Re: Need Help setting the PA Bias Current for sBITX
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Adjusting bias can in fact blow the finals, it's just part of it. THat is why equipment and procedure is important. Best is a DMM and a current limiting supply. Set the supply to give just above the current you need to run the radio plus the bias current target. BEFORE installing the new finals, make 100% sure the bias pots are set to give zero voltage on the gate pin. I also always check the voltage rise as the pot is turned to see where I think I'll be getting close.? Only then, install the finals (carefully, some are very static sensitive). Then, slowly up the bias pots while monitoring the voltage in the bias pin AND the current. The gate voltage will tell you when you are getting close to causing current?flow in the mosfet. Then slowly increase the pot until the current draw hits the target (again, the current draw with no finals installed plus the target bias current).
After blowing a couple of?+100USD mosfets, I have followed this practice and never blown another one.?
The above assumes you have checked and there are no faults in the circuit (resistors, caps, wiring in good shape). If you have another bad component, it can blow the finals quickly. A friend had a set of finals blown when the wire winding on the output transformer chaffed and shorted?to ground for example. It can be done without the current limiting supply, but the current limiting supply provides insurance against bad components or procedures.
Andre? W4SFZ
I have a v3.
Ok let me understand this, I had to replace the finals and when I had to do that I have to adjust the PA Bias Current for the finals. When I adjust the PA Bias Current I blow the finals and now I have to replace them again. Once I replace them I have to adjust the PA Bias Current for them which causes them to blow.?
Yeah this is a never ending rabbit hole deep into wacky land.??
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'72
Aaron?
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