Vintage Synth Repair

>I tried playing some patches from the ROM cartridge (which didn't
>suppose to be affected by the battery change), and programming some new
>patches, but no sound it heard on neither. Anyhow, I can hear the hiss
>coming from the amplification circuit...

Check the output of the DAC, before any lowpass filtering or anything is 
applied.  If you have a signal there, it's probably fairly simple to track 
down and fix the problem (maybe a muting circuit or something).  If not, it 
may be much more difficult.  I don't have schematics or anything but I sort 
of understand the basic workings.  If the display, buttons, etc. are 
operating, the CPU side of things should be in good shape.  The CPU should 
communicate with the voicing chips (labeled OPS and ENV- OPS is the FM part 
and ENV is the envelopes), probably by a one-way 8-bit buss, and the output 
of the OPS chip should go more or less directly to the DAC (I don't know 
anything more specific than that).  The OPS and ENV chips are custom so it's 
hard to find info on them and basically impossible to get replacements 
(unless you want to take them from a TX7 or something).  I think there 
aren't any other custom chips (at least in that section), so the basic 
signal path can be deduced by looking up datasheets and following traces on 
the board.  Of course, if you have a service manual, it's a lot easier.

Also, if there are any ribbon cables or connectors relevant to these 
portions of the circuit, it would definitely pay to check them...

_________________________________________________________________
Try the next generation of search with Windows Live Search today!  
http://imagine-windowslive.com/minisites/searchlaunch/?locale=en-us&source=hmtagline