[sdiy] yamaha spx-90 blown p/s caps

[Stewart Pye]

Hi Nate,

Yep, that's a switchmode supply alright. I vaguely recall repairing one 
of these years ago. I'm pretty sure they run OK without a load, so until 
you get it working correctly don't connect the outputs.

Since this is an off line SMPS the big cap in the middle will have the 
full mains supply across it. Here we have 240 volt mains so the cap has 
about 360 volts across it when charged. Be careful as the caps can hold 
charge for a long time. Assume that anything on the PCB could be at HIGH 
voltages. After you power it down and disconnect the unit from mains 
carefully remove the board without touching anything and measure the 
voltage across the cap. Discharge it if necessary.

It's probably just the caps, but it is a good idea to check all diodes, 
chopper transistor (on heatsink) etc.
Personally I'd replace all the caps if possible, and use low ESR caps if 
possible. It sounds like it's running to some degree.

I'm by no means an expert on SMPS but I have repaired a few. Let me know 
if you need more explanation on the above. Good luck and be careful.

Stew.



Tom Wiltshire wrote:
> I agree with Graham - that's far too complicated for a linear supply, 
> He's also right to point out that the caps often die in switched 
> supplies. In fact, I've got a non-functional wireless router here 
> which is down to the same thing - dead caps in the cheap switched supply.
> I'd recommend replacing all the electrolytics with high-temp/high 
> voltage replacements starting at the largest values and going as far 
> down as you can be bothered. The original caps will have been the 
> cheapest they could get that met the spec, rather than anything that 
> improves the life of the unit.  I'd have thought you can source some 
> caps that would be an improvement.
>
> The only other thing to watch out for is that power supply faults 
> sometimes fry a lot of chips on the other circuit boards (like that 
> 29V you mentioned?) so it might not be worth a repair at all. Can you 
> check by powering it up with the other power supply?
>
> T.
>
> On 8 Oct 2008, at 19:06, GRAHAM ATKINS wrote:
>
>> Nate,
>>
>> Thats far too complicated for a linear supply !. The Xformer would be 
>> much
>> bigger. Those are inductors. Yes the switching regulator does 
>> probably run
>> fairly hot. Often the capacitors break down on such devices. Do you 
>> have a
>> meter that can read capacitance ?. Certainly remove and replace the 
>> damaged
>> one but I would carefully remove a few more, the biggest first, and 
>> check them.
>> They may be badly out of spec. replace any suspect ones.....look for 
>> signs of
>> heating and/or bulging at the ends. If you do have to source any 
>> replacements
>> (Not in your spare) go for the higher temp 105 Deg caps rather than 
>> the 85 Deg
>> equivalents. Used frequently in VCRs etc, usually the caps fail.
>>
>> Now wheres that meter......
>>
>> Graham
>> On Oct 8, 2008, at 17:43, Nate Reeves wrote:
>>
>>> i think it is a linear supply but here is a pic
>>>
>>> http://picasaweb.google.com/natere2ster/YamahaSpx90Spx90II#
>>>
>>> i did notice that the heat sinked voltage regulator (?) was VERY hot 
>>> to the touch, with the unit being one for only 10 minutes
>>>
>>> my hope is that between the 2 boards i can make one fully 
>>> functional!  lol
>>>
>>> cheers,
>>>
>>> nate
>>
>>
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