A neighbour brought me this mixer in with a sudden fault: it doesn't boot and instead it keeps flashing all LEDs in the front board.
I started without any documentation at hand and I was quite lost. I deducted that ±15V from PSU were down for some reason. No shorts in consumer side. There were 5V and 48V (for mic phantom), but that's all.
I was about to give up, but as an act of despair I contacted Soundcraft headquarters requesting some help or documentation. Against all odds, I got a prompt reply from Rick Morris attaching schematics of both the mixer and its PSU. Well, that's a boost!
With the aid of PSU schematic I spotted the common link between +15V and -15V: /PWRGOOD signal (0V = "power good"). It was always 5V, preventing the system from booting and from enabling ±15V.
With the PSU isolated, I shorted PWRGOOD to GND and then both +15V -15V show up! Why it isn't pulled to low level by the PSU itself? The PSU generates 5V and phantom 48V fine, so it had to be a very silly thing. Following the /PWRGOOD signal to the primary side, LM393N comparator outputs 0V (good), so the photocoupler CNY17-3 should had to pull /PWRGOOD to ground at secondary side, but it didn't. Compared both CNY17-3 and IC1 reads different than IC2 in the diode side. Also R79 has changed color from blue to dark green due to heating. I replaced IC1 photocoupler with a equivalent LTV702VB and the problem was gone! Now system boots OK.
Thanks again to Rick Morris from Soundcraft. Without his help I wouldn't have succeeded.
I started without any documentation at hand and I was quite lost. I deducted that ±15V from PSU were down for some reason. No shorts in consumer side. There were 5V and 48V (for mic phantom), but that's all.
I was about to give up, but as an act of despair I contacted Soundcraft headquarters requesting some help or documentation. Against all odds, I got a prompt reply from Rick Morris attaching schematics of both the mixer and its PSU. Well, that's a boost!
With the aid of PSU schematic I spotted the common link between +15V and -15V: /PWRGOOD signal (0V = "power good"). It was always 5V, preventing the system from booting and from enabling ±15V.
With the PSU isolated, I shorted PWRGOOD to GND and then both +15V -15V show up! Why it isn't pulled to low level by the PSU itself? The PSU generates 5V and phantom 48V fine, so it had to be a very silly thing. Following the /PWRGOOD signal to the primary side, LM393N comparator outputs 0V (good), so the photocoupler CNY17-3 should had to pull /PWRGOOD to ground at secondary side, but it didn't. Compared both CNY17-3 and IC1 reads different than IC2 in the diode side. Also R79 has changed color from blue to dark green due to heating. I replaced IC1 photocoupler with a equivalent LTV702VB and the problem was gone! Now system boots OK.
Thanks again to Rick Morris from Soundcraft. Without his help I wouldn't have succeeded.
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