RCI-2970N2 / 2950 IRF520 bias problem repair.

[BayouRadioAmplifier]

While working on my first 2970N2, from another techs hands,
found the main transceiver unit was getting hot. low output. the
IRF520 bias settings were way to high, and one was acting squirrelly,
eventually damaging IRF520 Q62. With the shorting board and Q62 removed
from the board, I tested the bias voltage to the gate, and it does not adjust
correctly and stays up a few volts, too high.
So looking at the schematic at Q62 there is no resistor going to ground after
bias adj pot VR13 to make a correct adjustable voltage divider.
So I added a 100k ohm resistor to ground at this spot, to make it just like
the bias circuit on the at Q62 on the RCI-2995.
bias voltage for Q60/61 do adjust.

I searched and see a few posts about this bias problem on the 2970 2950 radios.
So what's up with this? I just starting working on these radios a few weeks ago.
is this all documented somewhere. hopefully this will help others.

now I need to get some more IRF520's and see if I can get this thing working.
and the biasing for the external high power amp sounds crazy. just set
the gate voltages to 4.

 

[BayouRadioAmplifier]

P.S. - now the bias voltage at Q62 adjusts from 2.5 to 4.4 volts,
and is stable.

 

[nomadradio]

Good place to start. I still insist that setting gate-bias voltage for desired drain current is the only viable method.

That's the whole point of that bias voltage, to achieve some DC circuit current in the MOSFET when there is no drive.

We use the "arithmetic" method for the power amp, since there is no place to break the drain circuit for each transistor. Can't measure current without breaking the circuit, and running the current through the meter.

The trimpots are all set for zero Volts as the starting point. A current meter goes between the radio's positive power lead and the power supply. Key the mike, and you'll see a reading around one Amp on SSB with no mike audio. This reading is now your reference level. Keep track of that.

Now advance the first of the linear's bias trimpots. Set it so the meter now reads 50 mA (0.05 Amp) above the reference reading. Move the next one, add another 50 mA to the reading.

Keep this up, 50 mA per trimpot until you're done.

And if you insist on poking a volt meter probe onto the MOSFET gate circuits, you see readings closer to 3.5 Volts more or less. Odds are they will not all read the same identical voltage.

YMMV.

73