samson xp308i won't turn on

[nosaj]

I have a samson xp308i you turn it on all lights flash fan spins then go out. I attached the service manual.
I am not really versed in SMPS but I understand there can be special rules for working on them.

I would like to know where I can separate the power supply to hook a load up to the SMPS to verify its is working or if it is the issue.

Thanks,
nosaj

[The Dude]

It sounds to me like it's going into protect right away. I'd first check R54, R55, R71, & R72. They carry supply to the output IC's. If any of those are burnt, the associated output IC is probably shorted. If that tells you nothing, check the PS outputs to ground and make sure no supplies are shorted.

[Enzo]

Just remember the SMPS is dangerous. The mains are directly rectified into DC. There are two "ground" symbols on the power supply. The common triangular three horizontal lines - like an upside down Xmas tree. And then the three slanted lines - looks like a garden rake. That Xmas tree is chassis ground - ground as we expect in an amp. But the rake is not grounded. DO NOT connect a scope probe ground to that rake ground. That rake sits at -170vDC relative to earth. Look at U2, upper left area. Just below it is ZD4, a 15v zener to power the IC. The IC runs on just 15v, but its "ground" pin is on that -170v rail. SO its +15 pin is really at -155v. If I am going to work on an SMPS, I always plug the thing into an iso transformer.

SMPS usually have a ton of self protection. Yours starts and then qquickly shuts down. Usually that means a short on the output of the power supply. The SMPS itself could be shorted. No promises, but generally if it will try to start and even power up something like a fan, then the left side of the power schematic is likely working. PAge 23, this means every thing left of the two main transformers center right. T2 and T3. So my first check is look for shorted rectifiers on the secondary side. D13, D14 upper right area.

Of course other things could eb wrong in this complex circuit.

The next suspect is a blown output IC on one of the speaker channels. Those are the two TDA8920 ICs on page 24.

As I read the schematic, the power supply and power amp are on the same board, so hard to separate them.

[Arbutt]

you used to be able to buy a new board in the uk for very cheap price, ive bought about 6 of them over the years but not one recently

[nosaj]

R54, R55, R71, & R72. D13, D14 test good
But now no lights at all on power up just a momentary high pitch whine.

nosaj