B5.4
T855 Fault Finding
M850-00
31/09/98 Copyright TEL
5.4 DC Checks
5.4.1 Power Rails
Refer to the test points & options diagrams in Section 6 for test point locations, and to
the regulator fault finding chart (Section 5.7.2) for fault diagnosis.
Check the 9V (TP602) and 13.8V (TP601) power supply test points in the regulator
compartment with a DMM.
Check the 20V regulator output at the test point (TP603) in the regulator compart-
ment.
Check the 5V regulator output at the test point (TP604) in the regulator compart-
ment and on IC310 pin 4.
Check the 5V digital regulator output at the junction of C611A (+) and IC610 pin 2
in the regulator compartment.
5.4.2 VCO Locking
Using a DMM, monitor the VCO control voltage on the long lead of L1 on the
VCO PCB.
If the synthesiser is locked and the VCO aligned, the voltage at this point should
be between 3 and 16V.
If the VCO is not locked, refer to the synthesiser fault finding charts (Section 5.7.3).
5.4.3 Mute Operation
The front panel "Gate" LED will show the status of the mute circuitry and will turn on
when a signal is received above the threshold level.
Check that PL250 is linked correctly:
noise mute 1-2
carrier mute 2-3.
Check that the mute gate opens as follows:
noise mute - rotate RV230 (front panel gating sensitivity) fully clock-
wise and check that the front panel "Gate" LED turns on;
carrier mute - rotate RV235 (carrier mute) fully clockwise and check that
the front panel "Gate" LED turns on.
If the mute fails to operate correctly, refer to the noise mute fault finding chart (Section
5.7.4) or the carrier mute fault finding chart (Section 5.7.5).
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