Monacor SAM500D - noise / stopped working

[SKE]

Dear audiophiles,

after looking for a decent subwoofer for quite some time I deciced to give a DIY project a try. The hardware I have is a 12 inch scanspeak that I wanted to power by a Monacor SAM500D. Yesterday I plugged the speaker to the amp and I was going to make a thread here how to get rid of the static noise I got. I know that class D amps tend to be somewhat noisy but It appeared to be a little loud. I would guess maybe like 50 dB of static noise (woofer just lying on the floor) while otherwise working fine. However, after 20-30 minutes of playing around the amp stopped working. You can see that the LED on the back turns green when an audio signal is present. However the little blue LED on the circuit side that I recon indicates active power section stays off. I cannot see any obvious damages like burns on the board. The speaker is fine. Do you have any suggestions how to proceed? Unfortunately I cannot return the amp.

 

[DVDdoug]

If it was me, I'd look for loose connections or may something burned and if nothing was obvious I'd cry and replace it with something different. sometime tapping on the circuit board with a screwdriver handle or flexing the circuit will reveal something. Of course, be careful with power line voltages!!!

Do you have a schematic and would you know what to do with it? Do you have a multimeter and possibly an oscilloscope?

A DIY repair might be difficult or impossible if there's something wrong with class-D switching circuit... Unless you could get a replacement circuit board. Switching power supplies (which it likely has) are also complicated and they also usually have fine-pitch surface mount ICs that are hard to change (impossible for me!). Both switching circuits are hard to troubleshoot. A blown MOSFET could be easy to change if you know how to solder and you'd likely be guessing about the problem.

I know that class D amps tend to be somewhat noisy

Not necessarily. And hiss won't be reproduced well by a woofer so you shouldn't be hearing ANY noise unless it's coming from whatever is plugged-in. Or MAYBE if you get your ear very close to it.

Sometimes you can get mechanical whine from a coil in a switching supply but if you do I'd call it defective.

Linear power supplies can hum but that will be filtered-out in a good supply. Or sometimes transformers can mechanically hum/vibrate if there is a DC offset on your power line. I had that once... I was getting a strong vibration that I could hear and feel when the TV was on. The amplifier outlived the TV and I haven't seen that problem since. At the time I didn't know that was caused by an offset so I didn't know what TV was doing to the power line...

 

[SKE]

I do have a Multimeter at hand and I can get an oscilloscope. I also can solder, but at the moment it is not apparent what needs to be soldered. So i switched the device on and did some poking around with a wooden stick. Unfortunately no findings there. So I used the multimeter which revealed that the speaker is not blown and the cables are actually connected to the power amp. From the power supply i get 220V and it appears that the main board deliverd approx 14V to the smaller board with the cinchs and potis. I cannot tell whether the small board feeds back the audio signal. The amp part on the bigger board seems to be fully inactive. I could not detect any voltage there. It might be worth detatching the whole thing from the base plate to see whether there is any bald soldering. However id have guessed that Monacor has some sort of quality control plus annoyingly the screws mainly got some extra glue (for safety or to prevend people from taking it off). I double checked visible defects on the power side but i could not see any. The current state is somehwat annoying since the board has never been in use so far. Unfortunately no schematics are provided.

 

[SKE]

So since Monacor did not seem to be willing help (except for send in and pay 87€+shipping) I detatched the main circuit board to check whether anything suspicious can be seen on the back side. While I could not see any obvious damages or cold solder joints I was shocked how low the maufacturing quality was. Look at that picture, it seems a monkey could have done that better…

 

[AnalogSteph]

While I could not see any obvious damages or cold solder joints I was shocked how low the maufacturing quality was. Look at that picture, it seems a monkey could have done that better…

Well, it does have a certain "hand-assembled prototype" charm to it, that's for sure.

There's your first bad solder joint... probably not the last one. Look around for more like this.

These look a bit dubious as well:

Probably needs a good going over of these kinds of big, high-stress joints at least.

I don't really like how the joints on the power MOSFETs haven't flowed through properly to the other side either.

The amp part on the bigger board seems to be fully inactive. I could not detect any voltage there.

That's definitely a problem then. Can you see where the supply is supposed to be coming from? There's got to be a fairly beefy power supply of either the switching or conventional variety which should be hard to miss when starting at the power connector.

 

[SKE]

Thats a very friendly way of describing it I guess
I did not like the soldering on the MOSFETS either but at least the multimeter beeped when I measured from the leg to there the solder should be on the board.
What is so bad about the part you circled? I was looking for balling structures (also those can be found here or there). If I find some time during the weekend I can try to resolder the suspicious areas. I am not sure now if I want to use that device at all… even if I could get it working

 

[AnalogSteph]

What is so bad about the part you circled?

Sharp rings of any kind tend to indicate a cracked joint. The legs of anything mechanically stressed are particularly prone to developing these. Transformers, 3-terminal regulators with heatsinks, that sort of stuff.

 

[SKE]

So last sunday I did resolder all of the suspicious parts (and then some more). However, this did not restore the operation. Does anyone have ideas?

 

[SKE]

I could not find an "edit" button. Now I got rid of the amp and will start over
I'll open a new thread though