KORG Tuner GT-2 not working

[pcardoso73]

Hi guys
I have a guitar tuner which I bought in 1995 I think. I like it a lot, since it has a nice analog VU meter :-)
Some years ago stopped working properly and I was told that the problem was the main IC (microcontroller of some kind).
It is powered by a 9V battery, but the auto led which should be solid red, kit is blinking red at the rate of 1Hz maybe. It is like the system is constantly rebooting.

From one of the images, you can see a voltage regulator (78L05) and 2 devices with the reference N44 122.
A guy told me that these should be 82N44 voltage detectors, but I don't know.

I've checked the output voltage of the regulator and it is 4.9V when I apply 12V in the input, but for 9V is only 4.2V. However 4.2V should be good enough to most micrcontrollers.


Questions:
1 - Do you have an idea how to approach this?
2 - If the problem is the uC, how can I discover a compatible part? I don't believe that Korg had these ICs made just for them.

By the end, if I cannot fix it, I will put an STM32 F4, which has DSP embedded and build a new tuner using the same enclosure.
Thanks in advance.
Best regards,

[redwire]

Re: KORG Tuner GT-2 not working

I don't see any reverse-polarity protection, so the 78L05 voltage reg might be damaged if somebody tapped the battery backwards. Or there is an overload on the 5V rail. It should output 5V even with 7.5V input. Forcing 12V to get 4.9V out I wouldn't do, that can make smoke, although the MCU should come out of reset but it's clear the IC is not regulating properly.
I would replace the 78L05 Vreg- assuming it's not running hot due to overload.
The voltage detector IC's go to the MCU, might be for reset and low battery detect.
The MCU is a Korg custom mask-ROM of Hitachi HD6305 family MCU, specifically for the GT-2. Korg KLM-1281 is the assembly part number.

[R_J]

Re: KORG Tuner GT-2 not working

The N44's are likely pre biased transistors (digital transistors) RT1N441S, looks like the base(s) are controlled by ic3 outputs, and the emitters connect to the micro.
There could be another regulator (IC1) near the crystal.

[redwire]

Re: KORG Tuner GT-2 not working

IC1 connects to the MCU reset pin 28. It's got 2KB ROM and 128 bytes of RAM.
Can't see a need for DSP lol

[stj]

Re: KORG Tuner GT-2 not working

thoughts,

the electrolytics have a 2000hour life rating.
the switch could probably do with some contact cleaner,
re-solder the crystal.

[pcardoso73]

Re: KORG Tuner GT-2 not working

I don't see any reverse-polarity protection, so the 78L05 voltage reg might be damaged if somebody tapped the battery backwards.

Thanks for your reply. You've got a good point there. I think I will put an 1N4148 (or even a schottky) in series the the input.

[pcardoso73]

Re: KORG Tuner GT-2 not working

The N44's are likely pre biased transistors (digital transistors) RT1N441S, looks like the base(s) are controlled by ic3 outputs, and the emitters connect to the micro.
There could be another regulator (IC1) near the crystal.

Thanks for your reply. The only ICs present are: MCU, 78L05, 2x N44 122 and a quad op-amp IC. Nothing more. I've measured the voltage at the N44 122 terminal's and it doesn't seem to be a voltage detector since, according to the data sheet, one of the pins must be grounded and all of them have voltage.
Moreover, I cannot see a 0.6/0.7 V drop across any 2 pins which made me think that those were not transistors. Maybe I am wrong and they are but now they are damaged.

[pcardoso73]

Re: KORG Tuner GT-2 not working

Can't see a need for DSP lol

Thanks for your reply. You are right, I only talked about DSP capabilities since the STM32 F4 has one built-in and I was wondering if that could be a good candidate for a guitar tuner, in case this one cannot be repaired.

[pcardoso73]

Re: KORG Tuner GT-2 not working

thoughts,

the electrolytics have a 2000hour life rating.
the switch could probably do with some contact cleaner,
re-solder the crystal.

Thanks for your reply. I will do that.

[R_J]

Re: KORG Tuner GT-2 not working

Thanks for your reply. The only ICs present are: MCU, 78L05, 2x N44 122 and a quad op-amp IC. Nothing more. I've measured the voltage at the N44 122 terminal's and it doesn't seem to be a voltage detector since, according to the data sheet, one of the pins must be grounded and all of them have voltage.
Moreover, I cannot see a 0.6/0.7 V drop across any 2 pins which made me think that those were not transistors. Maybe I am wrong and they are but now they are damaged.

There is a to92 labeled IC1 which is likley a reset or supervisor ic. The two "N44" are what I said they are RT1N441S, Not voltage detectors. They are labeled, DT1 and DT2 (refered to as Digital Transistors) They are self biased transistors (look at the datasheet so you will understand) If you look at how they are connected you will see that the base of the transistors are connected to the output of ic3 (BA10324A) and the emitters are connected to the D0 & D1 inputs of the microprocessor

[pcardoso73]

Re: KORG Tuner GT-2 not working

Hi,
I couldn't see the IC1.

You are a genius ! This is a 1951B 2402, which is a reset IC.

https://html.alldatasheet.com/html-p...1/M51951B.html

So, it makes sense replacing this one, I guess.
I've ordered a bunch of 1N4001 diodes and 78L05, now I will order this guy, as well. What do you think?

update: looking for the component above, I have found as well a 2SC1951B transistor.

Cheers,
Pedro

[R_J]

Re: KORG Tuner GT-2 not working

2SC1951B is a transistor NOT a reset ic
Check the voltage on the ic, The way the ic works is when the supply input gets more than 4.25v the output pin which is low, goes high-low-high, This is what RESETS the microprocessor. The output pin of IC1 should be +5v and not pulsing
Because you only have 4.2V on the input of IC1, that could be a problem as it's reset threshold is around 4.2V
I suspect the reset ic is ok, and either the +5v regulator is weak or something else is causing the 5 volts to be low.

It looks like someone has disconnected J6 at one time, The +5v is supplied to IC3 via this jumper, if you unsolder one end of J6 that would eliminate IC3 as a problem.
I would not apply +12v to the 9V terminals, one end of R5 connects to this voltage and the other end connects to one of the adjustment pots, and then to the micro via resistors R10~R13.

[pcardoso73]

Re: KORG Tuner GT-2 not working

I did exactly what you told me to do.
I've checked the voltage of IC1 and IC1 it is is not going up and down, it is stable.

Next I remove J6.
Voltages before and after removing J6:

Before -> reg out = 4.30V output of IC1=4V
After -> reg out =4.40V output of IC1=4.1V
Thanks.

[R_J]

Re: KORG Tuner GT-2 not working

Hopefully it is just the 5 volt regulator ic, if not I suspect the microprocessor is likely damaged. You could remove IC1 just to be sure it is not loading the +5v line.

[pcardoso73]

Re: KORG Tuner GT-2 not working

Hi
I soldered a new 78l05 and put a reverse biased diode between output and input and it worked.
Now it doesn't reset anymore, but the tuner is working so and so. When in auto mode, even with no sound, the needle moves almost to 75% with no apparent reason.
If I play, it will do its job and then returns to that position. When I was using a dc adapter, in the past, it was doing the same but I had assumed that it was noise coming from the cheap adapter. Now it does it on batteries as well. At least it works and no longer resets.
Thanks for your help.
Best regards
Pedro

[redwire]

Re: KORG Tuner GT-2 not working

To protect against reverse-polarity battery, I put a 1N4004 diode right across the battery input, so that shorts the battery but you can't really keep it on the snap for a long time. Or a series Schottky at the power input. But not a diode between input and output to protect against reverse polarity, that just makes damage worse with a backwards battery.
The unwanted reading might be from a dirty switch in the input jack, between 1/4" or microphone it might not be working clean i.e. the mic is always on?
The two trimpots, one seems to be to calibrate the panel meter, and the other is a bias or gain for the LM324 input amp. Tuning freq. is always precise due to the quartz crystal. I don't think it's the trimpots.

[pcardoso73]

Re: KORG Tuner GT-2 not working

Hi,
Thanks for your reply. I am having this reading, both with the guitar connected to the input jack and using the internal mic when tuning a classical guitar.

Regarding the diode, I will follow your suggestion and move it to the input supply rails. Can you, please, provide me with the reference of a Schottky diode suitable for this application ?

Best regards,
Pedro

[redwire]

Re: KORG Tuner GT-2 not working

I don't know what the meter should read with no signal. You can use a signal generator at 430, 440, 450Hz and check if the panel meter is accurate. Or play a youtube tuning video to it and see if it is accurate.
1N5819 or SMD SS14 are easy to find parts.