Midi I/O; Keypad Decoding; Metronome Output; Updates And Corrections - Alesis MMT-8 Service Manual

The tape output hardware is simply the 8031 output port P3.2 (pin 12), a pullup resistor
(R40), and a voltage divider (R39 and R39) for achieving a line level output. The output during tape
save or type sync out applications, will appear as a .5V pulse train, but only if the tape out is NOT
connected to a deck (the decks A.C. coupling will distort the output).
While we have heard many complaints regarding tape back up, we have actually found very
few actual tape failures. Most of the complaints arise from user error, so below is a list of successful
backup and tape sync strategies.
1. When attempting to save to a stereo cassette deck, use only the 1 channel (using both channels
may result in odd phase cancellations during playback).
2. Avoid using any noise reduction systems (i.e. Dolby, or DBX) as these can distort the timing of
the pulse train that contains the data.
3. Avoid using adapters for two reasons. 1> Some adapters contain built in attenuators that can
result in extremely reduced levels, both to and from the tape. 2> Oxidation and "wear and tear"
can cause adapters to become intermittent.
4. Always make several copies of each "save". It's especially smart to make copies on at least 2
different tapes as well. This reduces the chances that tape dropouts will cause loss of data.
5. Always use normal bias tapes, as high bias tapes actually end up recording noise, which could
make it past the wave shaping circuitry and cause false triggers.
6. Always verify tapes after saving them. This helps reduce the chances of bad saves. Note
however that the MMT-8 does not compare the tape to the contents of memory. It simply verifies
that the information on the tape is valid MMT-8 data.
7. Experimentation with record and playback levels usually lead to better results.
Trouble shooting tape problems should begin with listening to the data tape audibly. This can
help the technician determine if the problem occurs during tape save or load. If unusual dropouts
are heard then the problem is either just a bad tape, bad cable, or the tape save circuit. Normal
sounding tapes usually indicate a tape load problem. Only practice will help you determine what is
"normal".

5.00 MIDI I/O

The MIDI hardware is a standard implementation. MIDI out begins at the 8031's TXD port
(pin 11) and travels via R6 to the darlington pair Q1/Q2. Note that the 8031's internal pullup is not
very strong, and older units (revision A) may require the addition of an external pullup resistor for
the MIDI out to function correctly (see section 7.7).
MIDI in consists mostly of the opto isolator (U4), protection diode D6, pullup R7, and
threshold resistor R5. Note that the threshold resistor may need to be changed in order to eliminate
false MIDI triggers (see section 7.6).

6.00 Keypad Decoding

Keypad I/O is handled through a simple polling process Each row of the keypad matrix is
pulled low one at a time (via U14 which is memory mapped). If any button along the row is pressed,
the corresponding column input (U22) will appear high. If no buttons are pressed, all column inputs
will appear as a low. D9-D15 and R42-R47 provide protection for the outputs of U14. Use diagram 3
to localize individual button failures.

7.00 Metronome Output

8.00 Updates and Corrections

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