Eurorack UB1222FX-PRO troubleshooting help (power supply)?

I got a Eurorack UB1222FX-PRO sound mixer/whatever from my school a while ago. It was broken and wouldn't turn on so that's why I got it. I'm not exactly sure whats wrong with it but im pretty darn sure that something is up with the power supply as nothing on the board is even powering on. But there is some power coming out of it to the main board but im not sure if its just not enough power or what. I know my chances of anyone on here knowing whats up are pretty slim but I don't have anything else left to do, and it doesn't hurt to ask.

Im not exactly sure what to be testing or how to test things for that matter. It doesn't look like anything is wrong with it. I have a digital multimeter but it doesn't have many settings and im not sure if im testing parts on the right setting. sometimes i get readings and sometimes i dont. Anyway, if someone could maybe guide me through a troubleshooting process that could possible narrow the problem down that would be great. if not ill just scrap it and steal all the parts out of it. 

If you need more pictures or more information i'd be more than happy to provide them. 

EDIT: I doubt this will help any but here is the manual: http://www.behringer.com/assets/UB1222FX-PRO_M_EN.pdf

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iceng3 years ago

Good pic, I would eliminate the possibility of the barely visible gas smearing on the light blue component and possibly the over heat bulging of the big EL capacitor, not enough light to say from here.

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knex.gun.builder (author)  iceng3 years ago

I believe it is just dust on that light blue cap. As for the large cap, it did have a black shield over it that I popped out so I could look at it better.

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Regrettably that leaves the power devices as a tough to ID suspect.

These supplies rectify input AC and drive the transformer at high frequency also changing the voltage and use optical feedback.

Track the AC input to the rectifier and track the DC to the power transistors and see if there is AC on the transformer.

knex.gun.builder (author)  iceng3 years ago

Ok, well, I can't seem to get any sort of reading from the transformer; input or output. Also, I found a schematic for it. heres a link to the website if the uploaded one doesn't work. http://elektrotanya.com/behringer_ub_spsu2_power_supply_ub_mixer.pdf/download.html

so no continuity? That's very strange. I would suspect your meter! What was the state of the fuses again? I'd power it up if the fuses are OK and see if that 400-ish appears on the cap. The ic under the primary would blow before the transformer died.

knex.gun.builder (author)  steveastrouk3 years ago

My meter is fairly new and i've only used it a few times but, yes, it does seem to act up sometimes. But, it only seems to have happened when working on this item. Maybe I'm not on the proper setting? I'm pretty sure I am though. The settings are clearly labeled... I did just check the capacitor though. I'm only getting a 160 volt reading. Would the the board not being screwed in have anything to do with a lower voltage? There are contacts on the corners of the board that go directly to ground and they aren't connected because I can't get under the board without them being disconnected.

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Where are you in the world ? If you're in the USA, I'd expect to see roughly what you have.

It may well be your meter won't operate properly at the frequency the thing is working at, but I would expect you to be able to measure continuity through that transformer.

knex.gun.builder (author)  steveastrouk3 years ago

I'm in the US. So, I am now reading 160v on the input of the transformer. I'm not sure what I did wrong the first time or whatever but it's there now. As for the output I was getting a reading that continuously went from 0 volts to 200 volts in about 35 volt increments. Once it hit 200 it would say OL and start over. I'm not sure what that means..

OL= Overload !!!!

There's going to be upwards on 400V on a very large capacitor at the front end, if the fuses are intact. Usually the input transistors just after the cap fails, and we buy a new PSU....

knex.gun.builder (author)  steveastrouk3 years ago

I do have a 450v 150uf cap i could replace it with. not that it will make much of a difference but it will be able to take a little more voltage. I'm assuming the input transistors are the three on the side?

No, I'm just warning you, this thing has enough capacitance and voltage to kill instantly.

The "three on the side" are linear regulators - one is a 7805, I'll lay odds the others are a 7815 and a 7915.

Looking at the schematic, the others are the power devices for discrete voltage regulators (lots of bits)

Start simple and work your way up. First thing you'll want to do is check the fuse. There is a little door above the power plug that houses the fuse. Check it and replace if necessary then see if it works.

Only then do you want to actually pull the thing apart like you've done.

knex.gun.builder (author)  mpilchfamily3 years ago

Yes, that was the first thing I checked.