Cobra 29 ltd has gone crazy

[kaos513]

I have a Cobra 29ltd classic radio keeps blinking power on and off, here's is where it gets interesting when I remove center lead of the final radio power is restored to normal any help appreciate

 

[secret squirrel]

I am by no means a radio tech. In experience anytime I had an intermittent problem like that of something losing power its been a cold solder joint. Over time a poor factory made connection just breaks.

 

[Handy Andy]

It could be several reasons, including auto power-protection from the power supply. The power supply senses a short and trips, then attempts to restore power - only to trip again due to current draw is enough to make it think it's powering into a shorted system - so it will trip until the condition is cleared.

I'd check the Final - make sure it's not shorting it's center lead to chassis or case ground. Heat sink mounting hardware, etc - all of it - make sure it not shorting to ground.

 

[kaos513]

I hear the t2 transformer making a tick tock noise

 

[kaos513]

I tried another final does the same thing

 

[Handy Andy]

So they dropped a Ford Blinker Relay in where the Power Choke should be?

Better get more Blinker Fluid, this could be a long day...

 

[kaos513]

Found the issue r51 was shorted

 

[Handy Andy]

Speaker Wires not shorting to "spyder cage"?
Best to check MIC Pin 4 too...​

 

[kaos513]

Same again started same sputtering noise and d8 keeps exploding

 

[nomadradio]

Blown D8 tells you the fault is downstream from it. Pretty well narrows down to all the stuff connected to the driver and final transistors' output circuits. Don't see a lot of shorted capacitors in those parts of that radio. Not much else beyond the final and driver transistors left to blame.

Blown D8 also suggests the fuse on the radio was bigger than 2 Amps.

73

 

[Handy Andy]

Again, check Mounting hardware, - make sure there are no shorts - lift center leg of both Driver and Final - see if the trace that D8 works, is no longer shorted.

This may also indicate the Modulation Transformer may be going bad - a load on the coil, heats up the core of it, to the point where it can fail. May short a wind or two - but this wouldn't be the reason for a Blown D8 - more than likely there is something loading down that foil trace to the Final/Driver

 

[nomadradio]

A check with the multimeter set for 'continuity' might be less risky. Remove the shorted D8. Ground the black probe to the negative pin of the power socket, and probe the empty hole where the banded end of D8 will go when it gets replaced. Unsoldering the lead of each component on that foil trace one by one will eventually identify which part has the short to ground in it.

73

 

[guitar_199]

Just tagging in to say, that is a BRILLIANT strategy Nomad!

Certainly better than touching a car battery directly to ground AND that pad and let the shorted part "identify" itself!!!!!

Ground the black probe to the negative pin of the power socket, and probe the empty hole where the banded end of D8 will go when it gets replaced. Unsoldering the lead of each component on that foil trace one by one will eventually identify which part has the short to ground in it.

73

 

[nomadradio]

BRILLIANT strategy

I would say the guy who came up with that trick was brilliant. Just wasn't me. Been long enough I can't remember who I learned it from.

73