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Cobra 148 GTL driver bias help

L

LongRod

Member
Mar 29, 2013
2
0
11
Hello all,
I have a Cobra 148 Gtl made in the phillipines in 1992. I noticed that the driver is getting very hot during ssb tx. When I checked the bias current to the final it was around 50ma and about 650ma to the driver. Tests were done in USB channel 19 dynamike minimum. Even when I adjust vr9 to minimum, bias current is still 310ma and the transistor gets warm. With vr9 set to minimum I am getting reports back that I'm not swinging as hard as before. This radio is stock and unmolested and I would like to get it running right again. If more info is needed let me know and I will get it.
Thanks, 73's
 

Holy shit 650 ma.. did I read that right?? Driver 50ma final 100ma. Give me more details I'll try to help
 
That transistor may have gone bad. Do you have a replacement? If not check out weazle66 on eBay. Get 1 2sc2166 and one 2sc1969 might as well put a new final in it since you gotta order from England
 
Check R182. This resistor could be a 220 ohm, a 270 ohm, or a 330 ohm, depending on the year the radio was made. It should be a 1/2W resistor. Make sure this resistor hasn't gone short. If this resistor checks real low on ohms, or shorted, or if it's less than 1/2W, replace it with a 1/2W 220 ohm resistor.

Also, check what the number of the transistor is in the driver spot, it should be labelled C2166. If it was replaced with the wrong transistor, it can pull excessive current, and produce a lot of heat. Check out these couple things for me and let us know how you get on.




~Cheers~
 
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Hey 13, do you bias the final at 50/50 ma or 50/100? I can't get proper output at 50/50
 
this will happen when the bias diode goes bad.

the bias diode is the little square thingie that is either bolted to the heatsink near the driver or bolted directly to the front of the driver itself.

there is one for the driver and one for the final, so check the one that is closest to the driver.

it has two wires coming out of it that go to the PC board.
unsolder those two wires, pull them out, and check them with the diode check function of your multimeter.

it will probably read open.

i think its either an MV1Y or MV13Y. go to cbtricks and look at the parts list for the 148 to be sure.

i think you can get them from wearecb.com, at least you used to be able to.

good luck.
LC
 
LC!!!

Good to see you sir, been a bit since I've seen you, always enjoyed your posts. Hope to see you around here more! (y)




For the record, the diode is MV1Y on the driver. I had completely forgotten about the bias diodes, and while it's not too common for those to go fault, it's definitely possible.


~Cheers~
 
My vote is bias diode... that poor poor poor junction is having to handle all of that current...not good.
 
thanks xit13, ive been moving around quite a bit over the last year or so, and really havent had time to play radio in a long while.

i am somewhat getting settled, and do plan to be here more often.
LC
 
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