Sub-Power Supply Troubleshooting Guide - Toshiba 1997 G Series Troubleshooting Manual

11. Troubleshooting Sub-Power Supply.
Refer to fig 14. To troubleshoot the Sup-Power Sup-
ply, it must be isolated from the load. It is necessary to
hookup an external load to the secondary, and use a
Variac. With the Variac set to 40 Vpp and a 100W light
bulb hooked up for a load, the relay has to be shorted to
supply the 40Vpp to the Sub-Power supply.
Dead
First, check the 160 volts Main B+ and Fuse F850.
If the 160V is good and the fuse is open, the transistor
in IC802 maybe shorted. Check for shorts between pins
2,3, and 4 of IC802. If IC802 is good or after replacing
IC 802, make the following checks before restoring the
120V:
* Note: If IC802 is bad, the feedback circuit must be
checked.
Check for switching at pin 2 of IC801.
If the switching is missing, the Start-up circuit maybe
bad. Check R852 for an open, check Q851, Q850 and
Q852 for shorts, and check IC802 for a base emitter
open on Q1. Lastly, check T888 and C855. After re-
storing the switching, check the feedback: D856, R851,
R820, and C853.
If the switching is good, Check the feedback: D856,
R851,R850, and C853.
Wrong secondary voltage or improper regulation
A loss of feedback will cause the secondary voltage to
increase and can damage IC802 resulting in no power.
Check the following components of the Secondary volt-
ages are to low:
R853 and R854 the Over Current protect resistors.
Q850, Q851, and Q852 the Soft Start transistors and
surrounding circuit.
C855 Oscillator Capacitor.
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