                        ST 520/1040 - Stereo Modification                      
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Introduction.                                                                  
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     If you have a stereo sound monitor, e.g. the Philips CM 8833, you can     
modify the sound output of the ST to provide a true stereo signal.  The sound  
chip, YM2149, has three independent sound channels, A,B and C. In the          
production ST these are simply joined together, hence the monophonic or single 
channel output signal to the monitor (or TV via the modulator in the STM or    
STFM).  If you are highly experienced at electronics and are willing to modify 
your ST's circuit board, you might like to attempt the following work.  You    
will need to cut tracks on the board.  Obviously the internal metal screen has 
to be removed as well.  If your machine is still under warranty, this will     
certainly invalidate it.  I take no responsibility for any damage you cause to 
your machine.  It is entirely your responsibility if you decide to undertake   
the work.                                                                      
                                                                               
The circuit.                                                                   
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     A circuit has to be assembled on a small piece of circuit board.  I used  
a short strip of veroboard. The circuit is needed to place the three sound     
channels in different positions in the sound field.  Channel A will come out   
on the LEFT, channel B in the centre (i.e. fed to both LEFT and RIGHT equally, 
this is termed panning) and channel C on the RIGHT.  The result is a true      
spread of stereo sound between stereo speakers or headphones.                  
                                                                               
     I have not given a component layout or a list of components since there   
are so few.  The total cost should be less than 2 pounds and some of you might 
be able to build it from your spares box.  The circuit is given below.         
                                                                               
                                                                               
  YM2149   pin 4 >----------------:                                            
                                  |                                            
           pin 3 >----------------(---:  :---------------------------> left    
                      :-----------(---(--:                                     
          pin 38 >----(-----------(---(--------------O---[  ]-----O--> right   
                      |           |   |              |   10k      |            
                      O---[  ]----O   :----:         |            |            
                      |   10k     |        |         |            |            
                      O-----[  ]--(--------O---------(---[  ]-----O            
                      |     10k   |        |         |   10k      |            
                      |           |        |         |            |            
                     [ ]8k2      [ ]1k    [ ]1k     [ ]1k        [ ]8k2        
                     [ ]         [ ]      [ ]       [ ]          [ ]           
                      |           |        |         |            |            
                      |           |        |         |            |            
           pin 1 >----O-----------O--------O---------O------------O-->earth    
                                                                               
                                                                               
       [  ]  or  [ ]               ( bridge, no join      : bend               
                 [ ]  resistor                                                 
                                   O join                                      
                                                                               
                                                                               
                                                                               
ST modifications.                                                              
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     The circuit board around the YM2149 has to be cut carefully to separate   
the three channesl. Pins 3,4 and 38 are joined. 3 and 4 are joined at the pins 
on the top of the circuit board (chip side). Carefully scrape away the join.   
On the underside there is a strip joining pins 3 and 38. Scrape a gap in that  
too.  The last one is more difficult.  A track leaves pin 38 and goes across   
the board to join to the negative end of capacitor C12 (on the 1040 at least ) 
which is the coupling capacitor to the sound output amplifier. It is situated  
near the monitor socket.  If you trace its path, you will find it travels      
underneath the YM2149, between the rows of pins (underside of the board). This 
is a convenient place to break the circuit to the mono (normal output).        
                                                                               
Fitting the supplementary board.                                               
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     The extra circuit should be connected by short flying leads to the pins   
of the YM2149;  the appropriate leads to pins 1,3,4 and 38.  The stereo        
outputs LEFT, RIGHT and EARTH should be taken by screened leads to the monitor 
output socket.  I have broken the tracks at the 13 pin DIN socket so that pin  
1 (the normal mono sound output) becomes the left sound channel and pin 2 (the 
composite video output - not used since a stereo monitor is being used!)       
becomes the right sound channel.  Obviously you will have to make up a stereo  
monitor lead too.                                                              
                                                                               
And Finally!                                                                   
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     The stereo modification will give you glorious stereo on music and sound  
effects, where the programmer has made good use of the YM2149's three          
channels. On some arcade games the effect is stunning!  However, you will find 
that sound samples will give very hissy results.  This is because some         
samplers use the individual channels to take parts or bites(!) of the sound    
waveform.  Therefore the sound channels must be joined or added together i.e.  
you must revert to the channels being joined together!  What you need is a     
stereo/mono switch.  This is easily done.  Wire a two pole two way switch so   
that pin 3 can be joined simultaneously to pins 4 and 38 of the YM2149.  With  
the switch one way, the pins are separate (stereo) and the other way the are   
joined together (mono, for sampled sound effects).  Programs that use ST       
Replay will need to be played in mono.                                         
                                                                               
Good luck!  Copyright of Derek Green 1988. For use in public domain, NOT TO BE 
SOLD for commercial gain.                                                      
