Hello guys , i am a novice engineer and i have an old linear power supply for a video door phone system.
The P.S. has a 12VAC output (good) , a +15VDC output (bad) and a +18VDC (good). DC outputs share same GND.

The problem is when there is no load on GND and +15VDC output i measure a voltage of 12VDC and when i connect a load there is no voltage. +18VDC output works ok.

I have checked the components and changed the electrolytic capacitors but nothing seems to work. Also i changed the BDW94C with one from another donor P.S. that works but it's not working on this circuit.

Do you have any idea?

 

Do you have any idea?

Are you able to trace the board and create a schematic?

 

Better focused pics on the track side, then we can trace it out.

 

hi Andreas,
This image shows what appears to be a solder blob short??

E

 

I can't find any shortcircuit. The path for the +15VDC output is simple enough to trace but i don't understand why the BDW94C switches off when i add a load. I replaces the component with another from a working circuit but does the same.

I am confused!

 

If you can trace out the circuit ( reverse engineer it) the you will understand how it works.

 

How important is it for you to rescue the original power supply or are you just looking to get the video door phone system working again?

If the latter, does the phone system actually need the 12 VAC? I've always kept power supply/chargers for laptop computers because they typically have an output of around 19 to 19.5V DC and at least 2A current in a compact form. Your 18V requirement may tolerate 19V (or drop it with a diode or tw0) and you could use an LM317 linear regulator to drop 19V to 15V DC. If you need the 12VAC, maybe use the transformer you have in the existing supply.

 

The path for the +15VDC output is simple enough to trace but i don't understand why the BDW94C switches off when i add a load

What load? Maybe there is current limiting circuitry - if you are using the video phone 15V line, maybe the video phone is faulty, drawing too much current.

If the "+15VDC output is simple enough to trace" I urge you to sketch the whole schematic which doesn't look too complicated. There are lots of competent people on this site who are willing and kind enough to help but they've already made it clear that they need to see the whole circuit. It doesn't have to be a CAD drawing, a photo of a neat pencil sketch would do fine.