General Lee dual 520 bias question...

[Cable Guy]

VR23 and VR22 are both shorted together on the left side when in high position. This schematic is in error, so you are correct Brandon, and it should have a VR listed. I looked around here, all these rigs have the power knob. Maybe Hawkeye can grab some pics of that switch from behind for us.

 

[brandon7861]

VR23 and VR22 are both shorted together on the left side when in high position. This schematic is in error, so you are correct Brandon, and it should have a VR listed. I looked around here, all these rigs have the power knob. Maybe Hawkeye can grab some pics of that switch from behind for us.

I didn't even notice that! wow

 

[brandon7861]

I was figuring that VR12 is shared between medium and low and that vr14 went to the high side, but that vr22/23 thing makes me wonder if one was for medium instead. A picture would be cool for sure

Edit: Thanks Cable Guy! I thought I was losing it. That had me questioning my understanding of transistors again lol

 

[Cable Guy]

That had me questioning my understanding of transistors again lol

Haha, I have been there and done that, still learning and forgetting at every turn. If only the details would stay in there, but I think I have a slow brain leak around the head gasket.

 

[brandon7861]

but I think I have a slow brain leak around the head gasket.

same here lol

 

[BayouRadioAmplifier]

been following this thread.
it is fun to make these transmitters work!

so Hawkeye351 did you get this radio to transmit
without the dropping power problem?
and any modulation problem?

here is what I would do.

disable that AMC circuit. keep D92 or R249 cut out.
or adjust the pot so it does not kick in. that part of the
circuit has been messed with.

also remove, as you may have done, that "NPC MOD"
diode, and or diode and resistor.

then adjust for a little bias current on the IRF520's
20 to 50 mA each. measure the input current to the
radio. short out TP3, like this:

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

Apr 12, 2025

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...

• BayouRadioAmplifier
• Replies: 2
• Forum: CB and Export Equipment and Accessories

• 

then set fo 5.5 to 6 volts during high power at the
collector of the modulation transistor, or measure at
the bare wires at where the shorting board goes
on the radios with SSB mode.

then TX and watch your RF output power, and also
monitor the voltage at the modulator output,
making sure it does not drop.

there is DC feedback loop thru R240, TR49 - 50 - 51

 

[BayouRadioAmplifier]

and also monitor the 8 V transmit line someplace.

if it drops, then modulator drops.

 

[BayouRadioAmplifier]

mounted the 8.5 volts at the collector of transistor TR41.

 

[Hawkeye351]

Update:
Replaced both modulation VR's (checked them out of circuit also, both blown out).
Resoldered R249 back together.
Modulation set now.
Shorted out TP3 and removed jumpers from TP to set bias properly.

Driver - 50ma
Finals - 50ma each (100ma total)

World of difference. No power drop whatsoever. Locals say it's loud and strong. Nothing getting hot, just cool to warm.

Thank you all...

 

[nomadradio]

Rock on! Congratulations for sticking with it. If there's a fixed resistor in parallel with each blown trimpot it deserves a close look. Coulda been sideswiped, so to speak.

73