Galaxy DX 939 - RF output "520's" bias set up.

[BayouRadioAmplifier]

got a 939 for repair. it is modified for "TX audio direct injection"
on the back of the radio, and had a "golden screwdriver" turning
this and that.
It worked but ran so hot I was told they had to
put a fan in place of the speaker to keep it cool.



so yes, the "520's" bias was very high. (as usually is the case!)
I started over with my screwdrivers.

first, set it up, AM mode. turn mic gain all the way off (ccw).
use a screwdriver to short out TP3 to the nearby ground.
.


.
this kills the RF going to the mixer.

key up with a current meter inline and turn the bias pots (VR10/12) for
minimum current. measure the voltage at TP7/8/9 and turn the RF POWER
control all the way high cw and adjust VR14 for 6.2 V. then turn all the way low
ccw and adjust VR18 for 3 V. (this worked for me)

then put back to high power full cw 6.2 V at TP7/8/9.
and while keyed up watch the input current, and adjust
VR10 and VR12 for an increase of 50 mA for each "520"
I had about .75 A total into radio.

then take screwdriver short off of TP3, key up, and adjust cans
L44 L43 L42 L40 for maximum RF output.

I got about 6.5 watts output and "520" gate voltages of 3.1 and 3.6

turned RF POWER control for about 5 watts, turned mic gain up,
and radio works great.
no need for that ass-hat fan on a 5 watt radio!

 

[Cable Guy]

That tp3 trick is neat, faster than soldering a disc cap across the pre-driver.

 

[nomadradio]

Rock on! Gotta keep the gate bias in the Goldilocks zone. Not too hot, not too cold.

73