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2970N4 Mosfet adjustment question - one more time please

R

Rogerbird1

Well-Known Member
Apr 24, 2002
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rogerbird1.20megsfree.com
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.
 

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.
 
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
 
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nomadradio said:
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
Click to expand...
so Zero when keyed over idle current, then add each trimmer to 50mA on the power supply ammeter.
 
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Rogerbird1 said:
so Zero when keyed over idle current, then add each trimmer to 50mA on the power supply ammeter.
Click to expand...
I must be doing something wrong, Not seeing any rise in the current draw, its Idle at 322mA and when keyed goes to 460mA.
USB, Power knob down no audio, Shorting board pulled. the Driver control Vr13 makes no difference. (amp off)
 
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Rogerbird1 said:
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.
Click to expand...
BTW: I was not the one that replaced all the parts. Im now 3rd one looking at this.
 
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Rogerbird,

Yes, shorting board stays connected.

You need to put an ammeter in series with the positive wire going into the power jack on the back of the radio.

if you can't break the connection at the power supply then you'll have to de-solder the positive wire on the inside of the power jack of the radio.

remember, current is measured in series, so disconnect positive wire, put one lead of ammeter on the wire itself, and the other lead on the power source.

set the meter to whatever range will allow you to read more than say 5 amps.

set the radio to SSB mode. RF power control all the way up, mic gain all the way down.

Key the radio and make a note of the ammeter reading. this reading is the amount of current the radio uses to "idle" meaning there is no power output because there's no audio and you're on SSB but the driver and finals have been forward biased.

let's say this number is 575mA.
now, with the mic keyed, turn the driver bias pot in whatever direction causes that 575mA number to lower. turn the pot all the way in that direction.

now do the same for each of the final bias pots.

let's say that now your number when you key the mic is 350mA.

now you key the mic and turn up the driver bias pot until you raise that 350mA number by 50mA, in our example this would be 400mA.

now raise the first final bias pot by another 50mA, bringing our example to 450mA.
now raise the other final bias pot by another 50mA brining our example to 500mA.

you have now set your driver and final transistors so they will work well and not runaway.

you can check the gate voltage if you like just to see what it ends up at, but don't make any adjustments based on that voltage. it is what it is.
LC
 
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loosecannon said:
Rogerbird,

Yes, shorting board stays connected.

You need to put an ammeter in series with the positive wire going into the power jack on the back of the radio.

if you can't break the connection at the power supply then you'll have to de-solder the positive wire on the inside of the power jack of the radio.

remember, current is measured in series, so disconnect positive wire, put one lead of ammeter on the wire itself, and the other lead on the power source.

set the meter to whatever range will allow you to read more than say 5 amps.

set the radio to SSB mode. RF power control all the way up, mic gain all the way down.

Key the radio and make a note of the ammeter reading. this reading is the amount of current the radio uses to "idle" meaning there is no power output because there's no audio and you're on SSB but the driver and finals have been forward biased.

let's say this number is 575mA.
now, with the mic keyed, turn the driver bias pot in whatever direction causes that 575mA number to lower. turn the pot all the way in that direction.

now do the same for each of the final bias pots.

let's say that now your number when you key the mic is 350mA.

now you key the mic and turn up the driver bias pot until you raise that 350mA number by 50mA, in our example this would be 400mA.

now raise the first final bias pot by another 50mA, bringing our example to 450mA.
now raise the other final bias pot by another 50mA bringing our example to 500mA.

you have now set your driver and final transistors so they will work well and not runaway.

you can check the gate voltage if you like just to see what it ends up at, but don't make any adjustments based on that voltage. it is what it is.
LC
Click to expand...
Thank you!
That put all the information I had in the proper perspective.
I was able to see the correct measurements and adjustments and had proper results.
It now has good clean audio and a calm Spectrum Analyzer measurement.
Thanks for everyone's input!

Roger
 
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