Cobra 148 GTL

[dxing.bruce]

Hey: Guys. I picked up a newer Cobra 148GTL " made in China ".
I notice it seems to have weak receive compared to all my other radios.
I did the alignment for the receive inductors but no improvement.
I replace all th eleectrolytic capacitors also with no improvement in receive.

I also notice on USB & LSB the audio sounds like it coming through a 10'
plastic pipe, kinda tinny or hollow.

The radio transmits fine 4w on AM / 12w on SSB
the transmit audio sounds clear also.

Should I be looking anywhere in the circuit maybe for bad small signal
transistors in the receiver path or diodes ?

Any help or suggestions would eb greatly appreciated.

 

[999]

Witch variant is it? NW, or a ST?

 

[dxing.bruce]

Just a regular Cobra 148 GTL not sound tracker, or NW. On the nameplate on the radio rear it says Cobra 148 GTL D. Made in China.

 

[144inBama]

Did you buy it new or used? What the manufacture date on it? It could be as simple as a old, weak/bad capacitor in the circuit.

 

[NevadaHam]

Hey: Guys. I picked up a newer Cobra 148GTL " made in China ".
I notice it seems to have weak receive compared to all my other radios.
I did the alignment for the receive inductors but no improvement.
I replace all th eleectrolytic capacitors also with no improvement in receive.

I also notice on USB & LSB the audio sounds like it coming through a 10'
plastic pipe, kinda tinny or hollow.

The radio transmits fine 4w on AM / 12w on SSB
the transmit audio sounds clear also.

Should I be looking anywhere in the circuit maybe for bad small signal
transistors in the receiver path or diodes ?

Any help or suggestions would eb greatly appreciated.

Poor sensitivity/Low/Distorted audio is a I.F. alignment or a bad IF filter. Usually a small black block (Says MURATA on it) has three terminals.

 

[dxing.bruce]

Hi: Ya I bought it used. I replaced all the electrolytic capacitors and the receive is just low. Signals that come in aorund 9s on all other cb radios I have only come in about 2s on this China made Cobra 148 GTL. The signals sound clear but weak on AM & on SSB they are also week but unlike the usual SSB type sound the audio sounds like it coming through a 10' plastic pipe on SSB. I have never heard this from a cb before. I can replace the 455khz filter & try that. I was thinking maybe a weak transistor in the receive amplifier path but I have no idea. I also looked at the service manual online & tried to adjust all the receive variable inductors being careful to put them back to their original postions. It made no difference when adjusting any of them except to make the signal weaker so I put them back to where the signal sounded the strongest which is liek it was originally with very weak receive.

 

[Shadetree Mechanic]

Interesting problem, I probably won't be much help, but does the rf gain knob on the front of the radio do anything?

If you can post pictures of the front and back of the circuit board, someone might be able to spot something out of place.

 

[dxing.bruce]

Interesting problem, I probably won't be much help, but does the rf gain knob on the front of the radio do anything?

If you can post pictures of the front and back of the circuit board, someone might be able to spot something out of place.

Yes the rf gain functions as it should but due to the low receive issue it simply reduces the already low receive signal to zero. I did a bit more probing around the board with the radio connected to an antenna I lick my index finger which kinda makes it act like a semiconductor or capacitor & when I touch the solder side of the board by the transistors in the receiver path that are for IF amplification the receive gets louder, The transitors are all KTC3192 & are TR16, TR17, TR18, TR52. I don't have any of these to swap out new ones so I will have to search for another transitor that will be equivalant like an NTE part.
I would assume that if one of these IF amplifer transistors were bad that it would not make the signal louder when I touch the solder connections though. I will also replace the two Tantalum capacitors in this section to see if that helps I have read that they use one with barely enough voltage rating for the circuit.
BTW, the radio was bought from a cb shop so I'm gonna assume that it was not butchered even though it's a used cb. I did replace all the electrolytic caps on the board too.

 

[Shadetree Mechanic]

Yes the rf gain functions as it should but due to the low receive issue it simply reduces the already low receive signal to zero. I did a bit more probing around the board with the radio connected to an antenna I lick my index finger which kinda makes it act like a semiconductor or capacitor & when I touch the solder side of the board by the transistors in the receiver path that are for IF amplification the receive gets louder, The transitors are all KTC3192 & are TR16, TR17, TR18, TR52. I don't have any of these to swap out new ones so I will have to search for another transitor that will be equivalant like an NTE part.
I would assume that if one of these IF amplifer transistors were bad that it would not make the signal louder when I touch the solder connections though. I will also replace the two Tantalum capacitors in this section to see if that helps I have read that they use one with barely enough voltage rating for the circuit.
BTW, the radio was bought from a cb shop so I'm gonna assume that it was not butchered even though it's a used cb. I did replace all the electrolytic caps on the board too.

It might be possible that you could touch across a bad transistor and increase the signal to the next stage? Or maybe one of the adjustment cans has gone bad? I am not sure what mode they fail to, but some cans have capacitors inside them that can fail. Might just be the big cans in a tube radio, I am not sure.

 

[fm325]

Hi D. B.,
I just acquired a Cobra 148 that receives fine on AM but has the same low-level SSB receive that you described. I have yet to re-cap the unit but was not happy to hear that you did, and the issue still remains. Have you made any further progress in resolving the issue?

Regards,
Frank

 

[dxing.bruce]

Hi D. B.,
I just acquired a Cobra 148 that receives fine on AM but has the same low-level SSB receive that you described. I have yet to re-cap the unit but was not happy to hear that you did, and the issue still remains. Have you made any further progress in resolving the issue?

Regards,
Frank

Hi: Frank.

No I replaced all the electrolytic capacitors & a ton of the small signal transistors if the receive stage & it's the same. I eneded up giving the radio to another cb'er here in Toronto to let him figure it out as I was fed up with it. Lesson learned, stick with the old UNIDEN made Cobra 148 GTL'S as the new China ones are junk.

 

[fm325]

Hi: Frank.

No I replaced all the electrolytic capacitors & a ton of the small signal transistors if the receive stage & it's the same. I eneded up giving the radio to another cb'er here in Toronto to let him figure it out as I was fed up with it. Lesson learned, stick with the old UNIDEN made Cobra 148 GTL'S as the new China ones are junk.

Hi Bruce,
I wrote to Handy Andy and he said that the issue is with the 7.8MHz IF filter, the large gray one in shrink wrap. Andy sent me a nice write up that involved a bit more than I ended up doing. I'm hoping that he'll post it here.
What I did was I removed the shrink wrap and reflowed all the solder connections on the vertical PC board. The radio is now receiving SSB at the correct level and without that hollow sound! Tell your friend to look into doing the same! Good luck!

Regards,
Frank

BIG THANKS to HANDY ANDY!

 

[Holydvr]

Glad at least one is working the way it should. Way back when they were still making them a friend bought one. I was not impressed with them at all just based on the few corners they cut in the production of them. I do have a Nightwatch version that has only had the clarifier unlocked. It works well, but it does not get used much, if at all..