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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
SECTION 6 - MAINTENANCE 6-31
(see procedure below). Since much larger signals are fed to the output stage it is somewhat self-biasing and
does not need a high bias level. The bias level is given in amps of idle current rather than a voltage since the
turn-on voltage for various batches of MRF-141G MOSFET transistors varies widely. An initial setting of 2.4V
should be a safe starting point for all production lots of these devices. The bias level can be probed by measur-
ing voltage on the gate of the MRF-141G MOSFET.
6:99. Once an initial bias voltage level has been set, or if checking an operating AMP, the system should be
operated with 50 ohm terminators connected to the rear case RF1 and RF2 outputs. Attaching two probes, one
on each side of the surge limiting resistors R60/600, and setting the oscilloscope to an A-B display, multiply the
voltage drop observed by 10 (R60/600 is 0.1ohm) to get the idle current. Similarly, checking across R131/219
and multiplying by 20 gives the idle current in the output stage. If no A-B setting is available, the voltage refer-
enced to ground can be probed on each side of the resistors and the subtraction performed on paper. If adjust-
ment is necessary, turning the 15-turn potentiometers (R67/147/107 and 187) CCW increases the voltage and
CW decreases the voltage, at about 0.3V/turn.
6:100. Since the MRF-141G device is a matched pair of FETs it is possible that only one has failed. If a stage
is biased properly but does not make the gain it should, then check that a very similar signal can be detected at
each drain (output) tab on the suspected MRF-141. If the entire device has shorted, the 27V from the drain of-
ten feeds through to the gate (the input tab) which should normally have the 2.4V bias pulse on it. If this is the
case replace the FET.
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