Mackie 402VLZ4 mixer PSU

[RFmaster89]

Hello Forum

Background and Description
I've recently picked up a pair of faulty Mackie 402VLZ4 analog audio mixers. These are small units that are handy for small venues where one or two microphones and line audio can be used. These can be used for dance classes or a training room.  Both units suffered from a failed PSU boards.  PSU, part number CP2047468 (aka longjoin) takes +12V @ 2 Amps and converts it to +/- 15Vdc and -48V.  A portion of the PCBA is not populated - my guess the PSU board is used in other products where additional DC rail is needed.   PSU is based on DC-DC converter topology and employs LM5022.  There are only two LM5022 on this PCBA and other DC-DC controllers present - however I might be wrong!  One of the two LM5022 is used to generate -48V DC (phantom power) while the second iunit s used to generate +/- 15V rails.  I don't know what trickery engineers used to use a single boost controller to generate +/-15 voltages!  There are no schematics available and Mackie's help support is at best can be described as laughable.

Problems statement:
Both units exhibit no power symptom. PSU board is disconnected from analog PCBA for testing, no load. + 12 Vdc is applied from a lab supply, current limited. Visual inspection yielded no obvious signs of trauma.

Unit # 1 +12V input is loaded (2.5V @1.8Amps) - shorted to ground. Utilizing IR camera I was able to ID Q2 as pass to ground - 85C hot spot.  Q2 appears to be TSOP-6 device. It has marking ZU AC  as shown in the photo.  From TI's LM5022 data sheet Q2 should be a  N-channel MOSFET, per application circuit.  Pin 5 (OUT) ot LM5022 is coupled to Pin 3 of the transistor (gate). My search for ZU marking yielded nothing definitive except for Nexperia PMN27UP which is a 20V P-channel trench mosfet.   No output voltages. +/-15 and -48Vdc are down.

Unit #2  +12V input is at 0.056A  - no +/- 15Vdc but -48Volt is alive. without schematic further troubleshooting is problematic at best.  Any ideas or suggestions how to proceed further would be appreciated.

Thanks in advance.

[aeg]

Is the circuit similar to the DL806Mk2?

http://bee.mif.pg.gda.pl/ciasteczkowypotwor/%23pro_audio/Mackie/Mackie_DL806,_DL1608_Main_MKII_Schematics.pdf

Page 13 for the power supply.

[RFmaster89]

Thanks aeg.  Yes, PSU section for +/- 15Vdc is identical!  Case closed!

[RFmaster89]

Since I'm not an SMPS designer this app note covers use of generic boost converter to generate both positive and negative voltage rails.
https://www.analog.com/en/resources/design-notes/generating-negative-output-voltage-from-positive-input-voltage.html