Side mic cobra 148gtl instantly blows 3amp fuse

[red97k1500]

Hey guys, I recently purchased a 1989 Philippines cobra 148gtl from shop goodwill. Com. So I don’t know the history of the radio.

The radio appears to be bone stock, no channel mods/soldering in pll section, hacked up switch wiring..... no clipped resistors/transistors r131 etc,that I could see.
I spent about 30 mins closely looking over both sides of the board, before I ever tried to power it on for the first time.nothing looks burned up on either side of the board.The radio is clean on the inside, it does not smell like burned up parts.

When I turn the radio on it instantly blows the 3 amp fuse on the negative side of my double fused power cord. D55 tests good! I even removed and replaced it just for good measure.still blows the fuse.
Obviously some part is shorted somewhere...
I guess I need to look at d44 next, and around the voltage regulator.

Any advice from the pros would be greatly appreciated.

 

[ExitThirteen]

I had the same issue with a Philly 148GTL radio a while back, turned out to be an electrolytic capacitor that had dried up and failed short. There is an outside chance that the audio IC (TA7222) and the voltage regulator IC (MB3756) have failed short, but that's a pretty rare occurance if D55 checks good. I'd pull the 1000uF @25V electrolytic capacitor near the 13.8V plug and see if it's failed short and go from there.



~Cheers~

 

[red97k1500]

I had the same issue with a Philly 148GTL radio a while back, turned out to be an electrolytic capacitor that had dried up and failed short. There is an outside chance that the audio IC (TA7222) and the voltage regulator IC (MB3756) have failed short, but that's a pretty rare occurance if D55 checks good. I'd pull the 1000uF @25V electrolytic capacitor near the 13.8V plug and see if it's failed short and go from there.



~Cheers~

I pulled c172 and it is not shorted, I tested it with my dvom, and even charged and discharged it with a 9volt battery.... anybody know of any other culprits?

 

[Robb]

Had a problem like that.
Had to go thru the schematic and find every point where a device was connected directly to the 12v rail.
Turned out to be D44.
YMMV . . .

 

[red97k1500]

Thanks Robb, and exitthirteen, yeah it looks like I’m going to be checking parts one by one... the schematic I’m looking at shows d44 to be a 1n4003, but in real life it does not look like one, it looks like a clear glass diode?

 

[nomadradio]

They changed it to a 16-Volt (or 18-Volt?) zener diode after the first year of production.

Here's the tech bulletin.

[URL]http://www.cbtricks.com/radios/cobra/140gtl/graphics/cobra_140gtl_142gtl_148gtl_tb1226.pdf[/url]

The original schemo never got updated.

73

 

[red97k1500]

Thanks nomad, mouser does not even list an 18.2v zener diode, they have 18v and 18.32v. What is the proper diode to replace d44 with? if it is indeed bad, I can rob one out of another one of my radios for now, but then I will need to replace that one.

 

[Robb]

For the purposes of testing, you can remove that diode to see if it clears the short and the radio works . . .

 

[red97k1500]

D44 was indeed bad... i removed it and the radio powered on and came to life. I swapped in the d44 from my Malaysian 148 gtl for now. The radio seems to work and has receive, the channel display is a little bit flaky, but I think it will get better with some use...worst case scenario is I will have to clean the channel selector....

Now I just need to find a replacement diode for d44?

 

[ExitThirteen]

An 18V zener diode will be just fine in the D44 slot. Just make sure it's 18V @ 1W and you should be good to go.


~Cheers~

 

[WINKDOGG]

Had a problem like that.
Had to go thru the schematic and find every point where a device was connected directly to the 12v rail.
Turned out to be D44.
YMMV . . .

Unfortunately that's usually the case in my novice tech experience ;( start at the back and work forward lol , keep us in the loop !!

73's Brian 4-2-Zero