Allen & Heath Xone:02 (DJ Mixer) - Burnt out Resistor

[trickartt]

Hello Badcaps!

I am trying to troubleshoot my Allen & Heath Xone:02 DJ mixer. I have had this for somewhere close to 20 years and have played music through it in one form or another pretty much every single day since I've had it, though these days it's usually from a wireless streaming receptacle (Apple Airport) more than the turntables that are also connected to it.

This week my music stopped working, and after a bit of a wild goose chase, I realised my mixer was dead. I checked the 500mA fuse in the device beside the power supply and found it was blown, so I replaced it with a new one, and as soon as I tried to power it on again, it blew the new (tested good before installation) fuse immediately. I dismantled the device and got down to the main board, where I have found a very burnt-looking resistor, but everything else looks fine to me - no leaky caps or other fouling as far as I have noticed.

I've found the service manual and schematics for this device accordingly (link to manualslib), but I'm a bit out of my depth now. I could (and am willing to) replace this resistor and solder a new one in, but I think a resistor won't just burn out for no reason - presumably something is sending too much electricity to it further up the chain.

Per the service manual, the resistor is the one labelled in the schematics as R205, 82K 1W.

I would welcome your suggestions as to what is going on, and whether I should be replacing something else beside the resistor when I get the soldering iron out.

Thank you in advance for your advice.

[CapLeaker]

Naaaah… it's not the resistors fault. I think you got a short somewhere. Check that mosfet and see if it is shorted. The resistor could have gone higher in value, but it won’t cause a fuse to blow.

[trickartt]

Thank you for the reply! Before I go to bed, I have just given it a quick test with the multimeter (without removing the mosfet from the board) - there is continuity between the drain and the source, with the red and black cables either way around. That probably means it's toast, right?

[CapLeaker]

The burnt resistor needs a bit a cleaning and new solder on the bottom. So you replaced the fuse, plugged in the mixer into power mains and as soon as you press the power button, the fuse blows. Right? It screams there is a short in the primary section of the power supply. Could be a shorted bridge rectifier, blown mosfet, bad PWM (blown and stuck on due to mosfet fail), shorted main filter capacitor...
Sounds a bit taunting, but it isn't that bad.
You answered your own question, but this has to be under right test conditions. The quickest and shortest way to identify a bad mosfet is if all 3 legs are shorted together. Anything else but this, you got to take the mosfet out and properly test it with either a component tester or turn the mosfet on and off with your DMM in diode mode. How this is done? There are tons of YouTube vids about "How to test a mosfet". This mosfet has a body diode, so you should see it. If the mosfet is bad and look for a replacement, search for a "3N80" and pay attention if its tab exposed (metal) or encapsuled in plastic (full pack).
If you see the all 3 legs shorted, order a replacement for the UC3842 PWM just in case.
Check the diodes and remember: If something doesn't check out, take the component out or lift one leg and check it out of circuit.

[trickartt]

Sorry it's taken me so long to acknowledge the reply you were kind enough to send.

I pulled the mosfet from the board and tested it with my multimeter and it checked out as good. Eventually I plucked up the courage to order a whole bunch of components and trial-and-error it.

I replaced, in this order:
The mosfet
The PWM
The resistor that was showing the dark marks, just for the hell of it
Diode D6 (not sure how I decided on ordering one of those to replace, but I did)
The bridge rectifier

And testing after each replacement blew another fuse.

There is a separate little board that the power supply comes in on (which hosts the fuse that keeps blowing), and I tried powering the device on without this main board plugged into that board. It didn't blow the fuse, which makes me think the problem definitely is on the main board. After I did that, I replaced the D6 diode I mentioned, and plugging it in that time got a particularly spectacular fuse blow - it emitted a pop sound, and when I went to remove the fuse to check it, the glass was completely shattered. I wonder if that's a clue or a red herring?

I think if I want this mixer to live, I'm going to have to just drop the cash for an entire main board assembly replacement - I called the company today and they said they still have a couple in stock, but I'm waiting for them to get back to me with the price!

Thanks again for replying

[CapLeaker]

On the bridge rectifier measure with your DMM in diode mode pin 1 to pin 4. Do the same for pin 2 to pin 3.

I assume you put the new parts in the correct way, not backwards?

[trickartt]

I hope I put them in the right way - the one I was least confident in was the PWM, because I got the same part number for that one, but they looked different - the original one had a notch centre left (that was also marked on the circuit board) and the new one had a spot bottom left, so as both markings were on the left and the text on the chip was the same way up, I figured that was the right way.

On the bridge rectifier:
On the one I took out (KBP206G), measuring 1>4 gives me a reading that when I checked it was about 1.7 and climbing infinitely in one direction, and nothing in the other direction. Measuring 2>3 gets nothing in either direction.

I replaced it with a 2KBB60 - the service manual says the chip is a 2KBP06M, and the component shop I went to said that either the KBP206G or a 2KBB60 were a replacement for a 2KBP06M. I would've chosen the KBP206G if I had already pulled it from the board and saw that's actually what was there, but I took the wrong 50/50 choice! Oh well.

Measuring the 2KBBP60, which is still in the board, measuring 1>4 in either direction yields a number that drops rapidly to 001. Measuring 2>3 got me about .82 in one direction only.

I wish I understood what any of this means!

FWIW the bridge rectifier is the closest chip in the circuit to the toasty looking resistor I originally found.

Thanks again for still bothering to reply

[trickartt]

In which I concede I am a total noob - the 2KBB60 actually has a completely different leg layout (-~+~) to the KBP206G (+??-), despite having matching dog ears. I think I'll be ordering a new KBP206G!

Thanks for your time

[CapLeaker]

Yeah, you put in a wrong full wave bridge rectifier. This one has a different pin out. No wonder chit blows up more violently. That’s not good. Now you may have damaged more parts too… ugh!
Take the bridge rectifier out, replace the fuse and take the measurements again. If you do get a short on the DC side with the bridge rectifier removed (the outer 2 pins) you’ve blown something else up.