Cobra 148GTL - Philippine- AUG90

[Smokinone]

Having a weird issue with this radio. The 148GTL looks new on the inside and outside, and the sonybond(sp) that was on the inside of the radio was white and pliable. I did remove it though. I also re-capped to to make sure that was not an issue, however, all caps removed were right on tolerance...which I have not seen in the past. There was no change in doing this. It does have a frequency conversion with switches and that works as it should, and a 10 turn clarifier pot as it goes up and down pretty good. I've had it several years and it's never been mobile or out of my room. I believe this radio has been indoors in a controlled environment all it's life.

What started my investigation is that the SSB audio is low, both USB and LSB. AM this thing really works well, and when keying up with a power microphone I could be a good distance from the mic and still provided good swing and audio. Now to SSB. same distance from mic, and it barely moves the meter. I hooked up another 148 radio with the same working conditions, and that radio would swing great on both SSB and AM at a similar distance.

I played with the ALC and it did bring up and lower output using a 2 tone audio signal. However, not like the 148-B refence radio I'll call it.

Another weird thing is I cannot get the 10.240 at pin 8 of the PLL. I tried 2 different counters with the same results. But, when I key up and take the frequency sample off the output, I get the correct frequency I should be on. It receives good, and AM transmits loud and clear.

Is there an area I might look at to check this. I see reference to AM audio, but not the SSB on the Jackson schematic.

Thanks.

 

[nomadradio]

The one way to be sure the problem is not just alignment, feed a tone into the mike, keyed on sideband. Set the mike gain for around 1 or 2 Watts, then peak L45, 46, 47 and 48. The low drive level makes the peak sharper, and if any of them have a peak slug position that's DEAD EVEN with the rim of the hole, that's a component failure of a different color.

73

 

[Smokinone]

Thanks Nomadradio, I'll give it a shot. I did find a mistake I made during the recap, that brought back all my readings on the PLL. So I was able to get the PLL aligned which wasn't off by much. That got me to where I started, low SSB microphone gain.
Just need to grab some coffee and wake up a bit, 4AM, awakened by the pooch.

 

[Low_Boy]

Just curious... You said the meter does not swing. But what is the output power on the radio? There is a setting for AM and Sideband meter adjustment on that radio. With no mods that radio will only do about 18W possible 20W clean.

 

[Smokinone]

Just curious... You said the meter does not swing. But what is the output power on the radio? There is a setting for AM and Sideband meter adjustment on that radio. With no mods that radio will only do about 18W possible 20W clean.

I may have not stated it correctly. Both AM and SSB do have swing, and AM power DK at about 2.5W and swings about 15W. SSB swings to about 18W, but I have to be up on the mic with a power pic to get it to do so on SSB. On AM, I can be some distance from the power mic and still get good audio and swing. At the same distance with the same mic, on SSB swings very little. On another Cobra 148GTL, under the same conditions with the same mic I can get the swing on the mic at the same distance I can on AM. So it seems to me the SSB mic gain in the first radio isn't at the level it needs to be to get the audio and swing.

Thanks

 

[Low_Boy]

That would meat turn up VR11.

 

[Smokinone]

VR11 does allow me to set the output with the injected 2 tone. What seems to be happening, is that the SSB audio is getting pinched off or shunted maybe?
I don't know enough at this point to know which paths the audio travels. I see on the Jackson redraw schematic it points out the AM audio for TX, but not the SSB TX path...if it is different. Seems like the SSB audio amp, if there is one, may be failing or weak?

Maybe TR32 or TR34? SSB audio is just weak is another way to put it.
I just wish I was a little more knowledgeable, but I am trying.

Thanks.

The one way to be sure the problem is not just alignment, feed a tone into the mike, keyed on sideband. Set the mike gain for around 1 or 2 Watts, then peak L45, 46, 47 and 48. The low drive level makes the peak sharper, and if any of them have a peak slug position that's DEAD EVEN with the rim of the hole, that's a component failure of a different color.

I did give this a try, it peaked a little more, but didn't change the issue.
None of the coils appear to be topped out.
Thanks

 

[Low_Boy]

One more thought. I have had 148 GTL's that when the mic. gain is backed off a bit I get more modulation. I can see that on the Scope and power meter. Also that Rick Jackson schematic has mistakes in it.

 

[Smokinone]

One more thought. I have had 148 GTL's that when the mic. gain is backed off a bit I get more modulation. I can see that on the Scope and power meter. Also that Rick Jackson schematic has mistakes in it.

Thanks. I did try with the mic gain and it seems to do what it is supposed to. It reduces the gain on the mic input, showed no gains in any setting.
Thanks for the info on the Jackson schematic, I wasn't aware of any issues with it. It is about the clearest schematic I have.

Anyway, I still have the low SSB audio. I went through another alignment process, and as I hadn't adjusted the bias on the finals, I did find the driver to be twice the value it should have been. I corrected that, but it didn't change anything as far as the SSB.

Thanks.

 

[Low_Boy]

I have had many 148 GTL's and some do more power than others.

 

[kc8mob]

In SSB mode the final is powered with 12vdc.. In AM it is powered and modulated by the AM regulator.

A common mod is to "Volt" the final as they say. By feeding the final 12vdc constantly.

Commonly people tag the final feed aka "bias wire" on the web to 12v. The driver and final wires are tied into the PCB via two pin connectors.

Make sure the final has not been tied into the driver supply point. Either by accidental short or tied together improperly from the top which if someone was being lazy it'd be easy to do.

If this were the case for whatever reason. The final would be seeing half of its intended design voltage which given ohms law would produce exactly the numbers your seeing but it would also fangangle the way the RF is modulated (Audio in ssb)

This also would be bad for other reasons.

So make sure the final is being fed 12vdc in SSB mode. If it's not then peekaboo

 

[kc8mob]

Also were it to be a switching issue as far as the mode switch is concerned. Try USB and LSB. If only one mode is janky then it's probably the mode switch. If both are snaggletooth then I'd really suspect what I mentioned first. That would create a conflict with the different mode/modulation schemes used and would probably cause all sorts of not right, low and thin audio being one.

Phase cancelation who knows. Just going from rough memory here.

But your stated AM output numbers tell me the final isn't hardwired to 12vdc. But it needs 12vdc in SSB.

However. I very well could be wrong and not close.

 

[Smokinone]

Also were it to be a switching issue as far as the mode switch is concerned. Try USB and LSB. If only one mode is janky then it's probably the mode switch. If both are snaggletooth then I'd really suspect what I mentioned first. That would create a conflict with the different mode/modulation schemes used and would probably cause all sorts of not right, low and thin audio being one.

Phase cancelation who knows. Just going from rough memory here.

But your stated AM output numbers tell me the final isn't hardwired to 12vdc. But it needs 12vdc in SSB.

However. I very well could be wrong and not close.

LSB and USB act the same as far as audio. The driver and final are both fed from TP 7&8, and I have 13.7V at that point. TX voltage changes on keyup depending on AM mode or SSB mode, from about 5.6V AM to the 13.8V SSB.

Thanks. You brought up a good point.

 

[Low_Boy]

I think you are expecting too much out of a CB.

 

[Smokinone]

I think you are expecting too much out of a CB.

Well, maybe.
I have hooked up 2 other 148's, a Taiwan and a Malaysian built along side of this Philip made radio. I am using the same mic(s) (stock and a Turner +3), power supply, antenna/dummy. Essentially all the same, but the Philip make is the only one of the 3 that exhibits the lack of audio pickup from the mic on SSB. Even with a stock mic, it seems I still have to swallow the mic to get the audio to drive the SSB, and I have to get close to the power mic as well to get it to modulate. When I back off a bit, it drops. Like a weak SSB audio amp or something, if there even is one. In AM, no problem.
Just some goofy quirky stuff to me. I know you you stated you have had many 148's with different output, it just seems to me that audio input shouldn't vary between AM and SSB, particularly when 2 of 3 don't.

It drives me nuttier than I am, and if I had more hair I'd be pulling it out.

I really do appreciate all the help and comments.

Thanks.