power supply repair

[ognam2]

Hello everybody.
I try to repair my Roland Bluescube Amp. Obviously the power supply doesn't work. I am not able to measure any voltage at the output. The Fuse is ok. My question is how can I find the defective part?
Thank you!

Jonas

 

[JohnRoberts]

apparently some kind of switcher... dangerous to probe around a mains connected power supply with power connected.

Look for a faulty switching device, or control IC...

Good luck, not trivial.

JR 

 

[Khron]

I'd start with replacing the small electrolytic on the primary side (usually something in the range of 10-47uF), with a new, known-good, low-ESR one, of similar or slightly greater value (up to 100uF, no more). In that top photo, you can juuuuuust see it peeking out from "behind" the heatsink of Q1, along the left edge of the board.

Any photos of the bottom of the board, though?

 

[Bo Deadly]

You might have to load it to measure something on the output.

Look for parts that look discolored. A common failure in an SMPS like this is some IC or resistor that's handling a lot of power gets burned up inside.

 

[ognam2]

Thank you for your help so far. I loaded the disconnected board with 220V and measured the output (labeled like +20V -20V and so on). No DC there... can I actually be sure that the fault is in the power supply then? All the parts look ok from outside...

 

[Bo Deadly]

Loading does not mean putting voltage in, it means putting a "load" on the output(s). For example you could put 100 ohm resistors on each output. But they would have to be high wattage like 5W (if you used regular 1/4W, they would get very hot and maybe start to smoke). Or parallel 3-4 1K lower wattage resistors and only turn it on long enough to get a reading on the meter.

Otherwise, what is your skill level? Fixing a power supply is not for newbies because it's dealing with mains voltage and probably a lot of power so it could damage something or start to smoke and stink up your house or worse.

 

[moamps]

..The Fuse is ok. ....

Please check out also NTC's  resistance.

 

[Khron]

Well that can't be good news - all the control circuitry on the primary side seems to be inside that black "brick". Which may or may not be epoxy-potted...  :-\

 

[ognam2]

Perhaps it’s better to order a new power supply then. But are we sure that this one is faulty? Do I need to load the output with resistors to measure if it produces DC Voltage?

 

[Bo Deadly]

Probably no. Some of the SMPS chips are pretty sophisticated. For example it could pulse on briefly to see if there's a load. But I have never seen that first hand. That's largely a guess from looking at the things SMPS do in datasheets. It's something I would check but presumably the amp itself is perfectly suitable load but it's completely lifeless when you power it on? If yes, then either a critical connection broke, the power switch broke or the supply is in fact defunct. It could be possible to fix it but I think Khron is right that that black box is more than just a capacitor because from the bottom you can see there is an IC footprint. That could make it very difficult to fix even for a pro. And since it looks like a newish amp, you could probably get a replacement. It might not even be terribly expensive. That SMPS probably cost them $10. They'll charge you $40 plus shipping. Something like that. So yes, just get a replacement and see what happens.

 

[Khron]

If anything, SMPS controllers pulse the switching element(s) on the primary side (ie. working, but with a very low duty-cycle) in order to increase efficiency at low loads, but you WILL still get the full output voltage. The feedback, be it from the secondary side, or "just" an auxiliary primary winding, doesn't really work otherwise.

Or, well, i wouldn't be surprised if there are some oddball controllers that somehow manage to do some sort of load-sensing even without the output being at the voltage it's supposed to be, but likely not at the cheap end of things (as is this case). How would one determine the load that quickly, considering there's a few hundreds / thousands of uF of capacitance after the rectifiers? Current-sensing adds a lot of (arguably unnecessary) complication and cost (components, design time, board space etc).

 

[fazer]

This is my go to thread when looking at SMPS repair.  Sounds like you may be aware of it. 

https://groupdiy.com/index.php?topic=19733.msg895976#msg895976

Svart has not been here in awhile but nice thread never the less.