Hi EEVBlog;
I could use some help if you can spare some time. Not really sure what the process on this forum is, so apologies if I get this wrong.
I have a brand new Longwei branded PS-305D-M 30V 5A bench power supply.
So when the PS started at first, it was ok for a few seconds then the smell of magic smoke and then it showed 49.3V and 3.5 or 6.5A depending on the Hi/Low switch.
Now on startup, it is ok for a second then ramps right up to the 49V 6.5A.
There is a burnt out resistor at R144
who's value I do not know (68?, thanks gman4925). This resistor is one of two that feed non inverted input 1 to an LM324N.
There is a resistor adjacent to the LM324N in a spot marked D107. Not sure if label or resistor is wrong. The resistor is 150K. Suppersready has a diode at D107 on their board, but gman4925's board has a resistor like mine.
I've been working through the system slowly trying to understand each part. I'm also using photos to make an image of the traces with components.
Power to the board comes in at the bottom left and gets turned into +27 and --27V at the rectifier. Power then runs through an L7815CV and L7915CV which should regulate to +15V and -15V. From there power goes to power the LM324N quad op amp and HA17741 op amp. There is a short in between +15v and -15v somewhere.
The power pins 4 and 11 show a short (0.5 ohms) on the LM324N. When I test a different LM324N out of circuit there is no connection between the power pins.
I have replaced R144, L7915CV, D107 and the LM324N. Before powering on, the 15v system had a healthy resistance between + and -. Powering on produced the same symptoms as brand new and for 30 seconds everything worked perfectly until R144 burned out again and a short appeared in the 15 volt system.
Thanks for reading and I appreciate any comments.
EDIT: Higher Res images here
http://imgur.com/a/YUEIGEDIT2: The big power transistors 2N3055s never get warm. R106 150? is starting to burn up. I've added to the Imgur album an image that I have been working on for my use understanding the circuit.
EDIT3: Schematics from Longwei are attached.
EDIT4: Found a PNP A1015 in a spot for an NPN transistor at Q104. Anyone know what is supposed to go in Q104? Schematics don't have values on them.