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Motherboard shuts down (Korg AX3G Guitar processor)

Milos R

New Member
Hello guys,

I was wondering if you could give me some advice for my guitar processor. It's Korg AX3G and the main chip seems to be faulty. When I plug the power cable, processor gives a booting singal by blinking these 3 lines and showing 3G letters on display. In my case, it restarts itself after that, and goes in loop. When I let it loop like this it usually gets stable after few minutes, but still shuts down when I try to play something on guitar - it can't handle the input signal. I was hoping I will find some blown capacitors, because I assumed when I give the processor input signal the main board needs more power to procces it and it shuts down, but all capacitors seem fine. Nothing is burned or something. There are no cables at all. This problem only progressed by time. A year ago, processor restarted "only" few times and then I was able to play guitar without shutdowns. So, it's slowly getting worse.

Video demo of problem: https://drive.google.com/file/d/1OTigGodMnK8Gymr3Ir6M17Iw0tpKFIIj/view

The bumping sound is from guitar amp at the moment when processor shuts down. Do you have any idea for fix?

Thanks in advance,
Miloš
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Hi,

I'd guess the five pin device around top centre is a switching voltage regulator. That could be failing?
Can you see any numbers on it?

Just below the power socket, there is a diode, a brown looking disc then an electrolytic cap.
Is that disc connected between the socket and diode and is it getting warm? It could be a polyswitch type thermal fuse?

Closeup photos of both the topside and underside of the board could be useful, if it is a fault within the unit.


The external PSU you are running it on could also be failing - have you tried a different power source?

It looks like a "universal" psu in the video; is it a fully regulated type and what output voltage is it set for?
Some cheap ones are unregulated and the voltage can vary wildly as the load changes.

Can you measure the voltage and see if that is varying much as the unit keeps restarting? It should be approximately 9V at most and no less than 6V or so at a minimum.
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
all capacitors seem fine
A fault which develops slowly over time could well be due to a faulty capacitor (particularly an electrolytic one), even if it "seems fine". Have you actually measured the ESR of any capacitors in the unit?
Other likely suspects are high value resistors and oxidation/corrosion of connector contacts.
 

Milos R

New Member
rjenkinsgb, alec_t Thank you guys for answering!

Can you see any numbers on it?
Yes, it has three lines of label. It says:
* UTC
* UR3325L
* 00UHBP

Just below the power socket, there is a diode, a brown looking disc then an electrolytic cap.
Is that disc connected between the socket and diode and is it getting warm?
Yes, it is connected, but it's not getting warm (at least it wasn't after 5 minutes of chip working).

It looks like a "universal" psu in the video; is it a fully regulated type and what output voltage is it set for?
It's universal, I took a photo of how it's set (according to marks on processor case, this is the exact voltage for it). You can take a look at all pictures you asked for: https://drive.google.com/drive/folders/1d19nRXNz2_Sm6sX2XtNSLYbyC1qEjPb2


Unfortunatly I don't have the second power supply. I will try to figure out how to measure the voltage and ESR of capacitors 'cause I'm total noob. I do have a multimeter so I'll try to google it. Will come back to you if I manage to measure it.

Meanwhile, anything you can diagnose from these pictures?

Thanks for help and sorry I'm such a bad associate in this feild.

Miloš
 

rjenkinsgb

Well-Known Member
Most Helpful Member
OK, the disc is just a filter, not a thermal fuse.
The 5 pin IC is the voltage regulator.

Can you get a voltage reading from the PSU while the board is trying to start? You could hold the leads on the sleeve and tip of one of the jack plug tips.

If that is stable, try between the tab and the each pin of the 5 pin IC.

The pin nearest the power socket should be near the same as the PSU voltage,

The opposite end pin should have a stable 2.5V, or close to that.
The next one on from that (connecting to the big electrolytic and C21) should be showing as stable 3.3V, or close.

If those are right and not fluctuating, the fault is further in. If anything is varying at all there though, it's a power supply problem of some sort - which is usually a lot easier to fix!
 

Colonel666

New Member
The AC power adapter must be center post negative. Universal one like yours usually has polarity switch. The 'pop' you hear is you capacitors exploding. It shut down to prevent burnout. Look for symbol of center post negative polarity DC power 9volt at 300 mA with 50-60 Htz rating. The longer you use center positive adapter will destroy that device.
 
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