Joe,
Did you also replace Q11 and Q13?
The demise of the finals can be caused by excess bias voltage, so when
new finals are put in with no other checks or actions, the new finals
can be damaged quickly.
Make a quick check - measure the base voltage on Q7 or Q8 while doing a
quick TUNE - be certain you have connected a good dummy load with a
known good short piece of coax. If the base voltage is more than 0.62
volts, that is the likely cause of the PA failure, and either Q11 or Q13
are at fault.
You may have damaged the new PA transistors, but make the check anyway
and see what the results are. Do not stay in transmit for any more time
than necessary to check the base bias voltage.
The real check on the bias voltage is to remove Q7 and Q8 then do a TUNE
and check the voltage presented to the base at the Q7 and Q8 locations.
(the transistor itself can limit the voltage applied to the base at 0.63
volts or so - the base-emitter junction is just like a forward
conducting diode).
There are other possible causes for your symptoms, but start with the
checks I mentioned and we can go from there.
73,
Don W3FPR
Dickinson, Joe wrote:
> I burnt the finals, and just replaced them last night, put the heat sink on and fired it up at 2 watts.
>
> I immediately get over current.
>
> I turned the power all the way down, same symptom when I hit the paddle.
>
> Any ideas on how to troubleshoot this?
>
> I did resistance checks on Q6, Q7 and Q8 are all fine.
>
>
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