If your analog to digital converter fails to work properly make following checks.
Improperly Wired AtoD
Make sure you have not misread the orientation of the wiring for the AtoD chip. Pin 1 and pin 8 are on the end with the small notch in the top of the IC. Did you wire it correctly but insert it in the socket backwards?
Double check all wiring connections. Look for dull solder joints and reheat them if necessary. Check for solder bridges which can easily occur where solder connections appear close to each other as between the pins of the IC socket.
Make sure the connections to the 25 pin printer port connector are in the correct positions and that there are no bent or broken pins on the connector. Most of these connectors will have hard to read raised numbers next to the pins on at least one side of the connector.
With a voltmeter check to make sure that the voltage appearing on pins 1 and 3 of the MAX187 or pins 1 and 8 of the TLC548 is 5 volts with respect to ground. A variance of more than .1 volts could mean trouble. If no voltage is detected make sure that an appropriate input voltage is being fed to the voltage regulator. If the voltage regulator input voltage is correct, but the output voltage is wrong double check that you have the regulator inserted in the circuit correctly. If this is not the problem try removing the AtoD chip from its socket and measuring the voltages again. If the voltages now appear correct with the AtoD removed check all wiring connections to the AtoD. If the AtoD feels warm after being in the circuit it may be defective.
Look for Pulses
If you have an oscilloscope or pulse detecting logic probe look for the square wave pulses on the three data wires which feed to the computer printer port. Of course these pulse should only be present when the computer is trying to get data from the AtoD board so you will do this with the program logging data. If no pulses are detected suspect that something is wrong with the driver you are using. Make sure your printer port is mapped to the IO address that the driver file name indicates.
What are you feeding your AtoD?
Are you trying to feed the AtoD with audio directly from a speaker connection? This will not work. The audio will first have to pass through a simple detector circuit before the AtoD can register it correctly.
The two circuits shown above will translate the audio (AC) signal into a DC voltage which the AtoD can follow. The upper circuit is best for audio sources of low impedance such as external speaker circuits of 16 ohms or less. The lower version without the transformer will work with high impedance outputs. The diodes in both circuits are standard silicon switching diodes like 1N914s . The capacitor C and resistor R in each circuit are determined by the amount of smoothing or "integration" you want to apply to the signal. Increasing the value of either C or R will increase the integration. A good starting value might be a 10 microfarad electrolytic capacitor for C and a 100 kilohm resistor for R.