User manual SOUNDCRAFT CPS 250

[. . . ] CPS 250 CPS 275 USER AND TECHNICAL MANUAL SOUNDCRAFT CPS250 CPS275 Console Power Supply User and Technical Manual For your own safety and to avoid invalidation of the warranty all text marked with these Warning Symbols should be read carefully. IMPORTANT: please read this manual carefully before connecting your Soundcraft console power supply to the mains for the first time. © Harman International Industries Ltd. 1997, 2001 All rights reserved Parts of the design of this product may be protected by worldwide patents. Part No. ZM0195 Issue 4 Soundcraft is a trading division of Harman International Industries Ltd. Information in this manual is subject to change without notice and does not represent a commitment on the part of the vendor. [. . . ] The circuitry of the +17V regulator is described in detail below: The +17V Supply. The transformer secondary is fused by F1. Secondary fuses will normally only blow in the event of serious failure, such as a major short in the AC wiring or a failed bridge rectifier. The fullwave bridge rectifier is mounted on the heatsink, at the hottest end as it is the most heat-tolerant of the semiconductors. For safety reasons, the reservoir capacitor is discharged by bleeder resistors R39, 40 at switchoff; this takes about 10 seconds. The 723 IC contains a 7. 15V nominal Zener diode, defining the voltage that appears at Pin 6. R5 is in series with the positive input of the servo opamp (Pin 5) and is made roughly equal in resistance to the feedback divider R7, 11 etc to minimise bias-current offsets. The feedback divider R7, 8, 11, & PR1 derives a fraction of the output voltage and delivers it to the negative input of the servo opamp. (Pin 4) The negative feedback keeps this point also at 17. 15V, so the actual output voltage is determined by the feedback divider ratio, which is trimmed over a narrow range by PR1. The output of the servo opamp controls the output through TR1, an emitter follower driving the series-pass device TR3. The power supply to TR1 is taken from the +12V supply developed across C23. This has its 0V side connected to the positive unregulated DC supply of 26 - 29V, (at nominal mains) giving a total voltage of approx 38 - 41V for the driver; this also powers the 723 IC via R1. This subrail markedly reduces the minimum drop across the pass elements and therefore increases the efficiency. 10 Amp Schottky diode D25 on the output allows supplies to be paralleled for redundancy. This is only fitted for the CPS275 option. Overcurrent Protection. The power supply is protected against short-circuits and overload by a dual-mode protection system; brief overloads are dealt with by constant-current protection, while prolonged overload causes both 17V rails to shutdown. Reset is by turning the mains power off for not less than 6 seconds. 14 Technical Description The constant-current protection system works in the standard way; when an attempt is made to draw excessive current, the output voltage is reduced so that no more than a fixed maximum can be drawn. However, the high current capability of this PSU means that long-term constant-current protection alone is not practical as the dissipation in the pass devices is too high for the cooling system to deal with, and in time they will overheat. The constant-current system measures the output current by sensing the voltage drop across resistors R18, 19 in parallel. When it reaches approx 0. 60 V the internal protection transistor in IC1 is turned via R41, and the output voltage on pin 10 pulled down to limit the current supplied. If this condition persists for longer than one second, then both rails of the +/-17V supply are shut down by the Mutual Shutdown System, described below. The Mutual Shutdown System. Equipment containing some kinds of op-amp is vulnerable to damage when only one of the +/-17V rails fails, as in this case excessive supply currents can be drawn, damaging fuse resistors on the modules. When both 17V supplies are working normally, DZ1, DZ2 are conducting, LED1 is on and opto-couplers OP1, 2 have their photo- transistors conducting. If the -17V rail collapses, there is no longer enough voltage to keep DZ1, DZ2 on, and OP1 switches off. TR5 is turned on by R16, with R28, 30 and C40 providing a time-delay to prevent spurious operation on transients, or brief overloads which are dealt with by the constant-current protection system . [. . . ] Ensure that the insulating fuse covers are replaced with the new fuses. The extrusion/pcb assembly must be removed from the case to gain access to the underside of the pcb in order to unsolder and/or undo the appropriate component. They can be removed by desoldering the pins, where appropriate, and unscrewing the fixing nuts or the fixing screws, taking care to retain the small insulating bush(es). Retain carefully the mounting pads and plastic insulating bushes where fitted. [. . . ]