Corvette

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Manufacturer	System	Year	Model	Produced
Bally	WPC-Security	1994	50036	5,001

Corvette Playing Tips and Rule Sheet

Corvette Shop Guide

1 Game-Specific Issues

1.1 LT-5 Engine Electronics issues

The LT-5 engine electronics that cause the assembly to "shudder" when the engine revs is a closed loop servo system that can be quite difficult to troubleshoot. Documentation available for the electronics is sparse and sometimes incorrect. By combining the sparse information in the original release game manual with the information in manual amendment #16-9890, a bit more sense can be made of the system.

Ultimately, a collection of 4 coils powered by 20VDC flash lamp power cause the assembly to move. This is accomplished via the following components.

1. WPC-S MPU
2. Ribbon cable from WPC-S MPU to the "Motor Drive Master Board A-18532 or A-19625"
3. Motor Drive Master Board A-18532 or A-19625
4. Specially long ribbon cable from the Motor Drive Master board to the "Slave Board A-18533"
5. Slave Board A-18533
6. Hall Effect Sensor
7. Opto board under the LT-5 ramp
8. Hall Offset Adjustment PCB A-19706 also under the LT-5 ramp (not present in early production games)

Note that there is an error in the schematics for the Master Board. U2 on the master board is not a 74LS374 as indicated. It is in fact a 74LS175 Quad D Flip Flop. The only function performed by U2 is to disable the coils by shunting the enable signals to the TIP-102 drive transistors to ground. The LED signals shown in the schematics aren't present nor are any LEDs other than the "power present" LEDs.

Also not shown on the master board schematics is the voltage regulation circuit that accepts 20VDC and regulates it down to 12VDC via a TO-220 case 7812T.

System Concept of Operations
The system creates a signal called "V/2 Ref" which accommodates 5VDC power circuit fluctuation. The Hall Effect sensor voltage and the V/2 Reference voltage are provided as inputs to an LM324 op-amp at U1. This provides the first voltage reference.

This second voltage is created by the MPU sending 8-bit digital values to the master board. The master board buffers and relays those signals to the slave board. Via J3 on the master board, the ribbon cable, and J3 on the slave board, the digital values are conveyed from master to slave. These signals pull the normally high 12VDC for each data line (D0-D7) on the slave board to ground, creating input to the AD7524 digital-to-analog (DAC) converter. The AD7524 outputs an analog voltage at pin 15.

The first and second voltage references are "mixed" via additional gates on U1 (U1B). That voltage acts as the drive voltage for additional gates on U1 as well as U3 (another LM324) which amplify or attenuate the signal relative to the V/2 reference and a "bias" voltage that is created similarly to V/2 reference. Eventually, voltages reach a level necessary to turn on/turn off the TIP-102 drive transistors.

Probable failure points
At least two instances of the 7812 voltage regulator failing have resulted in U1 (74LS374) and U2 (74LS175) on the master board failing as well as the AD7524 on the slave board. ULN2803s and LM324s are comparatively beefy and haven't been seen to fail as frequently.

Systematic failure diagnosis
under construction...

1. Test each of the 74XX ICs via the diode test. If any gates test bad, even if not connected on the board, replace the IC.
2. Measure the output of the 7812 voltage regulator on the master board.
3. Follow the analog signal provided by the hall effect sensor through the LM324 gates.
4. Test the "disable" signal from the 74LS175 that shunts enables to the TIP-102s to ground.
5. Measure the analog signal output from the AD7524. During T.16, the output voltage should start at about 2.5VDC, increase to about 3.5VDC, decrease to 1.5VDC, then return to 2.5VDC.

http://passionforpinball.com/lt5adjust.htm

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