Soundcraft EFX12 power supply

[Khron]

Initial status: no output, fuse intact.
Initial fault: track broken right near one of the solder pads for the IEC connector.

Second stage: PSU powered on, all outputs were present, but quite high
Second fault: during measurement of the UC3842's Vcc, i managed to short it to the output of the chip, blowing it, the switching FET (STP13NK60ZFP 13A 600v) and a few other resistors in the process (R3 unknown, R6 10R, R2 2x1R in parallel), plus the BC546 driving the TIP31A that regulates the phantom-power output.

Third stage: soldered in a KA3843 i had at hand, a slightly lower-rated FET (FQPF8N60C 8A 600v), and replaced the blown resistors (R3=2.2k, R6=10R, R2=0R3 2W). Plugged in the IEC *without* a bulb tester , pressed the (mechanical) power switch.
Third fault: same components blown.

I'm slightly at a loss here...

Details: UNREGULATED design; outputs are +/-15v for the analog stuff, +48v for the phantom-power, +8v for the digital board (which contains some linear regulators for 5v / 3.3v). After the initial round of replacements, the voltages were around +/-22v, 61v and 17v respectively.

[Escort Eagle]

Re: Soundcraft EFX12 power supply

Is your PCB Connection Lines are OK ? Specially - make sure that FET's Source Pin connection line to the Ground is OK and check other ground connection lines also .

[Khron]

Re: Soundcraft EFX12 power supply

Well, with some invaluable guidance from Th3_un1qu3, i ended up removing that resistive divider going into the current sense pin of the chip, placing a 680ohm resistor in series with that input, and tested the PSU with several source resistor values.

3.9 ohms = +/-15v outputs were wobbling between 0.8-2v, 8v output was almost zero, unloaded phantom-power rail (before the discrete regulator) a steady 10v
1.2 ohms = 2.9-5.5v, pre-reg phantom about 21v
680 ohm -> 2.2k (going into the current sense pin)
Outputs = +/-13.7 stable, 6.7v stable, 47v stable
0.68 ohm source resistor => 15.2-15.3v, 8v, 52v (post-reg)

Despite the original FET being a 13A 600v, the closest one i had left was a 5A 900v 2SK2717.

All these tests were conducted with 40ohm loads on the +/-15v rails (375mA) and 20ohm on the 8v rail (400mA). Numbers also looked good with 60ohm / 30ohm loads (250mA and 260mA respectively).

All in all...