Soundcraft 200/400 PSU problem

[Chrome Heart]

Well Ive decided to rebuild this power supply... again. But Id like to know if anyone can suggest any mods/improvements.
And if anyone has an idea, the PSU seems to drop the -17v rail down to -.6v when its under a load. Seems OK when its not plugged into the board. And its not the board, I checked it with an JLM AC/DC supply and it functions like it should with one channel and the master section plugged in. So definitely a PSU problem. I replaced the caps 3 years ago and they look OK. Sadly there are a few obsolete transistors in it too.
cheers

 

[wolfgang]

Maybe you should test all channels of your board with the working JLM PSU! One Channel after the other one.
Maybe a single channel shorts the negative rail???

regards,
wolf

 

[Chrome Heart]

I tested with one channel and the master section with BOTH supplies. The JLM worked and the SC PSU displayed the drop in -17 voltage and pinned VU's. Seems pretty obvious to me that its the power supply if one worked and the other didnt.

 

[MarkusBass]

Most probably you are right that -17V rail is not working. But just to make sure that this is the case you may disconnect the mixer and connect 33 Ohms/5W resistor to the -17V rail and the ground (only for a while). Measure the voltage on the resistor. If it drops below 17V, the PSU is not working. There is not much that can be wrong - this rail consists of two resistors, one capacitor and LM337. The most possible cause of the problem is LM337. Its price is below $0.5. Pay attention that it has to be isolated from the heatsink. Of course you may also look at other components (two resistors) and check the printed board for shorts.
There are few very simpe measurements that would help you to find out the reason of the problem. You can measure the voltage directly after the rectifier (on C2) and voltage on the voltage divider (R3/R4). What are the voltages there?
The obsolete transistors are used in +48V rail and I hope this rail is working correctly. Appart from that, the transistors can be replaced by almost any transistors available on the market so you don't have to worry about the fact that they are obsolete.

Mark

 

[Chrome Heart]

Thanks Markus. I think Im just going to replace everything on the pcb. The adjustment resistors are carbon and Im going to use some matched 1% in there. What Im interested in is any cap value upgrades. I think Ive read some people changing the 22uf to 47. Ive found the LM337 and 317's. Everything looks pretty cheap so I guess theres no reason not to completely rebuild as long as Im in there.