through to pin 10, it is likely that a solder blob is shorting a couple of pins out, causing the PLL to jump to the wrong frequency, or there is some other fault causing an incorrect binary code.

So what you need to do is tell me what the binary code is for every channel, measure the voltage, anything below 1.5V is "0", above that is "1", you should see a distinct voltage difference between a "low" and a "high".

Example:
Ch 1: 1 1 1 1 0 0 1
Ch 2: 0 0 0 0 1 0 1

okay... i got a question... when i touch the lead to each pll pin... should the voltage be floating??? if so do i hold it until it settles??

[The Defpom]

You may want to re-check the clarifier mod, in case it is shorting the TX and RX lines together, it sounds like the MB3756 may have failed.

Make sure you connect your negative test lead to one of the metal cans on the PCB, and not the chassis, as the chassis is not a true 0V.

[frogman296]

You may want to re-check the clarifier mod, in case it is shorting the TX and RX lines together, it sounds like the MB3756 may have failed.

Make sure you connect your negative test lead to one of the metal cans on the PCB, and not the chassis, as the chassis is not a true 0V.

i will recheck the clarifier mod to be sure.... and will get the voltages...

thanks in advance scott

[The Defpom]

Also make sure that any diodes or resistors on the clarifier circuit have been restored and are not bridged or anything, often they are, which could put too much strain the the MB3756 if the clarifier is turned fully in one direction.

[frogman296]

1. 1 1 0 1 1 1 1
2. 1 1 1 0 0 0 0
3. 1 1 1 0 0 0 1
4. 1 1 1 0 0 1 1
5. 1 1 1 0 1 0 0
6. 1 1 1 0 1 0 1
7. 1 1 1 0 1 1 0
8. 1 1 1 1 0 0 0
9. 1 1 1 1 0 0 1
10. 1 1 1 1 0 1 0
11. 1 1 1 1 0 1 1
12. 1 1 1 1 1 0 1
13. 1 1 1 1 1 1 0
14. 1 1 1 1 1 1 1
15. 1 1 0 0 0 0 0
16. 1 1 0 0 0 1 0
17. 1 1 0 0 0 1 1
18. 1 1 0 0 1 0 0
19. 1 1 0 0 1 0 1
20. 1 1 0 0 1 1 1
21. 1 1 0 1 0 0 0
22. 1 1 0 1 0 0 1
23. 1 1 0 1 1 0 0
24. 1 1 0 1 0 1 0
25. 1 1 0 1 0 1 1
26. 1 1 0 1 1 0 1
27. 1 1 0 1 1 1 0
28. 1 1 0 1 1 1 1
29. 1 1 1 0 0 0 0
30. 1 1 1 0 0 0 1
31. 1 1 1 0 0 1 0
32. 1 1 1 0 0 1 1
33. 1 1 1 0 1 0 0
34. 1 1 1 0 1 0 1
35. 1 1 1 0 1 1 0
36. 1 1 1 0 1 1 1
37. 1 1 1 1 0 0 0
38. 1 1 1 1 0 0 1
39. 1 1 1 1 0 1 0
40. 1 1 1 1 0 1 1

[The Defpom]

There is an error with the N codes, the ch1 code should be 1001111, not 1101111, check for problems with the connections for pin 11, try checking the diode matrix that is used for the CH9 funtion, or lifting the wire from the ch selector for that pin.

[frogman296]

the radio does not have the instant channel 9 function.

so i should check pin 11 of the pll and its path through the radio??

[The Defpom]

Of course, no CH9.. but yes, check the path and source of the voltage feeding pin 11, if you disconnect it from the ch selector and test the ch selector side for voltage it should say whether the selector is at fault, or if there is a short circuit somewhere (it should be feeding 0V), the PLL pin will measure high when disconnected, as the PLL uses internal pull up resistors on its pins, if you connect pin 11 through a 2K resistor to 0V if should drag it low, if not, the PLL could have an internal fault.

[frogman296]

Of course, no CH9.. but yes, check the path and source of the voltage feeding pin 11, if you disconnect it from the ch selector and test the ch selector side for voltage it should say whether the selector is at fault, or if there is a short circuit somewhere (it should be feeding 0V), the PLL pin will measure high when disconnected, as the PLL uses internal pull up resistors on its pins, if you connect pin 11 through a 2K resistor to 0V if should drag it low, if not, the PLL could have an internal fault.

selector checks out

adding the resistor from pin 11 to 0v doesn't drag it low...

should i replace the pll??

[frogman296]

replaced pll

now i can align the RX on freq... but i transmit off freq.... it seems as though there is not enough adjustment to the tx freq VR 5

on a side note... when i unlocked the clarifier.... the radio will align...

looks like the pll crapped out... thanks a million Scott

[The Defpom]

The TX alignment issue is probably due to the original clarifier mod, check for correct component values for the resistors that are connected by VR5.

On the TX side R187 should be 2.7K, R186 should be 100 Ohm, there should also be diodes in D75 and D52.

On the RX side R175 should be 3.3K, R174 should be 12K, R44 should be 8.2K.

[frogman296]

The TX alignment issue is probably due to the original clarifier mod, check for correct component values for the resistors that are connected by VR5.

On the TX side R187 should be 2.7K, R186 should be 100 Ohm, there should also be diodes in D75 and D52.

On the RX side R175 should be 3.3K, R174 should be 12K, R44 should be 8.2K.

i don't even see a r175 in this radio... i know where the location is... but i haven't been able to actually find the darn thing

will check the others

http://www.radiomods.co.nz/cobra148gtlmods.html

the above link is the clarifier unlock that i used... even though i have no r175 to change out... the mod works perfect

[frogman296]

checked the others... added correct values.... still no luck... i went back to the unlocked clarifier...

radio works A+