Marshall Stockwell teardown

Yesterday a friend of mine gave me his Marshall Stockwell wireless audio player/amplifier to try to debug and fix. The battery in it would take a charge, and the charging LED would pulse, but the unit itself wouldn't start.

Taking the unit apart and removing the small PCB covering the top left portion of the main PCB, we are greeted with this:

Picture of the main PCB in the Marshall Stockwell

Different lighting:

Another picture of the same PCB, with different lighting

The two wires and the through-hole resistor are bodges of mine done during testing. I also had to snip the IC to the right of the red bodge wire, which was the 3.3V DC/DC regulator.

Some initial measurements showed that the 5V rail was dead and that Q4, the P-channel MOSFET that enables the 15V supply for the rest of the unit, would not turn on. Some time during my investigations the 3.3V switching power supply also broke.

Q4 was easy enough to turn on, just a 10k resistor from its gate to ground. I hooked up an external power supply for the 3.3V rail and let the unit run for a while.

After some time had passed, I noticed the current draw on the 3.3V rail went from 500 mA down to around 400 mA. Most of this current was going to a single 5.8 Ohm resistor, R25, which appears to be connected to a diode elsewhere. Unsoldering one of R25's legs reduced the current to around 80 mA, much more reasonable.

Despite turning on Q4 and feeding the 3.3V rail the 5V regulator would not turn on. The pin on U2 which I suspect to be the "enable" pin measured 0V. The "input" LED would however light up.

I have some more tricks up my sleeve to try and resurrect this thing. For now hopes are slim.


The board revision is "Stockwell V.A2 160630".

I have summarized what components I have identified on the board in the hopes that it might be useful for someone in the future:

Ref Markings Part name Function Links
? MP26123 MP26123 2A,24V Input, 600kHz 2/3-Cell Switching Li-ion Battery Charger datasheet
Q4 ME50P06-6 ME50P06-6 P-Channel Power MOSFET datasheet
? NTP-8230 NTP-8230 High Performance, High Fidelity Power Driver Integrated Full Digital Audio Amplifier datasheet
U3 YJ33 SGM2019 3.3V linear regulator for audio datasheet
U4 PCM1808 PCM1808 Single-Ended, Analog-Input 24-Bit, 96-kHz Stereo ADC datasheet
U2 04=34P (hard to read) LM2734Z/-Q1 (qualified guess) 5V DC/DC for USB datasheet
? TPS5401 TPS5401 3.3V DC/DC datasheet
U5 N79E814AT28 N79E814AT28 8051 clone product page, user manual
U1 HEF4052BT
HEF4052BT 2-channel 4-to-1 analog mux datasheet
U14 4560
NJM4560 Dual Operational Amplifier datasheet

The unit has two 3.3V supplies: one linear, connected to all analog audio chips, and one switching, connected to the logic, buttons and potentiometers.