midiutils.org

These schematics were given to me by an anonymous person to post here on the site.

NOTES ABOUT THE SCHEMATICS:

- I have tried very hard to reproduce them accurately, and to decipher to the best of my ability, the parts that were unclear. However, screen fatigue sets in when you're looking intently for many hours, so I may have missed a few things. I don't think so, but for anyone wishing to proof read them, it is essential that you all download the originals, which I will also be providing here as well.

- there were a few components that were almost unreadable. I have a bit of a knowledge of electronics, so I had an educated guess as to how it should have read. These parts are:

- transistors Q1 and Q2, near the speaker. I think I have fairly well interpreted what was supposed to go there, but if someone with an extensive electronics knowledge could confirm or correct, I would appreciate it.

- The tech guy who I got them from drew a pinout diagram for the LCD DISPLAY chip, underneath it. It is almost impossible to read, so I removed it. It is still on the original, however.

- Errata: R25 should be connected to the CPU TxD.

POINTS OF INTEREST:

1. POWER SUPPLY CIRCUIT

- Although the circuit is powered by a 9V AC plug pack, it runs internally off +5V DC, and +10V DC in various places. It is very possible to convert it to run off a 9VDC plug pack. Input power jack J1 connects to the power switch, which then feeds a power diode D2 and voltage regulator VR1. The diode simply bocks the negative component of the AC sinewave, and passes onto VR1 a pulsed DC voltage. This is then regulated and smoothed to +5VDC by the capacitors C7.
If you were to plug in a 9V DC power supply, provided that the polarity was correct, with the tip of the plug as +9V, then diode D2 would not need to block any negative voltages; in fact, it would probably work as a reverse polarity protection on a DC plug pack, if it was. VR1 would then simply take the +9V DC and regulate it to +5V DC.
NOTES FOR ANYONE WISHING TO TRY IT:
- I TAKE NO RESPONSIBILITY FOR ANY DAMAGE TO ANY OF YOUR EQUIPMENT, DO THIS AT YOUR OWN RISK!!!!

SUGGESTION:
- Make sure the MMT8 is switched OFF. Start by plugging into it a multi-DC plugpack that has several selectable voltages, from 3 to 12V DC maximum. Check that the TIP is POSITIVE (CHECK THIS WITH A MULTIMETER - OR ASK SOMEONE ELECTRONICALLY KNOWLEDGEABLE). Set it to a low voltage, eg. 4.5v or 6v DC. Make sure the MMT8 is switched OFF. Plug it into the mains power, switch the plug pack on. Now switch the MMT8 on. At this low voltage, it may not function correctly, or at all. If it does light up, but will not correctly function, switch the MMT8 off. Increase the plugpack voltage to 7.5V. Try again. If no luck, keep going up to 9V DC until the MMT8 functions stably. Then leave it at that voltage. Don't go higher, if it is working properly. If it will not switch on at all, even at 9V DC, check the polarity of the plug pack again.

IMPLICATIONS:
- It may then be possible to run the MMT8 off BATTERIES!!!!

2. SPEAKER CLICK DISABLE MODIFICATION
- It is very easy to connect an on/off switch for the speaker, when using the click out. Simply disconnect one terminal from the speaker, install a switch in between. Now you can run the click into a mixer/headphones, and switch the internal speaker off.

3. MEMORY BATTERY
- note where it is, and that the circuit is reverse polarity protected by diode D14. Just for reference, when changing it.

4. LCD BRIGHTNESS/CONTAST ADJUSTMENT R67, on the LCD DISPLAY CHIP.
- varying this resistor or trimpot adjusts the display intensity.

Hope this info helps anyone repair or develop this great sequencer!!!