DRAGON'S LAIR / SPACE ACE PCB SOUND VOLUME CONTROL HACK - September 13, 2002
Written By Shaun Wood

INTRODUCTION

Every move made while playing Dragon's Lair or Space Ace is accompanied by either a beep or a buzz. The volume for these sound effects was fixed for noisy arcade environments. Most of these machines are now located in private homes, and the sound level is much louder than necessary (annoying our wives and waking the children). This document will show you step-by-step instructions to add a control to the main board to adjust the volume of the sound effects. Upon completion, you will be able to adjust the volume in a range from zero to 150% of normal (200% with optional resistor).

PARTS AND TOOLS NEEDED

1K Potentiometer
3.3K resistor (optional)
Soldering Iron
Solder
Vacuum de-soldering tool (recommended)
Wire
Two-pin in-line connector (optional - method 2)

THEORY OF OPERATION

U19 is a General Instruments AY-3-8910 sound chip. Its three independent analog channels are tied together on pins 3, 4 & 38. This output signal is then controlled by a small network of resistors and capacitors (schematic page 2, grids B3 & B4). R3 provides impedance matching. C17 bleeds away any high frequency noise. C18 is a DC blocking capacitor. Resistors R20 and R21 form a voltage divider circuit providing the fixed volume setting. The AC signal across R21 is passed through C18 to the right amplifier U29.

It is the ratio of R21 to the total resistance, which determines the volume level.

In the factory setting:

    Signal out = Signal in * R21 / ( R21 + R20 ) = Signal in * 510/ (510 + 5100)
    Signal out = Signal in * .091

A little more than 9% of the original signal is amplified.

To make the volume adjustable, we need to replace R21 with a 1K variable resistor. With this VR adjusted to zero ohms, the signal out is zero. With the VR set at 1000 ohms:

    Signal out = Signal in * VR / ( VR + R20 ) = Signal in * 1000/ (1000 + 5100)
    Signal out = Signal in * .164

A little more than 16% of the original signal will be amplified.

Option: If your primary interest is to increase the volume of the beeps, then you can also Replace R20 with a 3.3K resistor. Along with the VR, you will achieve a maximum output signal of 23%.

CONSTRUCTION

Method 1 - Mounted on the board:

(Click on the pictures for a larger view.)

 

For this method, your potentiometer should be a board-mount miniature type with the middle (adjustable) lead forward from the other two.

1) Desolder and remove resistor R21.
2) Fill the hole nearer to the middle of the board with solder to close it.
3) Use a vacuum de-soldering tool to remove any remaining solder and open the hole nearer the edge of the board.
4) Align the middle lead of the potentiometer over the open hole and, using a felt tip marker, mark the points on the fringe of the circuit board for the other two leads.
5) Verify that the marked points are over the unused edge of the board and not over any circuit tracings.
6) Use a 1/16" bit to drill the holes.
7) Insert the potentiometer, and solder the middle lead.
8) Solder a wire from ground to one of the side leads.
9) Solder a small wire from the middle lead to the remaining side lead.
10) Set the pot to the mid-point and reassemble your board.

Method 2 - Full size potentiometer to be mounted with the other volume controls:

1) Desolder and remove R21
2) Using lightweight speaker wire (or equivalent), solder two wires into the holes from R21.
3) Solder the other ends of the two wires to the two outside leads on the potentiometer.
4) Solder a small jumper wire from one of the side leads to the middle lead.
5) Option - Install a two-pin in-line connector a few inches from the board. This will allow for easy removal of the board while leaving the volume control in place.
6) Reassemble the circuit board.
7) On the volume-control mounting bracket, drill the appropriate sized hole and mount your potentiometer.

Warning: If the two-pin connector is disconnected during boot-up, 100% of the sound signal will be supplied to the audio amplifier.

DISCLAIMER OF LIABILITY

With respect to documents available from this server, neither the author nor The Dragon's Lair Project, makes any warranty or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process.


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