TTI QPX1200 Service Manual page 14

The signal across the current sense resistors R12, R13, R14 and R15 is fed into pin5 of IC15-B.
With the output current set to 20A, the signal at pin 5 is nulled by the current in R91/VR4 from the
negative voltage generated by zener D49. The null point setting is aided by comparator IC15-A
which drives two LEDs, the null being achieved when both LEDs are off .
Having nulled the voltage generated by the 20A output current in the current sense resistors, the
increasing current generates a voltage at the output of IC15B and this is fed to the Z input of the
analogue multiplier. The required level of correction is set by VR5.
The supply rails for the front panel control circuit are generated from an auxiliary winding on the
converter transformer T1 terminated on pins 13 and 14.
A V+ rail of approximately 10V is generated by D33, D34 and filter L4, C40.
A +15V rail is generated by D35, C43, R51 and D36.
A -8V rail Is generated by D38, C44, R55 and D37.
Photograph 1
Photograph 2
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