Sega Turbo arcade pcb repair
Sega Turbo is quite a complex pcb, certainly very complex for 1981, and also a bit of pain to repair. The video output isn’t compatible with my usual Sony PVM test monitor (hence the red only display), but also it requires -12V as well as +12V which not all modern PSUs supply. The problem with this board is stretched sprites – in fact all sprites are stretched to their maximum value.
The way the sprite scaling works is quite interesting – unlike the digital processing in most arcade games, the scaling is effectively analog. The clock signal used to read the sprite data can be sped up or slowed down with respect to the scanline pixel clock. So if the sprite clock is slower then the sprite will effectively be scaled bigger across the scanline as the sprite pixels ‘smear’ across multiple scanline pixels.
The current scale value is held in the LS373 at IC1 on the video board – the drives a digital to analog convertor – UPC624D which then drives a UPC159. Notice how the two analog components require -12V – without that the sprite clock will not run at all and there will be no sprites on screen. If the LS373 fails with all outputs high the board will display the maximum scale, likewise if the UPC624D fails with high output you get the same effect as both cases lead to the UPC159 getting maximum voltage. On this board the UPC624D had failed and was replaced to fix the problem.