If your MX 70 or MX 80 tape recorder has no audio on all tracks
the odds are that you have just one bad Audio Card that is MUTING all the others..
This is due to a fail-safe design error in the MX70 / MX80 audio cards:
the MUTE line goes directly to the AUDIO OUTPUT IC on each Audio card
but the Audio Output IC's internal mute design doesn't bufer the Mute Command line
- which means if the Audio Output IC goes bad - and they often DO go bad -
the MUTE command line is pulled to ground or pulled high....
meaning ALL of the other audio cards go into the MUTE mode !!
Thus ALL audio cards mute audio and cannot work because of just ONE card's fault.
The fix is easy:
Turn the machine OFF... wait 15 seconds, then pull just ONE audio card
Turn the machine back ON and see if all the other cards are now working or not...
You will need to pull ONE audio card at a time until you find the card that has
gone bad and is forcing the others into the MUTE mode. Once you find the
bad card, you will need to cut the MUTE command input pin at the Audio Output IC
(on most MX70 cards this should be pin 14 on IC #6) Check your
specific machine's schematics as there are 2 diferent Audio Output IC units and
the pin number on the Mute line could be diferent on your machine.
Once you've cut the Mute Command pin at the Audio Output IC, Turn your machine OFF,
wait 15 seconds, and put the "bad' card back in the machine, turn her ON,
and if you're very lucky, you may find that the Audio Output IC is working again,
but usually you will find that the Audio Ouput IC is blown and doesn't work on the "bad" card.
In that case you will have to replace the Output IC in the 'bad' card.
I suggest that you buy a couple good spare Audio Cards if you can find them
and without fail, buy a few Audio Output ICs for the future - they are special
ICs made for Otari and are only available through OTARI.
Note the MX 70 Audio Card schematic is up as a Free Download in the Manuals area
and the MUTE line is marked in red.
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