Audio aplifier with faulty 12VDC Power supply

[turrikasd]

Hey. I'm repairing a valuable car audio amplifier that is in almost perfect condition except for the power supply.

It's a switching power supply that is supposed to take 12VDC as input and output 25V & -25V DC. However the output is 1.5V and -1.5V.
The four PSU FETs output measure about half of the voltage they should. The highest voltage FET's gate resistor has 10% lower resistance than the rest, however impossible to tell if it is by design or not since the really really small SMDs are unreadable.

Visibly everything is in perfect condition, especially all capacitors.

Any ideas what gives? My repair skills cut off when there is no visibly damaged components or obvious shorts.

 

[Arouse1973]

Got any clear photo?
cheers Adam

 

[turrikasd]

Got any clear photo?
cheers Adam

Sure. Added photo

 

[Arouse1973]

Nice one

 

[Arouse1973]

No crispy components then Have you tried to get a service manual? Whats the make of the AMP? Could be the power mosfets.

 

[turrikasd]

No crispy components then Have you tried to get a service manual? Whats the make of the AMP?

It's labelled "VDO Dayton" but I believe they went out of business or changed their niche so no luck with manual or any support.

 

[turrikasd]

Could be the power mosfets.

For replacement FETs, what values matter and what don't? I don't think I can find the original ones.

 

[Arouse1973]

I'll have a quick look. Does it have a model number or do you think VDO dayton is it?

 

[Arouse1973]

VDO is still going by the looks of it. They might not do audio amps anymore.

 

[Arouse1973]

Looks like they might be BJT devices.

 

[Arouse1973]

Found some origional transistors. But using tablet and cant work out how to copy link. I will post tomorrow

 

[Arouse1973]

Transistors
http://www.digikey.com/product-sear...oducts/transistors-bjt-single/1376376?k=C4468
http://www.digikey.com/product-search/en?vendor=0&keywords=A1695

I would try and test the transistors first. I always prefer to remove them from the PCB first and then test them. But before you do that can you try and measure the voltage rails. This will make sure your not removing something that might not be faulty.

Adam

 

[turrikasd]

Transistors
http://www.digikey.com/product-sear...oducts/transistors-bjt-single/1376376?k=C4468
http://www.digikey.com/product-search/en?vendor=0&keywords=A1695

I would try and test the transistors first. I always prefer to remove them from the PCB first and then test them. But before you do that can you try and measure the voltage rails. This will make sure your not removing something that might not be faulty.

Adam

The output transistors are fine. The problem is in the power supply, which isn't producing the correct voltages.
Thanks.

 

[Arouse1973]

Ok cool. I am guessing it has some sort of transistor/s to switch the coil of the SMPS. Could that be the issue?