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FAULTFINDING NOTES.
SAFETY: These safety notes are directed to those
testing and repairing this power supply. Legal
requirements mean that we must not encourage
untrained personnel to take the lid off.
+/- 17V RAILS.
1) A convenient position for attaching a test meter to 0V is the top leg of bleeder
resistor R9 (top leg of R47 on RH PCB) Remember these resistors run hot.
2) If one side of the +/-17V supply is not working, the mutual shutdown system will
close down both sides of the supply. This makes faultfinding difficult as it is often not
obvious which side has failed. The mutual shutdown can be disabled by putting on
jumpers JMP1 & JMP3 on PCBs at Issue 6 and above.
IT IS ESSENTIAL THAT THESE JUMPERS ARE
REMOVED AFTER SERVICING IS COMPLETED!
FAILURE TO DO THIS MAY CAUSE SEVERE CONSOLE
DAMAGE IF ONLY ONE 17V RAIL SHUTS DOWN.
3) If the wireform connecting the two PCBs is disconnected, the mutual shutdown
will close down both sides of the supply. The Headroom Alarm dots will also show
on the DPM as an active low from the RH PCB is required to suppress them, and this
is now missing.
4) If there is no power to the 723 IC (eg if R11 or R48 is open-circuit) the output will
remain low, but no damage occurs.
5) It is possible to swop over the two 2-pin connections on the LH PCB in error. The
fan will not run, but no circuit damage occurs.
+48V SUPPLY.
1) If TR17 fails short-circuit, the regulator will appear to work, but since all voltage
is dropped across the TL783 regulator, this will overheat and shut down at high
currents. The TL783 should not be damaged if this is the only fault present.
2) If zener DZ5 fails short-circuit, the regulator will not turn on, and there will be
only 300 mV approx at the output.
3) As usual, the unregulated supply to the +48V regulator can reach +90V, and should
be treated with some respect.
+8V SUPPLY.
1) This is a completely standard IC regulator circuit. The regulator REG3 is the only
part that is likely to fail.
2) This supply contains mains voltages on the voltage select PCB, the mains switch,
etc, and the usual precautions must be taken.
3) The main reservoir capacitors have a capacity of one tenth of a Farad, and are
charged to +26V.
22 Circuit Description