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Special Force (Bally 1986)

Complete Bally 6803 platform build from scripting through 4K toolkit release. Rich in platform-specific knowledge including solenoid multiplexing, ROM-activated bumpers, and physics tuning methodology. Built by apophis79, sixtoe, and bord1947.

Build Story

Special Force served as a learning ground for the Bally 6803 platform in VPW, documenting platform quirks that differ significantly from the more common WPC builds. The build evolved through a full toolkit conversion with extensive physics calibration against real machine video.

Table-Specific Details

Mechanisms & Hardware

  • Bally 6803 solenoid multiplexing: limited hardware outputs use a relay/expander board to drive multiple physical solenoids from single outputs. VPinMAME solenoid numbers may not map 1:1 to physical solenoids.
  • ROM-activated bumpers fire rapidly ("machine gun effect") via solenoid commands without ball contact during certain game states. Initially broken in older PinMAME builds, fixed in VPinMAME 3.5.
  • Fleep custom bumper sounds needed for ROM-triggered (no-ball) activation since standard Fleep hooks into ball-hit events. Each bumper should have its own distinct sound.
  • 6803 light mapping uses timer intervals rather than directly addressed lamp numbers.

Art & Visuals

  • Barrel distortion correction for playfield photos: Photoshop auto-corrects via Filter > Lens Correction if EXIF data contains the lens model.
  • Transparent/tinted ramps (brown camouflage) appear too dark when toolkit's light level system adds black + alpha. Solutions: use newer plastics material with AO, adjust layer separator, add slight opacity to reduce pure transparency.
  • Bevel modifiers must include -no exp (or -NoExp) in names to prevent geometry export bloat.

Physics & Gameplay

  • 6803-era flippers are "notoriously overpowered" with ~10 degrees more EOS angle than typical.
  • Far right ramp is extremely difficult -- even from cradled ball on left flipper, success rate is very low on real machine.
  • No smooth rails leading to flippers (just plastic + rubber), causing more ball bounce in drain lanes. Ball cradling is easy due to high flipper angle.
  • Physical post Z-height must be 25 (not 0). Origin is at volumetric center, so Z=0 places half below playfield, causing exaggerated bouncing and airborne shots.
  • Standup targets use hidden collision primitives (like sw38o) tilted slightly backward to naturally add bounce and prevent ball pass-through during fast shots.

Known Issues

  • VLM toolkit use_bake subdivision bug inflated triangle counts by 50x (100K to 3.9M). Fixed by -no exp suffix on subdivision modifiers.
  • VPX 10.8 playfield_mesh naming forces static rendering -- playfield will not respond to dynamic lighting changes. Use a different name if dynamic lighting is needed.

Techniques Developed Here

See Also