Casio MT-800 not powering on

[belzrebuth]

Hi all,

I got a MT-800 that's not powering on and I'm trying to repair it.

I can see power consumption when the switch is on but no activity whatsoever and nothing seems to get noticeably hot.

I measured a couple of 1n4001 like diodes and all the transistors and what appears to be a power supply and speaker amp supply board, they all checked ok.

An opamp has about  6.3V on its positive rail so there's DC going in but since there is no service manual for it available I can't troubleshoot it properly.

I guess there is no negative rail so there's only the 6.3 so far..

I wonder if this should be 5 instead of 6.3 because there is actually a 5V6 diode there.

Is there any other service manual available that could help me with this? I was hoping for another model with the same power section or even more similarities.

I believe that if I manage to get the power section working properly it would be a great first step.

The problem is that there were some corrosion on the other PCB which has indeed severed some connections.

I repaired what I could see but there may be more traces that are cut.

 

 

 

Edited September 20, 2024 by belzrebuth

[pianokeyjoe]

Hi, firstly check that you are using a NEGATIVE TIP CENTER polarity 7.5VDC or 9VDC power supply with at least 500mA. Thats the first thing. ALL older CASIO keyboards use NEGATIVE CENTER PIN polarity power supplies, no exception. Once you have tried to use the properly negative polarized power supply, the next thing to check in the jack itself and the solder points on the jack. You WILL notice solder pad/pin lifting if the jack is loose or wiggly or a hairline crack around that area of the board. The next thing is the most obvious.. the battery compartment for corrosion and greenish or brownish corrosion spots on the boards. Then check the clear mylar flexi ribbon cables that connect between the POWER/AUDIO AMP board and the main BIG SYNTH board. The most obscure thing to check as I have indeed checked this before and was lost as to how it worked but indeed it WAS my issue on a CZ101, the POWER SWITCH section! On the MT800 I believe the switch is a single reddish color switch on the side cheek of the board? I could be wrong but check. That switch has several things goin on besides a simple make or break 2 wire circuit for power. It has several circuits that also redirect battery power and the auto shut off feature and power led signal.

[belzrebuth]

There was a fat trace, gnd I guess that didn't have continuity between the voice board and the panel board.

Restoring that connection revived it so now it works!

I should have dissasembled it completely from the start. There  was some corrosion on the back of the button panel board that wasn't visible without removing the other boards.

[pianokeyjoe]

Glad you did a more thorough investigation and got it resolved!