Yaesu FT-901DM Maintenance Service Manual page 29

TECHNICAL NOTES
SIGNAL TRACING IN THE FT-901
Because the signal path may change considerably
when the mode is changed, we have included
augmented block diagrams on page 2-3 through
2-9, in order to assist you in understanding the
function of the FT-901.
Below you will find a board-to-board block dia-
gram, showing the TX and RX signal paths through
the transceiver.
It should be noted that the internal calibrator
(Marker) is an extremely useful signal for pre-
liminary fault localization. In a properly function-
ing FT-901, the S-meter should read approximately
S9 + 10 dB, when the preselector is peaked on
14.000 MHz SSB. Minor variations from this
number are not unusual, but a blown RF amplifier
FET will cause this indication to be practically nil.
By using the internal calibrator, an experienced
technician can peak practically all circuits on the
receiver side, without the use of an external signal
generator.
PBA706
CARRIER
UNIT
TX MODE
PB-i703 PB-I716
NB = FILTER
UNIT UNIT.