
The Bally 2518-32 Sound Board, was used in several early games. When these fail, one (of many) things that can fail is U9, the LM741. Sometimes, replacing that IC with a new one does not fix the problem. That is because Bally was using the LM741 out of spec.
The LM741 is designed to run with a + and – DC supply. V+ should be on pin 7 and V- on pin 4.
In the Bally 2518-32 Sound Board, Bally grounded pin 4 and applied +12V DC to pin 7. The Bally board does not have a -12 power supply, so this appeared to be expedient.



In the -50 sound board, Bally simulated the + and – supplies by raising the input, pin 3 with a bridge resistor network, to about 6 V. This ‘makes the LM741 think’ that it has a +6 and a -6 power supply applied, since pin 3 (or 2) is the reference voltage.
If replacing a blown LM741 on a Bally 2518-32 sound board, it maybe necessary to make this modification. From some testing, it appears that Fairchild will not work, but National Semiconductor chips will work without this modification.
Additionally, solder a 1 uF cap on pin 3 of the 741, negative to ground, as shown in the -50 schematic.
With this modification, any LM741 will work on the -32 board, regardless of the manufacturer.
The original discussion for this has been archived on the Newsgroup, rec.games.pin and is accessible from Google Groups.