Casiotone CT-460 Buzzy, quiet sound

[J12345]

Hello, I am trying to repair a Casio CT-460 for a friend. I am equipped with an multimeter and a home made audio probe.

 

The keyboard is making very little noise that is also buried under static. I can hear the tones play faintly when I press a key. This also applies to the 'Demo" track that plays when the demo key is pressed.

 

I also cannot find any point on any of the boards that doesn't have a layer of static. I've probed pretty much everywhere (including places I shouldn't have) though there are placed where the tone is much louder. Specifically on the D23C module on the M5257 Board shown below.

 

I had the idea to probe all the conductors in the Key to Board ribbon shown below, but the tones I got out of there were strange and nothing like what the speaker outputs.

 

I've removed a speaker because I kept having to flip it with the board and that got annoying. I'll put it back on when/if the keyboard is repaired, but I didn't want the loose wires to confuse anyone.

 

If anyone has any guesses as to what should be checked/replaced, any suggestions would be appreciated!

 

Underside of the Full Keyboard:

 

 

M4254 Board:

 

M5257 Board:

Edited August 19, 2020 by J12345
Added more to M4254 Board

[IanB]

Hmm, I can't find a service manual/schematic online. My first guess is a ground connection that is absent, or maybe a power supply fault.

[IanB]

Also, does the volume control work? What happens to the sound as you raise and lower it?

[J12345]

The volume control doesn't seem to change the volume. I am getting 9.07v at the power input, which was swapped out by the owner of the keyboard.

[Jokeyman123]

Hard to say without closer inspection and chematic like Ian said. but if the volume control isn't working-maybe that pot is bad, or is it a slider? Also could be the amp chip-i think many of the earlier Casios had a dedicated amplifier chip to power the speakers and headphone jack. Justa wild guess. Try jumpering the volume control leads if you can access those, and see what happens. If you get full volume with the right leads jumpered, you've found your problem.

[J12345]

I didn't pay the volume control much mind, but I'll check it out a little further. Since I wasn't getting any clear frequencies going into it I didn't expect it to be the issue. Same goes with the op amp. I looked up the data sheet here and my input and output signals are pretty much identical and I am getting 2.72 volts across my voltage inputs.

[IanB]

Do the front panel controls work? Do the LEDs light up/go out as appropriate? What voltage appears at the power pins of chips like the op amp?

[Jokeyman123]

Another uneducated guess-transistors tend to be pretty frail-can go bad pretty easily-even one bad transistor in a circuit can create problems up and down a circuit-I don't see many in the pictures-and without taking these out of the circuit hard to test. You can test for voltages in circuit-I forget exactly what you need to look for-like a diode there has to be specific voltages for each of 2 pins in circuit with power on-these can blow like a fuse or diode. also check forward and reverse voltages if there are any  diodes in the circuit although these are usually pretty reliable. Make sure your IC's are getting 3-5V typical for this type of circuitry I think-again without schematics I am guessing, And the voltage regulator chips (3-pin with a heat sink) should be putting out either 5v or 12V although I don't see any in your pictures on these boards.

[J12345]

I was getting 2.72v across my op amp. I suppose I could just desolder and test each component individually. But I was thinking, since I can't find a clear tone out of any part of the board with my audio probe, would that mean it is whatever generates the sound in the first place? I'm not sure what component does that.

 

Though, since the tones I'm getting out of the Key to Board ribbon don't seem to be a 'final product', maybe it's feeding the 'processor' of tone bad signals to begin with?

 

(to be clear, nothing I get out of the Key to Board ribbon shown above in the Full Keyboard image sounds like the scratchy tones I'm getting elsewhere on the board, it just sounds like madness)

 

edit: for everyones info, this is a 9v board. I tested one side to ground and got 2.72v, followed that back to a resistor that had an incoming 5v, then back to the module on the right side of the M4254 Board image, above the big capacitor, with the heat sink. I'm not sure what that module does, but the pin with the highest voltage on that module was around 8v.

Edited August 18, 2020 by J12345
Elaboration

[IanB]

Hi J12345, you're going to need to be a bit more precise in your descriptions. When you say "2.72V across the op amp", which pins? The power? The two inputs (+ and -)? One input and the output? "This is a 9V board", do you mean the whole keyboard or a particular PCB? Etc.

 

The ribbon to the keyboard will be carrying digital control signals, you won't get audio out of that. To be precise it's a rapidly switching set of wires that test each key one at a time for pressed or not pressed.

 

The module you indicate on M4254 may be a power regulator IC, or a power amp IC. So the 5 volts may be coming from it, or just feeding it then passing on to the rest of the machine. Are there any markings on that module? Please tell us what they are. M4254 appears surely to be the main power board, it may also have the speaker amplifier on it, it depends what that module (single in line IC) and the one in the middle are. One or both are power regulators.

 

As a general guide, with dual in line ICs, the ones with a row of pins down each side, there will be an indicator of the "top" end which is either a dot or a half circle impressed into the plastic at one of the short sides. If you treat that as the top edge, pin 1 is "top left", then the numbers increase down the left side, then up the right side, and end at "top right". So for an 8 pin IC, pin 1 is top left, pin 4 is bottom left, pin 5 is bottom right and pin 8 is top right. It'll help when we are discussing particular pins and the signals on them to use that convention.

 

So, when you're measuring this 2.72V, which pins are your two meter probes connected to?

[J12345]

I've been going off of this datasheet, which agrees with what you've said. I'm probing the bottom left and top right pins, 4 and 8 respectively.

 

I have also discovered that when I probe pin 8 (VCC) against ground I get my 2.72 volts. However, if I probe 4 (VEE) against ground, I get .02 volts. I don't know enough about op amps to determine if that acceptable. My understanding (from YouTube) says they should have equal and opposite charges, so it needs a negative charge? Excuse my electrical ignorance, but is that even possible with a standard wall outlet?

 

The text on the module I was talking about having 5v reads 'Casio CA6722 801 423'. I've updated the image in the original post to be clear. The only reason I traced the voltage back to this module was because I thought 2.72v was a little on the low side when I first probed it. I found that it went CA6722 > 5v > resistor > 2.72v > pin 8 of op amp. This information may be relevant.

 

Edit: words are hard.

Edited August 19, 2020 by J12345
I can't do words good.

[IanB]

Yes, words are hard

 

2.72V is way too low for the power supply to the op-amp (it's that one on our right of M5257, yes?). It needs a minimum of +4V on a single rail supply. There's not a good reason its power would come from a resistor either, it seems more likely to me that that resistor is actually connecting from the power rail to something else. Basically the PCB trace connected to pin 8 should be a power supply rail and I think it's unpowered, so your 2.72V is just backfeeding from somewhere, maybe through the op-amp. Where is the resistor you are referring to and what value is it?

 

The IC labelled LC7880M is a DAC. Its outputs are pins 1 and 20 (top left and right corners). This is presumably the audio source. Pins 1 and 20 should be connecting down to the op amp which is their output buffer (it's a dual design). Pins 4 and 10 of the DAC should be the power supply to it. What voltages do you measure there? Do you get anything like audio from pins 1 or 20?

 

Also try moving the dryer sheet to a different location where it may be more effective.

[CYBERYOGI =CO=Windler]

The hardware is a relative of MT-540, which service manual AFAIK is on Elektrotanya.

 

And yes, keep that dryer sheet out - if it is something heavily perfumed, the odours may decompose rubber contacts over time or even crawl into chips and damage their package (like known from mothball chemicals based on benzol).