behringer eurorack ub2442fx-pro

[Off-Beat]

i had mentioned in a previous post I had what I thought was a peavy mixer coming in for repair as it is blowing fuses. Turns out it was a behringer eurorack ub2442fx-pro. I really don't know how to go about this one with my lack of knowledge. It took over an hour just to get inside the thing! With the internal connections being glued onto the boards it was a nightmare to pull it apart.
It blows the fuse as soon as its powered up. Nothing on the mixer lights up even for that split second. Everything is put together was a lot of difficulty to take apart, I.e the power board where the fuse blows has its own little chassis which is a nightmare to get at. Should this be the first place I look?

[drewl]

Always start with the power supply.

See if you can disconnect the plugs to the preamp and power amp to see if the power supply works without a load.

Check ac lines coming in, see if there is anything shorted going to the power supply.

I bought a 2222fx way back when they came out which had a bad power switch.
Bypassed it and it's still going strong 15 years later.

[Off-Beat]

Thanks I will try that. As I am still learning I am never sure if it's ok to disconnect something and switch the power on without causing more damage. I did notice when putting the power cable into the back of the fixer it felt a bit loose so il have to look at that also.

[lowell]

This is a switch mode power supply correct? If so, it should be approached very cautiously! I'm still a novice with SMPS, but I have read they may or may not work with no load. I've even read they can be damaged by over voltage! Someone, like RG or Enzo, will hopefully chime in here.

[Off-Beat]

Enzo did give a few tips on a previous post but he was unaware of the make and model at the time.

[Enzo]

This is not a powered mixer. But it does have an SMPS. If you are not experienced servicing the SMPS, be VERY CAREFUL. The mains voltage is directly rectified and these can kill you. The primary side circuits are NOT isolated, and they are also NOT referenced to ground. The "ground" in the primary side circuits is actually the negative side of the rectified mains.

There is a small 6-pin connector to the mixer circuits, and a small two pin for 12v. I'd unplug those and see if the SMPS will power up alone. Since it is blowing fuses, I suspect not, the SMPS itself likely has failed. But we need to make sure.

On any SMPS that blows fuses, I always check the secondary side rectifiers first, if they short, it can do that. Those are D4-9. There is no discrete switching transistor, it is inside the little IC2. See if pins 4 and 7 there are shorted together. If so it is toast. The bridge rectifier could be shorted, so check, and a lot less likely but worth checking is the main filter cap shorting. There is a MOV right across the mains input to the SMPS board, it may have given its life.

But before you do anything to it, unplug it and make sure the main filter cap is discharged.

[lowell]

Mental note, first thing Enzo checks in SMPS is diodes on secondary. Will be the first thing I check from now on. Been really getting into SMPS theory. Very cool, and fun. I'd love to know them well enough to be able to fix one. Currently designing one with RGs help. Good stuff. Sorry, slightly off topic.

[Off-Beat]

I disconnected the 6 pin and 2 pin connections and switched the SMPS on, like u said it still blew the fuse. One of the diodes (d8 as far as I can work out) seems to be open. The IC only has 6 pins but they don't seem to be shorted. So I'm hoping replacing D8 will fix the problem. Now I just have to hunt one down as I don't have any lying around.

[Off-Beat]

I'm a bit confused with these diodes actually... I am using the diode continuity test on my DMM. The meter beeps for a split second when I test across the diode then it reads normal. When test it the opposite way they read very high then drops to 1. Any thoughts?

[Off-Beat]

Sorry for the multiple posts, I'm having a look at it now and discovering new things. I found an SCK103 Power thermistor which has a little hole in it and is oc. Is it possible this is causing the fault all on its own or maybe something else has gone wrong and took this out along with it?

[Jazz P Bass]

Attached is the UB power supply schematic.

There are only a few items that will cause the fuse to blow.

Trace it through.

VT1, C22 & C21, C8 & C& should be checked.
If any diode on the bridge rectifier is shorted, result: blown fuse.

If the internal mosfet of IC2 (pin 7 & 4) is shorted, result: blown fuse.

[lowell]

Diodes sound fine. Sometimes a capacitor will cause the meter to read something at first touch, then settle. It sounds like they're fine. I usually go with the SETTLED reading.

[Off-Beat]

Thanks for the info. Maybe it is the power thermistor Causing it since its blew. I believe they're used to stop a rush of current coming in when the power is switched on? Il get a look at the schematic when I am home. When tou say check pins 4 and 7 the IC on this one only has 6 pins.

[52 Bill]

When tou say check pins 4 and 7 the IC on this one only has 6 pins.

If you look at the schematic that Jazz posted, the chip uses 6 pins of what was probably an 8 pin package. See if two of your six pins connect to the same places that the schematic shows as pins 4 and 7.

I don't know what part you are talking about having a hole in it. Please tell us where in the circuit it is located. Even better would be if you could find it on the schematic.

[Jazz P Bass]

The thermister is labeled as "10 ohms".
That is when the amp is cold.
It then drops to near zero ohms at startup.
I do not see that item causing excess current draw.

Here is the pinout for the TOP245Y (IC2)

TOP245Y.pdf