Yaesu FT-901DM Maintenance Service Manual page 88

SERVICIN
FAULT IDENTIFICATION AND
LOCALIZATION
Initially, turn on the receiver, and check out only
the RX side. Any malfunctions you detect on the
receiver side should be repaired before you check
out the transmitter. In doing this, you may well
cure the entire problem, as much circuitry is shared
on TX and RX,
The logical process of fault identification is to
determine the missing function (no RX on LSB),
then the board at fault (CARRIER UNIT), then
the bad circuit (LSB oscillator), then the mal-
functioning part (X701),
If, after the receiver inspection is completed, all
appears OK, switch to the transmit side, following
the same logical procedure (function — board —
circuit — component). Concentrate on those
sections unique to the transmit side, as you have
already performed a thorough checkout of all
receiver and shared circuits (hopefully), Use only a
dummy load. NEVER troubleshoot using an
antenna.
In this manual, we will provide troubleshooting
advice which leads you directly to suspect
components. As there are some 2,800 parts in the
FT-901DM, though, it obviously is impossible for
us to trace the path of every possible malfunction
in the radio. Therefore, if your tips do not lead to
identification of the trouble, the logical
elimination process is the way to go.
In all troubleshooting, an "extender board" is
essential for quick and easy voltage testing. A
major advantage of Yaesu equipment from a
service standpoint is that you don't have to snake
a probe into the innards of a rat's nest of wires to
get to a critical test point. With the plug-in circuit
boards, you can quickly examine a board and move
on to another potential trouble area. But don't be
without extender boards if you do any servicing.
They're cheap, and they cut service time dramatic-
ally.