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.
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.