STF Stereo Modification

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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

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

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

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!

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.

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