DX-959 Bias too high

[eyecue]

DX-959 the mosfet driver voltage wont go down with VR-12. The corresponding mA rating is therefore too high. IRF-520's are used and the driver tests fine out of the radio. The final can be adjusted with no issues. VR-12 tests 100K out of circuit.

 

[999]

DX-959 the mosfet driver voltage wont go down with VR-12. The corresponding mA rating is therefore too high. IRF-520's are used and the driver tests fine out of the radio. The final can be adjusted with no issues. VR-12 tests 100K out of circuit.

I would check the values of these components.

 

[BayouRadioAmplifier]

I check the bias adjust voltage on these IRF520 radios first.

 

[BayouRadioAmplifier]

I check the bias adjust voltage circuit on these IRF520
MOSFET RF output radios first.

take out that T.P. 7/8/9 shorting board.
key-up radio in SSB mode, mic turned all the way down,
no audio, thus no RF to the IRF520's.
then measure the voltage at the gate of the IRF520 Q49.
on this radio, according to the diagram, this gate
voltage should vary from about 3.2 to 5.4 V when adjusting VR12
back and forth.

maybe the diagram is incorrect. if you can go to 3.2 V at the gate,
and there is too much bias current, then maybe IRF520 Q49
needs to be changed. it is not bad, just kicks in earlier

this bias voltage circuit is a little different on each radio.
on the final Q47 you can turn down VR10 for zero volts
on the gate.

on the RCI 2950/2970 board there is a 100k resistor missing
so the voltage can't be adjust on the driver, so the current
does not adjust. I never worked on a 959.

- - Radio Phreak - -