Korg Forums

[Invader32]

Hi, I've had a korg t3 ex for a while in working condition and the buttons were getting old and unresponsive so after replacing them and testing them the keyboard no longer boots up, just some lines on the screen none of the buttons light up and no sound from the outputs either. The battery is the original one from factory and is obviously dead but I figured it should show at least something if the battery is dead. I've never repaired synths before but I'm not new to electronic repairs either so I don't know where and what to look at to check the problem. I also tried to power it on with the front panel cable disconnected in case it was causing the issue but no luck.
Any help or general guidance will be appreciated for me to have a shot at fixing this thing
Also I've checked for bulging caps on the psu and they seem to be okay too.

[Pasali]

Let's see if I understand you correctly. Have you changed the faulty buttons yourself by unsoldering the old ones and soldering the new ones and after this it doesn't start, or has it been someone else? .When the battery is low, a message usually appears on the screen, but if the fault has occurred just after changing the buttons, either there is a wrong connector or a track or similar on the board where they have been changed has been damaged.

If the fault was immediately after changing the buttons, this is almost certainly the cause of the failure to start.

I would change the battery (it is a very cheap CR2032) and check that all the connectors are tight and also that no tracks have been crossed with the solder or something like that around the place where the buttons have been changed.
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Korg T1 and T3EX

[Invader32]

Finally a reply!, yes I did change them myself and changed them with the proper part, I checked the entire panel myself again for any cold solder joints or excess flux and then put it back in. The floppy drive in this keyboard wasn't working beforehand so I just didn't use it but the keyboard just doesn't even want to boot now. I think I saw a similar post of someone who changed their front panel buttons and the keyboard locked up on the power up screen. While diagnosing I noticed that the capacitor on the top right of the motherboard was not giving any reading on my multimeter (yes I know they can't be tested accurately like this) but i figured it might be broken because I had some spares of that capacitor (50v 1uf) and tested them and they seemed to give a reading so I replaced it too, the capacitor traces were going into a transistor that seemed to control the reset line of the chips. I also checked the motherboard and while replacing the capacitor checked the supply voltages and cleaned the connectors and narrowed it down to be the motherboard being the problem but I don't know where on the motherboard to look at, there is no corrosion on it either. Also thought I should mention the front panel ribbon cable is getting worn down and the traces might not be making a good contact but the keyboard should still boot without it, there are no i/o chips on the front panel board to tell the motherboard if it is connected or not.

[Pasali]

[Invader32]

I really appreciate your determination to help me buddy and I appreciate it, Although asking for voltages from a working t3 is not needed because all service manuals have voltages which I have verified from, what you can do to assist me is check if your t3 will boot without the front panel connector plugged in. This way I can conclude if my problem lies in the worn cable or not, Also I completely understand if you are not comfortable with doing this or don't want to do it I am really thankful to you for even being interested in my rather uncommon repair.

Cheers.

[Pasali]

Hi there. My knowledge of electronics is very limited but I suspect that if I try to start the keyboard with an open connector it could cause some damage to the electronic part and in that case due to the age of the keyboard I find it extremely difficult to find someone who can repair the keyboard. You know the electronical diagram of the keyboard, could you assure that started in that way the electronic part will not suffer any damage?

On the other hand I have located a T3 in Williamston, South Carolina and that currently has an auction price of 140.5 USD. Maybe you would be interested in bidding on it to make test and also have spare parts. This is the link: https://www.ebay.com/itm/284457268232?hash=item423af97408:g:NzUAAOSw4M9hNTf8
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Korg T1 and T3EX

[Invader32]

I completely understand your concern and like I said you don't have to do it, thank you for your help, I will indeed look into this other korg but I doubt it will ship to asia where I live, thank you for reviving this post. I hope I get my t3 fixed up one day. That said if you could provide me with an all data dump from your working korg sounds and all it would be helpful because the keyboard has been non functional for some time and because it's not turning on there's no way for me to backup sysex files from it and the data dumps available online are missing some patches. I understand if you don't have time for this so please let me know either way.


Cheers.

[Pasali]

Count on it. Send me your mail by PM
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Korg T1 and T3EX

[Invader32]

Hello again, I've found what seems to be the problem on the logic board of my korg, a reset IC (responsible for resetting the cpu and mpu reset lines) is pulling the cpu reset line high constantly and thus the boot sequence is not getting initiated. Normally the IC would pull the line high and low with 200ms timing for boot sequence but the line is constantly high. I read the data sheet for the IC the Mitsubishi m5 1951b and it should provide an output of 5v if the input voltage gets higher than 4.25v volts and if negative voltage is used the same will happen but output will be negative volts, but the problem with this IC is that the input on it on the board is 3.8v so the line should be low but it still outputs high. I read some more and found out an inverse ic which will output 5.0v if the input is lower than 4.25v and lower it if the voltage raises. So far this looks to be the problem on my board and unfortunately there is no way for me to fix it due to the part not being available because it is out of production and not available where I live. So bottom line, if your t3 powers on but no lights turn on and it shows lines on the screen, it may have something to do with the reset circuitry on the top right of the board.
As for the motherboard voltages here they are

From left to right, (for the motherboard) pin 1 to 7

+5volts
-5volts
Ground
Ground
+12volts
Ground
-12volts

(For the jack board) left to right pin 1 to 6

-12volts
Ground
Ground
-12volts
+5volts
Ground

If any troubleshooters come across this post and require some info on fixing a korg t3 please let me know and I will respond to as many as I can.

Cheers and goodluck.

[T3owner]

Some observations of your diagnosis...

1) There is no negative 5 volt power supply in the T-series models. Both pin 1 and pin 2 of the 7-pin power connectors (at the supply board and the KLM-1370 main board) should be at positive 5 volts.

2) The "input" to the M51951B should be the +5 supply (as filtered on the KLM-1370). If you are measuring 3.8 volts there but seeing 5 volts at the power supply, I suggest that you determine where/why the voltage drop is occurring.

3) If you are using only a voltmeter to check voltages, if an oscilloscope is available you might want to check how "clean" the DC is. Failing filter capacitors can lead to excessive high-frequency ripple, resulting in misleading readings by voltmeters.

[Invader32]

Wow thanks for this info, I'll check it again asap I may have checked the voltages wrong, but my main concern is that should a voltage drop like this be able to cause the keyboard to not boot? I guess there's really only one way to know, I will have to dig in the circuit diagram to know why this is happening and also the capacitors have all been replaced by the same values, I ordered a kit and replaced them all except for the non polar caps on the jack board. Also is it possible that the reset IC is causing the line to drop voltage? Should I remove it and then check the input pin?

[Invader32]

It seems as though I've been an idiot and to correct my mistake the IC is receiving 5.0 volts at input and outputs 3.8v which then feeds into the positive terminal of the 50v 1uf capacitor. It may be that I'm still wrong so please correct me, but with this conclusion it still seems that the IC is broken. I will respond after verifying the voltages again.

[T3owner]

Thanks for the corrected information. While it's possible the M51951B (IC46) has failed, I wouldn't jump to that conclusion based on what you've determined so far.

Just to make sure you have a proper understanding of the reset function...
Reset is "active low" - that is, the reset pins have to be pulled low for a short time after the T3 is powered up, and then go high and stay there while the T3 remains on. At turn-on while the supply ramps up to 5 volts the IC46 output is low. Once the supply reaches about 4.25 volts, the approximately 200 µsec low is triggered, and then the IC46 output should go high. If you check the IC46 output beyond a fraction of a second after turn-on it will be high (the 3.8 volts you're seeing). Unless you use a scope - best a storage type - you probably won't be able to detect the short low state.

By the way, output of IC46 doesn't directly go to the reset line, it's buffered by two sections of IC45 (a 74HCU04 six-section inverter) in series. Pin 10 of IC45 should also be high when IC46 output is high - let us know what the voltage is at IC45 pin 10.
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Korg: T3EX, 05R/W | Yamaha: Motif XF6 and XS6, A3000V2, A4000, YS200 | Fender Chroma Polaris | Roland U-220 | Etc.

[Invader32]

I verified and indeed pin 10 of IC45 which is also going into the mpu is 5 volts and the psu is working fine.

[T3owner]