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Help needed for Cobra 148 NW ST

C

CTStallion

Active Member
Aug 29, 2010
196
21
28
Connecticut
I've got a used, (and Factory Reconditioned, no less) Cobra 148 NW ST that is giving me fits. Of course, the NW doesn't work, but that's not the issue.

The issue is: when I first got the radio (eBay of course) it wouldn't transmit or receive on channels 30 - 33. PLL and everything else checks out Okay, so I realigned the radio according to the Factory Service Manual on CB Tricks, and that did not solve the issue. So, thinking it must be a 'worn' channel selector, I sprayed it inside with tuner cleaner and then rotated it back and forth. Finally got channels 30 and 33 working, (although I have to transmit FIRST before they will come into receive), but channels 31 and 32 just transmit NOISE (at full power) on about 27.323 MHz (or thereabouts). So, still thinking it was the channel selector, I took the whole thing down, unsoldered the Channel selector, and replaced it with a KNOWN good one. Turns out that wasn't the problem; rig still wont TX/RX on channels 31 and 32 ONLY.

Any thoughts on WHERE I should be looking next?
 

Try playing with the VCO voltage (lower) and see if that doesn't bring it back.

Keep you freq counter on the loop osc and see if it becomes unlocked when voltage changes.

WARNING: these radios are very sensitive to changes and I have burned a VCO chip because of too much voltage.
 
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Robb said:
Try playing with the VCO voltage (lower) and see if that doesn't bring it back.

Keep you freq counter on the loop osc and see if it becomes unlocked when voltage changes.

WARNING: these radios are very sensitive to changes and I have burned a VCO chip because of too much voltage.
Click to expand...



Ha.... thanks for the warning..... NOW it looks like that (burning out the VCO chip) is precisely what I have done, as I can no longer adjust the VCO voltage via L19 any longer. It is now fixed at approx. 3.11 VDC, and I no longer can get a lock, or ANY transmissions on ANY channels.

Oh well, guess it has now become a parts rig, as it's not worth fixing. Don't think the UH1C070 from a 148 GTL will work in THIS particular IC2 position as a replacement.

No wonder they scrapped production of THIS particular model. What a P.O.S.
 
If this radio has the newer RCI 8719 PLL, Look around the PLL and see if there's a resistor going to Pin 9. It should be a small 1/8 watt 10 ohm resistor. This resistor fails, and Cobra has a service bulletin on it, I believe. Replace this resistor with a 10 ohm, 1/4 or 1/2 watt resistor, and see what that does. I've had several of these radios come across my bench with PLL issues and it ends up being this resistor. The resistor drops the voltage down, as the newer RCI 8719's use a different control voltage than the older MB8719's.

(edit) Forgot to add, this resistor will be inline with a wire, it won't actually be located on the PCB.


Good luck!


~Cheers~
 
Last edited:
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ExitThirteen said:
If this radio has the newer RCI 8719 PLL, Look around the PLL and see if there's a resistor going to Pin 9. It should be a small 1/8 watt 10 ohm resistor. This resistor fails, and Cobra has a service bulletin on it, I believe. Replace this resistor with a 10 ohm, 1/4 or 1/2 watt resistor, and see what that does. I've had several of these radios come across my bench with PLL issues and it ends up being this resistor. The resistor drops the voltage down, as the newer RCI 8719's use a different control voltage than the older MB8719's.

(edit) Forgot to add, this resistor will be inline with a wire, it won't actually be located on the PCB.


Good luck!


~Cheers~
Click to expand...

Thanks, but no such animal that I could find on this particular model.
 
ExitThirteen said:
Does the radio have the RCI 8719 PLL in it?

(edit) The resistor in question is R130.



~Cheers~
Click to expand...

MMB81719

And yes, I 'think' I found the resistor; I followed the trace off of Pin 7 to an unmarked resistor against the edge of the PC board, which is unmarked and presumably R130.

Checked the radio again on ALL channels, and it is INDEED putting out fully wattage, only fixed on a tx freq of 27.347 MHz, so this may be the problem.

I still think this rig is a POS, HOWEVER, even though it has a Sound Tracker, it actually receives quite well. First radio I've had where the Sound Tracker ACTUALLY WORKS as intended.

Eyes getting a lil tired right now, so will change out the resistor tomorrow.

The one I tested IN THE CIRCUIT, is reading 100 ohms instead of 10.
 
The RCI8719 and the MMB8719 are the same breed of cat, both have a different control voltage than the MB8719. Odd that you're getting 100 ohms across the resistor.


~Cheers~
 
This is great info guys; much appreciated.

I am going to pull out that pos that I have and see if that is it, as I have one of these clunkers too that failed the same way. Thought it was the VCO chip - lol!

While we are at it, is there any other tech tips for this radio that are important for this version?
 
Last edited:
ExitThirteen said:
The RCI8719 and the MMB8719 are the same breed of cat, both have a different control voltage than the MB8719. Odd that you're getting 100 ohms across the resistor.


~Cheers~
Click to expand...

Nope, it wasn't R130 (which, from the factory is 10 ohms)..... it was R105 that I was looking at, and it's supposed to be 100 ohms.

Can't seem to find R130.

Don't know of the "FACTORY RECONDITIONING" included all the freaking HOT GLUE they put in this radio.... can't even get to the parts, and if I try to pry the hot glue off, then, off come the parts with it!

Also, can't find R130 (which I really THINK is my problem), and also can't find either a schematic for it, or a PICTURE of it on the internet either.

Unbelievable.
 
Robb said:
This is great info guys; much appreciated.

I am going to pull out that pos that I have and see if that is it, as I have one of these clunkers too that failed the same way. Thought it was the VCO chip - lol!

While we are at it, is there any other tech tips for this radio that is important?
Click to expand...

Good on ya, and GOOD LUCK!

If you find R130, PLEASE let me know the position of it. (I can't locate it on the board anywhere.)
 
Supposed to be on the left corner of the board near the PLL. Says so in that service bulletin. Hot glue is a pain to work around; but a blow torch will fix that - and the radio - lol.

ACTION: Replace R130, which is located at the left edge of the PCB, near the PLL chip
(IC1, MB8719). Use a 10 ohm 1/2W resistor.
Click to expand...
 

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