cobra 148 GTL Philly version Weird happennings

[skybolt58]

Here is a good one for you cobra guru's? When I turn the mic gain on the radio, it not only changes the mic gain, it changes tone out of my speaker? what could be wrong>?

 

[Needle Bender]

My best guess would be that you're loading down the audio amplifier chip by using a 4-pin mic on a 5-pin radio and not adding the appropriate resistance between pins 2 & 4 while using them in parallel.

I usually find that by keeping the mic gain around 1:00-2:00 that audio distortion on both TX & RX disappears.

 

[skybolt58]

Well I am using a 5 pin plug, but the mic I am using it the 4 wire super side kick, and it don't do it on the 5 pin washington? with the same mic?

My best guess would be that you're loading down the audio amplifier chip by using a 4-pin mic on a 5-pin radio and not adding the appropriate resistance between pins 2 & 4 while using them in parallel.

I usually find that by keeping the mic gain around 1:00-2:00 that audio distortion on both TX & RX disappears.

 

[slprydk]

Looking at the schematic Needle Bender could still be right, Uniden and Cobra used 2 different audio grounding methods at the mic jack. Beyond that, the mic gain is fairly well isolated from the RX audio section.

Jim

 

[ExitThirteen]

If you have that Turner mic wired up 5 pin to match the Cobra, you need to solder a wire between pins 2 and 4 on the mic plug itself. The PCB and mic grounds are seperate. That's why your audio tone changes.

You can also put a wire between pins 2 and 4 on the mic jack in the radio, so if you decide to run another mic on the radio that only has 4 wires, it will work regardless if you have the ground wire on pin 2 or pin 4 on the mic plug. Hope this helps!



~Cheers~

 

[skybolt58]

Thanks, I wired up my silver eagle and it stopped doing it, so I sort of figured it had something to do with the mic since it had to be plugged in to rec, BUT, it didn't do it on my Uniden Washington 5 pin?

so you say just a jumper between those 2 pins?

Also, I had to remove a roger beep, and I did not know where all the wires went back on that little mic PC board, I think I got it with the schematic. but a picture is a thousand words if I can find one!

If you have that Turner mic wired up 5 pin to match the Cobra, you need to solder a wire between pins 2 and 4 on the mic plug itself. The PCB and mic grounds are seperate. That's why your audio tone changes.

You can also put a wire between pins 2 and 4 on the mic jack in the radio, so if you decide to run another mic on the radio that only has 4 wires, it will work regardless if you have the ground wire on pin 2 or pin 4 on the mic plug. Hope this helps!



~Cheers~

 

[ExitThirteen]

Yep, just put a jumper between pins 2 and 4 on either the mic plug itself, or on the mic jack in the radio. Doing it on the mic jack will allow you to run any 4 wire mic on the radio without having to worry about jumpering pins 2 and 4 on the mic plug itself. You can simply attach the ground wire on either pin 2 OR pin 4 on the mic plug if you mod the mic jack. I do this on all 5 pin radios that come across my bench for repair.



~Cheers~

 

[kb9ikh]

cobra 148gtl with motorboating in ssb/am audio

i have a 148 cobra that seems to have what sounds like ac ripple in the am/ssb audio.. i have swapped out power supplies with 3 different supplies and the 148 still has the rippling sound in the audio...it's more predominate in the ssb but is there on am as well...this is with a stock cobra mike as well..thought it may have been a grounding issue but not sure....and ideas??


john

 

[skybolt58]

148 GTL

OK I put the jumper on pins 2+4 and it seems ok now, BUT on a radio check my friend said it sounded fuzzy, I think I found out why,,, transistors 24 + 26 are gone, resistors 130 + 131 are gone. and a wire from r130 the long leg side to vr 10, I have no Idea what that did? does anybody know, or should I just rip out the wire and put both resistors back in and both transistors?

Thanks

 

[ExitThirteen]

Pull the wire off VR10, and replace both TR24 (C945) and TR26 (C945). Replace R131 with a 47K ohm resistor. R131 is supposed to be a 10k (brown/black/orange), but I usually put the 47K in (yellow/violet/orange is 47K color code). The wire that is located at R130 (connected to the emitter leg of TR26) is supposed to go to VR7, not VR10. It connects to the clockwise side wiper of VR7. The center wiper of VR7 connects to a 1N4148 switching diode (D57), which goes to ground. VR7 adjusts your AM modulation.

While you're at it, make sure R124 and R126 haven't been pulled out and jumpered. Also check to make sure TR34 hasn't been pulled (C1675). That's another common practice. Oh, and for SSB, run your mic gain about 1-2 o'clock. You'll sound great with that after you get all the parts re-installed. Good luck!


~Cheers~

 

[skybolt58]

148 gtl

OK great, I will be doing all that, thanks, I already replaced TR 24, I need to fins another one, I have 4.7k resistors, common one to stock, and also yes, the wire goes from r130 to vr7 am I suppose to remove this wire from R130 to VR7. because it is in R130's hole and I can't get a resistor back in there with the wire in it? OH and actually that wire in R130 Is on the Base side of TR26

Thanks again!

 

[ExitThirteen]

Keep the wire that goes from TR26 to VR7 in place. That is supposed to be there on a Cobra 148 GTL. It's different on a Cobra 2000 GTL (uses same board) because the 2000 uses VR7 to adjust the modulation meter. And you're right it being connected to the base of the transistor, I forgot it orients the other way on the board.


If you have an extra 10K resisitor for R131, you can drop it in. Changing R131 to a 47K just opens up your modulation a bit on AM. It doesn't affect SSB output, that's where TR24 and TR34 come into play.


~Cheers~

 

[skybolt58]

148 Gtl

OK thanks, BUT, what about R130, I cant put that in with the wire in one of it's holes! I got 131 in I got both transistors back in. I am trying to get this old philly back to stock. thank got the clarifier has not been touched!

Keep the wire that goes from TR26 to VR7 in place. That is supposed to be there on a Cobra 148 GTL. It's different on a Cobra 2000 GTL (uses same board) because the 2000 uses VR7 to adjust the modulation meter. And you're right it being connected to the base of the transistor, I forgot it orients the other way on the board.


If you have an extra 10K resisitor for R131, you can drop it in. Changing R131 to a 47K just opens up your modulation a bit on AM. It doesn't affect SSB output, that's where TR24 and TR34 come into play.


~Cheers~

 

[ExitThirteen]

A resistor doesn't go in there from factory. The wire that is in there now is supposed to be there. No need to install anything at R130. That part of it is ok. As long as the wire (normally white) that is coming off the base leg of TR26 goes to VR7, you're good. Sorry for not clarifying that earlier.



~Cheers~

 

[skybolt58]

148 gtl

OL I got the mic area all replaced, and working like it should stock! now I cant get more then 12 watts out of it, I tried to tune it to get max, also I tried the adjust the bias for both and whe I turn vr 8 & 9 for tp 7 & 8 I get no change, like both are stick on or about 32ma ? I also noticed it still has the 2sc1419 in it?? Anny sugustions?