Power issue with Behringer synthesizer - Higher current trips device to turn on

[Otto W]

A friend of mine asked me to have a look at a Behringer MS-101 synthesizer with a power issue.

Symptom: When powered on with the original mains adapter (output 9VDC @ 600 mA) the system tries to power on but indicator LED barely lights up and current draw is at rated maximum and voltage measured at the jack drops to around 5V. The power adapter is working, and I also tested with another one I have and the same happens. These are one of those negative-center music equipment barrel jack adapters used with most guitar pedals etc.. Anyhow, it's not the mains adapter, so my gut feeling says something is shorted to ground.

Visual inspection reveals no charred or obviously broken components. One diode seems a little hot (D5, circled with red in the image below), but not so much that I couldn't hold my finger on it.





After some in-circuit testing for shorted caps and diodes I tried to locate the short by hooking the unit to a lab supply with voltage limited to 9V and turning up the current to see if something would start giving smoke or smell signs. At around 1100 mA something unexpected happens and the device turns on, current draw drops to normal and everything works normally!

I'm somewhat clueless on what to look for next, because once it's on everything is working... Maybe a tranny that's struggling to operate at normal spec, but when given some extra current it decides to do its job? Any suggestions or hints on what to look for would be much appreciated.

Thanks!

[SeanB]

At 1A there will be something getting hot, so feel for something hot, like a ceramic capacitor, or a linear regulator that is getting toasty. If you have a thermal camera this only takes a few seconds to locate.

[SiliconWizard]

For instance, I'd look for a switching regulator drawing much higher current than normal to start. Maybe due to a leaky cap. Or, maybe it's actually the mains adapter that was able to provide higher current peaks (which would allow said regulator to start) and which is not anymore.

[Teledog]

Was it plugged into a positive center high current wall-wart? Diode might be toast. ( & maybe more)

[Otto W]

Was it plugged into a positive center high current wall-wart? Diode might be toast. ( & maybe more)

I doubt it, but can of course ask him. According to his story the issue started all of a sudden, and not e.g., as a result of connecting or fiddling with cables.

For instance, I'd look for a switching regulator drawing much higher current than normal to start. Maybe due to a leaky cap. Or, maybe it's actually the mains adapter that was able to provide higher current peaks (which would allow said regulator to start) and which is not anymore.

Thanks for the suggestion. I was indeed able to pinpoint an IC that generates a lot of heat while in the faulty power-up cycle. See below for more info.

At 1A there will be something getting hot, so feel for something hot, like a ceramic capacitor, or a linear regulator that is getting toasty. If you have a thermal camera this only takes a few seconds to locate.

Second time around looking for hot components I was able to narrow down the current sink. It is in fact IC4 that gets scorching hot. D5 warms up just as a side effect. IC4 appears to be a SOT-232, but the marking (S40B) doesn't give many hits.



From what I can measure in circuit the pinout looks as follows:
1: short (~0.5ohm) to pin 6
2: ground
3&4: Positive lead on pin 3 and negative on 4 reads like a 1.3V diode 
5: (no pin)
6: short to pin 1

When stuck in the faulty power-up cycle, pins measure as follows:
1:
2: ground
3: noise, averaging at 0.02V
4: 2V
5: (no pin)
6: 2.1V

When cranking the current and having the device operate normally same pins measure as follows:
1: 8.1V
2: ground
3: -1.2V (?)
4: 8V
5: (no pin)
6: 8.1V (same as pin 1)

To me this look like pin 1-to-6 isn't a direct short, since pin 1 looks like wanting to go up while in the faulty power-up cycle.