2970N4 Mosfet adjustment question - one more time please

[Rogerbird1]

Ok, sorry to beat this one once again, I have read many comments all over with the proper way to do this.
But since each thread I have read does it differently...

When replacing a IRF520N, what is the proper measurement to keep these from either exploding or dying again?

I have one, that just had all 3 IRF520N's replaced and the regulator.
The owner said it never worked quite correctly. Now it sounds mushy on SSB and the power output is no where near where it was originally. (amp or or amp on)
My first find was the Driver current was way too high and would not adjust below 800mA. at minimum power. (full CCW). The 2 final Mosfets seemed to adjust properly. First would adjust to 50mA second to 100mA. at minimum power (full CCW) .

Several adjustment documents say to adjust the gate voltage to 3.6v and 3.4v respectively.. OK.. with the replaced driver it adjusts to 3.6v with no issue, the Finals both adj to 3.4v with no issue.

Her is where I am confused: One document says just to set these voltages as stated.
another says to ground the meter to Chassis Ground, which, increases the adjustment positions dramatically..
Any help here? I am nervous to do this improperly and send it back.
Lack of any factory adjustment document that specifically is for the MOSFET version of the N4..I am not sure I am getting the correct procedure.

Main board is Ranger EPT69015BR.

 

[Lost Ram]

I wonder if you need a RF ground and not chassis ground or maybe the mosfet source. Also be leery of the mA draw on 520 clones. Not sure if I would rely on chassis ground for this but I am no expert on this deal, I just fallow the flow chart as you do.

 

[AudioShockwav]

Hello Roger
@nomadradio has been over this one, you should set the bias by current draw, not voltage.
They don't all have the same turn on threshold.
I will look for the thread in a little while here when I get home or he should see this.

73
Jeff

 

[nomadradio]

Lacking a way to isolate the final stage's drain supply, we put a meter on the 20 Amp DC scale in line with the radio's positive power lead. Mike gain to zero, set to one or the other sideband. Key the mike and set the bias trimmers for minimum current. The idea is to find a "zero current" reference point for each trimpot. The reading you see now represents what the rest of the radio draws from the power supply, and becomes your zero-reference figure. Advance each bias trimmer to add 50 mA apiece to the reference reading.

Should do the trick. Got some advice from RCI decades ago about (then) new 2995 radios popping the radio finals. Said to set bias current to 10 mA per final. We settled on 25 mA per transistor for the 4- and 8-transistor MOSFET amplifiers.

Should be enough idle current to clean up sideband audio and prevent thermal runaway issues.

73

 

[Rogerbird1]

Lacking a way to isolate the final stage's drain supply, we put a meter on the 20 Amp DC scale in line with the radio's positive power lead. Mike gain to zero, set to one or the other sideband. Key the mike and set the bias trimmers for minimum current. The idea is to find a "zero current" reference point for each trimpot. The reading you see now represents what the rest of the radio draws from the power supply, and becomes your zero-reference figure. Advance each bias trimmer to add 50 mA apiece to the reference reading.

Should do the trick. Got some advice from RCI decades ago about (then) new 2995 radios popping the radio finals. Said to set bias current to 10 mA per final. We settled on 25 mA per transistor for the 4- and 8-transistor MOSFET amplifiers.

Should be enough idle current to clean up sideband audio and prevent thermal runaway issues.

73

so Zero when keyed over idle current, then add each trimmer to 50mA on the power supply ammeter.