Cobra 2000 GTL Final Output Transistor Problem

D49, it is two diodes in one, so expect a 1.2v drop, not .6v. If that became an open, the rest of it gets throttled. And lift a leg to test it as the two transistor junctions can give the appearance the diode is ok.

Probate said:

Hello,

Never had a problem with my radio in the past, SWR is flat, keeps blowing 2sc1969 within 10 seconds of transmitting, any ideas on where i should check.

Thanks in Advance!

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Post a photo of the 2SC1969's you are using.

Probate said:

Hi The Original was an OEM 2sc1969, I replaced with HG1969. Is there a better source for these transistors, also drivers?

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I have not had a good experience with HG.

What about the checking the final bias to see if its in ballpark ~50ma. Might point to an issue in the bias circuit.

Ghetto said:

Might point to an issue in the bias circuit.

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Nine times out of ten, a brand-new final that pukes immediately is the victim of other faults in the final transistor's base circuit. Turning the bias trimpot to the zero-voltage end and placing that all-important current meter in line with the test jumper serves to provide a safe starting point when you power up the radio with a new final for the first time. We leave the base lead of the new final unsoldered and check the DC voltage at the base lead's solder pad. If the trimpot can't reduce it to zero, that can indicate a fault. And if it reads more than 6/10 of a Volt you definitely have a failure in the base circuit that needs attention. When a new final dies immediately like this you know it tried to pull too much DC current. The AM modulator transistor probably suffered the same surge current as the final transistor. It's worth checking TR41 to see that it's not damaged. Any time we find TR41 damaged we also replace TR42, the 2SC945 transistor that drives it. Cheap insurance.

73

nomadradio said:

leave the base lead of the new final unsoldered and check the DC voltage at the base lead's solder pad. If the trimpot can't reduce it to zero, that can indicate a fault. And if it reads more than 6/10 of a Volt you definitely have a failure in the base circuit that needs attention.

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drinking my morning coffee here,

do as mentioned above.

I was given a 2000 about two weeks ago with this same
problem, TR36 blowing out.

there was about 10 volts at TR36-base. but I could adjust the voltage at
TR37-base to proper levels.

so cut out capacitor C152. it was leaking DC. after this I was able to
adjust the bias voltage at TR36-base. put in another C152, checked
bias V again. Then soldered in TR36. All OK now.

with the modification of the Tone control to be in place of VR10,
was able to get 10 watts carrier. of course with downward
modulation. set to 4 W carrier. all OK.

I have only recently worked on a few of these types of radios,
and they all had this variable power modification.

Good tip Nomad - I will do that in the future.