Trying to repair a KORG KP3's blown power rail. Am I on the right track? - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: August 01, 2023, 05:02:27 am
content: I have a KORG KP3 Kaoss Pad that has been plugged into 220V by mistake.The schematics are here. Power supply comes in bottom right of page 5:https://www.manualslib.com/manual/1061558/Korg-Kp3.html?page=5#manualThe AC adapter got fried, and a couple of ferrite beads (L8 and L12) physically blew up, but apparently something else got damaged as well. When plugged into my bench power supply the current limit kicks in (700mA) at about 6.8V (then voltage goes down slowly). The unit normally requires a 12V 700mA AC adapter. So I'm guessing there's a blown component or two that short to ground, or no longer ground as they should.I've poked around the the power circuit on the rails leading to the A, B, C, D or E connections, and nothing appears obviously faulty (to my newbie eyes). However, the leg going up into R1, ZD1, WD1 and Q1 may be where the problem is located because there's nothing coming out of Q1 pin 3. I find the same limited voltage on both pin 1 and 2 of Q1.I was expecting some voltage there, to power some other part of the circuit. I think I understand the 10V voltage regulation part of this circuit, but not the role of Q1 (2SA812 PNP).I tried to replicate this bit of the circuit on Falstad to visualize what it should be doing, and to my surprise there's no real voltage showing up on pin 3 or Q1 on the simulation either. Though my simulation setup may not be correct:https://tinyurl.com/28w3md2qAm I on the right track? Have I taken 2 wrong turns? Any hints?Any insights appreciated! Thanks!

author: cincin
author_rank: Regular Contributor
timestamp: August 01, 2023, 06:47:11 pm
content: Upon closer inspection, that part of the circuit appears to be the mute drive. Which makes sense the way it goes to the 4 transistors at the line out and and phone out... So then it would be normal for there to be not voltage out of pin 3 of Q1 until the mute drive is activated. I'm not sure how that happens though.I guess I'm back to square one.Any tips on how to trouble shoot this further is greatly appreciated.Cheers!

author: Manul
author_rank: Super Contributor
timestamp: August 01, 2023, 07:47:44 pm
content: Could you feed 6V and write down all A,B,C,D,E voltages? If current is high, don't hold it on for a long time, put your multimeter probe and briefly turn on the supply, and so for every voltage test point. After the measurments are done we will think.

author: cincin
author_rank: Regular Contributor
timestamp: August 01, 2023, 10:12:42 pm
content: Absolutely!When feeding it 6V the current is about 0.323A and I get the following voltages at the various power rail connections:A: 4.063VB: 2.695VC: 2.695VD: 5.397VE: 2.07V5.397V at D is the same voltage as I get coming out of the power switch. That suggests to me that maybe there's something wrong in or around F1 or IC8 (D2, C40, C35, C25), since that part of the circuit should be taking 12V down to 3.3V.I'm not sure I understand why/how F1 (CPH6316, a MOSFET) and IC8 (S-8520F33MC, a 3.3V switching step down regulator) should play together in parallel to achieve the 3.3V. But since the voltage reduction is what's not happening, I'm starting to suspect IC8. Correct?Thanks for the assistance!

author: Manul
author_rank: Super Contributor
timestamp: August 01, 2023, 10:52:02 pm
content: IC8 and F1 are not "in parallel". IC8 is PWM controller, it simply controls the external switching mosfet (F1) which together with L3, D2 and output capacitors forms what is known as buck converter.With the unit unpowered, check the resistance between F1 drain and source. If the mosfet is shorted, then replace both the mosfet and IC8. If it is not shorted, it might be that the replacement of IC8 will be enough.BUT the problem is that because of this failure full input voltage went into 3.3V rail. 3.3V is used for many digital circuits. If you ask me, it is more then likely that they are all fried...

author: cincin
author_rank: Regular Contributor
timestamp: August 02, 2023, 01:56:56 am
content: The MOSFET is indeed shorted. Thanks for the help in figuring that out.So you say chances are pretty good that there's more damage downstream. That would make sense. I'm tempted to replace F1 and IC8 just to see what is else got zapped but, looking downstream from the 3.3VD rail, the ACD probably got it, but then the LED controller probably too, and maybe the LEDs all blew up too... yikes.I will look into testing the bits downstream if possible, ad I'll look at the cost of these parts to see how much I'm willing to spend on replacing everything.Much appreciated!

author: Manul
author_rank: Super Contributor
timestamp: August 02, 2023, 10:46:23 am
content: Also point E (DSP core supply?) is much too high. Most likely the DSP is toast, backfeeding to the 1.26V supply. Or maybe (less likely) IC6 is bad. In any case, lots of problems. Likely not economical to repair.