https://philwip.com/2024/03/22/pcb-repair-speed-buggy-buggy-boy/ * Home * About * Contact [ ] Search Menu PhilWIP [ ] Search Menu PCB Repair: Speed Buggy/Buggy Boy Problem No sprites displayed: Diagnosis Buggy Boy is a game close to my heart as it was one of the first I added to MAME back in 2006. It is a very large, incredibly complex boardset: [Buggy-A] [Buggy-B] Luckily, full schematics are available. Probing around the sprite hardware (TC-042 sheets 1-4) with the 'scope, I found that most of it was inactive. Eventually I traced the source of inactivity back to the FMA3 signal, which was stuck low and was gating the other timing signals. [BUGGYBOY_RRCLD] FMA3 is a signal derived from some of the memory refresh outputs (MA [0:13]) of the 6845 CRTC controller on the CPU PCB. These outputs (MA [0:4]) were floating. [BUGGYBOY_CRTC] [BUGGYBOY_CRTC] Replacing the CRTC resurrected the sprites, albeit with some glitches: [BUGGY_GLITCH] Within the space of 5 minutes, a fault developed which turned the tilemap layer completely black and forced it in front of the other layers: [BUGGY_BLACK] The LS378 at IC20 on the video board, which holds the tilemap output color index, had majorly failed, causing all of its outputs to float. It may not surprise some of you that it was a Fujitsu IC. IC192, another Fujitsu LS378, also had two floating outputs, likely causing the jail bars in some sprites. I removed all 8 LS378s, ridding the PCB entirely of Fujitsu ICs to prevent further misery down the road. [IMG_2311-2-2] [IMG_2331-2-1] After fitting new LS378s, things improved somewhat. There was still a fault affecting the scaling of the sprites: The scaling part of the sprite hardware is built around a 6-bit 7497 binary rate multiplier. It produces a pulse train with a frequency proportional to the value written to its inputs. Well, under normal circumstances it does. This particular IC had busted, floating outputs... [Screenshot-2024-03-21-at-09] [IMG_2341-2] A new SN7497N, courtesy of Mouser, fixed the sprite scaling issue and ended this repair... ...until something else failed while the game was left running overnight, causing sprites to glitch when exceeding a certain scale factor: There was now an issue when the x scale factor exceeded a certain value. I suspected one of the gates around the 7497: [image-35] When gate 1 of IC201, a 74S00 quad 2-input NAND gate, was observed with the logic analyzer, the output was behaving like a NOR gate! [image-36] My programmer's IC test facility confirmed this: [Screenshot-2024-03-21-at-14] Replacing IC201 fixed the damn thing once again. A 24-hour burn-in ensured that nothing further broke. Fixes Replace HD68A45SP at IC179. Replace 74LS378 at IC20. Replace 74LS378 at IC192. Replace SN7479N at IC199. Replace 74S00 at IC201. Philip Bennett Philip Bennett Share this: * Twitter * Facebook * Like this: Like Loading... * March 22, 2024 * CategoriesArcade PCB Repair PCB Repair: Narc Leave a ReplyCancel reply Search for: [ ] [Search] Archives * March 2024 1 * January 2024 2 * December 2023 1 * November 2019 3 * September 2019 4 * February 2019 3 * September 2018 2 * August 2018 6 * July 2018 3 * June 2018 6 * May 2018 4 * April 2018 6 * March 2018 4 * November 2017 1 Categories * Arcade PCB Repair * Uncategorized Authors * 1 John Bennett * 1 Philip Bennett Follow Blog via Email Enter your email address to follow this blog and receive notifications of new posts by email. Email Address [ ] Follow Follow me on Twitter My Tweets Powered by WordPress.com. Discover more from PhilWIP Subscribe now to keep reading and get access to the full archive. Type your email... [ ] Subscribe Continue reading Loading Comments... Write a Comment... [ ] Email (Required) [ ] Name (Required) [ ] Website [ ] [Post Comment] %d