Cobra 148gtl shorted final on RX

[357]

Its not the final though.
Pulled every cap from the am regulator to the driver, still shorted.
final gets hot, am reg removed, driver removed, still gets hot, l39 wants to melt.

Anyone have any clues?
Was working fine then heard static crashes and smelled melting l39.
tried everything, only does it if both bias wires are in place.
removed bias diodes, bias regulator, tried everything.
stumped

 

[kaos513]

Which model is it? has the caps been replaced?

 

[ExitThirteen]

Unsolder and lift the base leg of the final off the PCB and power up the radio. If the final does not get hot, replace C151 and C152. Both are disc caps that are located just left and beside the final on the component side of the board. C151 is a .0047uf value (4700pf) and C152 is a 470pf value. Both of these capacitors can go short and that puts +12V directly on the base of the transistor and will cause the transistor to get extremely hot (and smoke L39). Let us know how you get on.


~Cheers~

 

[357]

Unsolder and lift the base leg of the final off the PCB and power up the radio. If the final does not get hot, replace C151 and C152. Both are disc caps that are located just left and beside the final on the component side of the board. C151 is a .0047uf value (4700pf) and C152 is a 470pf value. Both of these capacitors can go short and that puts +12V directly on the base of the transistor and will cause the transistor to get extremely hot (and smoke L39). Let us know how you get on.


~Cheers~

Exactly what it was...C152 4K ohms, probably less once it was running power through it.
I'll replace 151 and 152.....and the rest I started yanking out with pliers in frustration.

thanks a bunch

 

[loosecannon]

DAMN!
Exit13 be like:

LC

 

[ExitThirteen]

Anytime, man! Glad I was able to help.


~Cheers~

 

[357]

Anytime, man! Glad I was able to help.


~Cheers~

I got it going, I'm having trouble locating (in my junk) c72 c74 33 and 47uf tantalum's I pulverized instead of unsoldering so the RX is not complete.
I've been stressing out lately.



Anyone have a schematic that shows VR7 and C90?
This board is a PC412AD
Seems C90 and VR7 are not shown.

thanks

 

[ExitThirteen]

C90 is a 1uf, 50V electrolytic capacitor, and VR7 is a 500 ohm trimmer potentiometer (labeled 501 on the resistor itself). Both are located right next to C172, which is the big 1000uf capacitor. C90 and VR7 are used as part of the AMC circuit, to adjust AM modulation.

Replace C72 and C74 with the same value electrolytic capacitors and it will still work fine.


~Cheers~

 

[357]

OK I got the radio working again.
Only problem now is the AM receive is non existent.
SSB is great, TX is fine on all modes but there is some scratchy distortion when using the letter "s".
Like scratchy, speakers.

** pretty sure its the CFW455 filter...now I gotta find one this one is in about 10 pieces.
***yep it was and amazingly had 1 in my junk ashtray

still scratchy but its working.

 

[ExitThirteen]

TX is still fuzzy/scratchy sounding? Let me know, I may have a solution.

~Cheers~

 

[357]

TX is still fuzzy/scratchy sounding? Let me know, I may have a solution.

~Cheers~

ya on ssb, s's sound like crap.

 

[ExitThirteen]

Ok, if that's the case, the problem is the lack of bias current on the final.

Remove the green wire from TP7, which is located by C172, and connect a multimeter that shows mA current inline. Observe polarity. Chances are, you're not getting any current to the base of the final. You should be able to adjust VR6 to about 50mA of bias current. If adjusting VR6 does not work, either VR6 needs cleaned or replaced, or TR37 has failed (labeled C496) and will need replacement. Also would not hurt to replace L39 too, if you have not done so. Do this check for me and let us know how you get on.

~Cheers~

 

[357]

thanks for your help.
I did do the bias twice now, 30ma and 50ma.
Maybe I should do a few more caps in the audio chain.
Its a pretty old radio, ser # 7300XXX
If I can find a extra L39, I assume its the one that got hot.

 

[ExitThirteen]

Check the driver bias and make sure you have about 20mA on the driver. If you did check for this and I missed it I apologize, but make sure you do have bias current to both the driver and final. Also make sure TR24 is in place as well, that’s the modulation limiter. Let us know how you get on.

~Cheers~