Problem with modification CB radio build PCB EPT360012B installation MOSFET transistor IRF520.

[amon86]

Hello everyone.

I have problem with modification my old CB radio. Superchetach was build on PCB EPT360012B. I found it mod on cbtricks:

https://cbtricks.org/miscellaneous/fet_papers/mosfet_conv/graphics/3600xxx_ssb_mosfet.pdf

But when I installed all the components as described, I encountered a problem.
I've analyzed the description/modifications. Currently, I'm not quite sure where I went wrong. I'm powering it with a GSV3000 power supply. I set it up as described, i.e., without the TX jumper, I set 6V on TP9. Then, with the jumper in place, I set the voltage on TR44 to 3.8V. Up to that point, everything is OK. When I adjust the voltage on TR43 to 3.6V, the current draw starts to increase to around 10A. And immediately, the TR51 and TR43 modulators heat up.

 

[brandon7861]

Never set mosfet bias by measuring gate voltage. You need to set each ones gate voltage to whatever gets you a drain current of 50mA each. Not all IRF520s are made the same. Some might reach 50mA at 3.2v gate, some might take closer to 3.8v, but one thing is for sure, if you take the first example that does 50mA at 3.2v and give it 3.8v, its going to get hot really fast. Point is, any service manual or write-up that tells you to adjust for a specific gate voltage, just ignore it. Always set voltage to whatever is required for the drain current you want. I've had good luck with 50mA (each).

 

[loosecannon]

an easy way to do what Brandon is telling you, and it is the correct way, is to put a current meter on the power wire going into the radio.

put the radio in SSB mode with mic gain all the way down.
install the "mirror board" that goes in front of the final and driver.

now key the mic while watching the current meter.
turn all the bias pots (driver and finals) to their minimums, noted by the lowering of the idle current the radio is pulling.

for example, your radio is pulling 528mA with the mic keyed.
after you turned all the bias pots down its reading 302mA.

you would then raise the driver bias pot by 50mA.
then the first final by another 50mA.
then the other final by another 50mA.

in my example you would now be pulling 452mA of idle current.
LC