Leon Borre Bally -51 and -56 Sound Repair

Bally sound board type AS2518-51 and AS2518-56.

This type of sound board can be found in the following Bally games: Nitro Groundshaker, Future Spa, Silverball Mania, Space Invaders, Rolling Stones, Mystic, Viking, Hot Doggin', Frontier, Skateball, Speakeasy, BMX, Grand Slam, Goldball, Xenon, and some versions of Flash Gordon. On the board we find a push button to launch a self-test of the sound-board. BUT some games do not have the self test programmed into their ROMs, and there is no self-test at these boards. The games missing the self test are: Nitro Groundshaker, Future Spa, Space Invaders, Xenon, and Speakeasy. On the other hand, the self-test is different in some games. It will launch only a single beep, while the sound is repeated in other games.

The difference between the -51 and the -56 model is, that the -56 model can be connected to a Vocalizer board, while it is not possible to connect the -51 to such a board. On the -56 are some jumpers to convert the -56 board into a -51 model.( downgrading, the jumper settings are indicated on the schematic )

A -56 type board , at the red arrow in the front the connector for the Vocalizer board .

The test .

The test EPROM will test the memory chip U10, the U2 PIA chip, and of course the U3 CPU chip. Either a 6808 or a 6802 CPU chip can be used. A jumper chooses which one to use. If you have a 6808, you need a external memory chip 6810. If you have a 6802 as CPU, the 6810 is not needed. The 6802 has an internal RAM portion, which takes care of the needed memory space. Also, C27 has to be removed if you test the board without a Vocalizer board attached. All the notes about the jumpers and C27 can be found on the schematic.

Place the test EPROM into the socket of the U4 sound rom. If the tension 5 volts is applied, the test will start immediately. A control LED connected at pin 15 of the U3 CPU chip will blink, if the test runs properly. You can then check all outputs of the U2 PIA (6820 or 6821). The outputs should go up and down from 0 to 5 volts. If ok, push the button on the board, this launches the memory test. The blinking of the control LED stops, and if ok, the LED will start blinking again after two seconds. If not ok, the led remains in its state, either a steady off or on. The test will run the memory test continuously, which gives us the opportunity to control the signals at the memory chip.

Start.

All we need is a 5 volts supply, a speaker, and the test EPROM. The test EPROM image is here ... (2716 ) and here (2732) Either burn the appropriate images into a 2716 or a 2732 EPROM. Both will work with the same jumpers and connections. Both images are available, because it is sometimes more easy to come by a 2732 than a 2716.

Everything connected

5v connects to pin 5 of J1 and the ground at pin 6. The speaker is plugged in at J2. We make a jumper between TP1 and TP2. This is because the 12 volts is not being used for the amplifier chip. For test purposes, the amplifier chip works loud enough using only 5 volts. We of course have a control LED connected between ground and pin 15 of the U3 CPU chip.

At the right of connector J1, the red arrow points at the 6 volt connection. The yellow arrow points at the speaker connection. The red wire is the jumper between TP1 and TP2, and above the connected control LED.

The jumpers for a -56 board are; B,C,G,E and H. For a -51 board, B and C. In both cases when using a 6802 CPU chip, jumper B changes to position A, and no U10 (6810) is needed.

Control led

The control LED is a normal led in series with a 470ohm resistor. It is used in many applications on this site. To make one, look at the page, "Repair of Bally/Stern CPU boards." Here's the photo ..

Control led

Practical

When tension is applied, the test starts immediately, and the control LED starts blinking. Also, we hear a smooth but fast heartbeat sound in the speaker. This means that the test program is running fine. If this is not the case, we will look at some basic signals at U3 to find out why. On pins 2, 3, 4, and 40, there must be 5 volts present. The clock signals are at pins 37 and 39. the signal is a strong 2.5 volts at pin 37, and 1.5 volts at pin 39. When these tensions are ok and the clocks are present, the CPU should start. If not, remove the test EPROM (U4), the PIA (U2), and the memory chip U10. Now, check the signals again to see if the CPU is working. There should be signals at the address bus pins 9 to 25, except pin 21, which is ground. Find the signals between 0.5 and 2.0 volts, the same on the pins 26 to 33. There are two possibilities. If the signals are there, the CPU is ok. If there are no signals, replace the CPU.

Now if the CPU runs fine as a stand alone, but the control LED does not blink with the test EPROM in place, see below.

We again remove the test EPROM, and look at the signals arriving on its socket. Are the different data and address signals are there? If not, there may be bad contact at the socket. Look at the selection signals. We need a 0.5 volt signal at pin 19 and a 4.5 volt signal at pin 20. Use a universal voltmeter, a digital test lamp, or a scope to control these tensions and signals. If there is a missing signal, look at the schematic to see where it originates from. Always suspect the sockets first. If it is a address or data bus signal that is missing, bend up the pin of the CPU where it originates from. If the now bent up pin delivers the signal, that means you have a short on that address or data line. You will have to look that up in the schematics. Normally you did find now the fault and can start controlling the PIA outputs. These will "dance" up and down from 0 to 5 volts. If some are missing the PIA is bad. If all are missing, check the socket connections again. All the signals to the PIA including the selection signals are data or address bus signals. Since these were already checked, if there are no outputs at all, the PIA is bad. At this point you must have found the fault.

Memory test

We start the memory test by pressing the push button on the board. The control LED will stop blinking, and restart blinking after about two seconds if all is ok. If not ok, the control LED will stay in his current state. This can be a steady on or off. Anyway the test continuous checking the memory and allow us to measure the arriving signals at the memory chip U10. On pins 1, 14, and 15, we find ground on pin 24 5 volts. All other pins receive signals between 1.5 and 3.0 volts. If that is ok, and the test indicates bad memory, you can be sure that the chip is bad. Otherwise, again suspect the socket. At this point you must have found the fault in the board.

Amplifier and sound generator chip.

To test the amplifier, we just will inject some noise by putting our finger on C5 and C2. This will result in humming from the speaker. If ok at C5, the TDA2002 U9 is good. If humming is heard at C2, the pre-amp U8 LM3900 is good. The humming is louder at C2, and about half as loud at C5. When the board still does not work in the pinball, the only thing remaining is the U1 sound generator. To test U1, we will need a game sound ROM that supports the self test. Install a ROM if it supports the self test (see above), otherwise install one with this ROM image - Burn it in a 2716. Change the test EPROM with the game sound ROM, and start by using the push button. You will hear a sound like this ... If not replace the AY-3-8910.

Remarque

On the schematic for the -51 board, you'll find a pulse generator formed around U7. At the bottom of the schematics, the "notes" say that this IC is not always mounted on the board. In fact, I have never seen a board with U7. If you have a board with U7 stuffed, the control of this circuit is easy. Pulses are supposed to come out continuously from pin 3. If not, replace the 555. The rest of the pulse generator being only three resistors and two condensers.