Yaesu FT-901DM Maintenance Service Manual page 72

KEY CLICK MODIFICATION
In some FT-901 transceivers, when operating in the
CW mode,the keying waveshape could become too
"hard" if the drive control (CARR) were advanced
too far. The following modification should
eliminate this difficulty.
Modification Procedure:
RECTIFIER C UNIT MODIFICATION
The FT-901 series has been modified, starting
with the No. 8 production lot, as described below.
The change was made in order to provide pro-
tection against the loss of Rigo3 caused by un-
balance in Ciso2 and C;s03. While the problem
was highly isolated, the failure of Rigo3 could
cause the destruction of the filter capacitors. In
order to catch this problem before it causes
damage in the field, we recommend the following
modification:
8
as 28
ee os
x x
_o
3 3§
— @Q
& z
SERVICIN(
'T 2SA639
6 DI803 D804
—=—Add CAP 0.0047uF
180s
e
RECT C UNIT
RI8IS 470
RIBI2 4.7K
Change Riso3 to 390 ohms, 2 watts.
Add a new Risi9 (39 ohms, | watt) in series
with Diso1, as shown.
Diso1 is being changed in production to type
SM1-12, but this change should not be neces-
sary in the field. Modification is now complete.
is to add another 10D10 diode in series VINYL TUBE
with D1801. No further modification is ines