Bally / Midway MCR¶
Tron, Satan's Hollow, Kick, Journey, Two Tigers. The American base of the early eighties — the one behind one of the most famous arcade games there is. A Z80 clocked by a crystal whose value you would never guess, and a sound stage of its own.
Identification¶
| Field | Value |
|---|---|
| Manufacturer | Bally / Midway (USA) |
| Years | 1981-1984 |
| Master crystal | 19.968 MHz |
| CPU | Z80 @ 2.496 MHz (19.968 ÷ 8) |
| Timer | Z80 CTC — on the same clock as the CPU |
| Sound | dedicated SSIO board |
| PCB format | proprietary |
19.968 MHz — a value you would not guess
It is neither a colour crystal nor a round number. 19.968 MHz is specific to this platform, and the Z80 takes its 2.496 MHz from it by division by 8.
In other words: measuring ~2.5 MHz on an MCR board's Z80 is normal — do not go looking for a standard 4 MHz.
Expected clocks — measurement table ✅¶
| Component | Expected clock | Role |
|---|---|---|
| Z80 | 2.496 MHz | Main CPU |
| Z80 CTC | 2.496 MHz | timer — same clock as the CPU |
The CTC shares the CPU's clock
The driver says so explicitly (same as “maincpu”). A clock missing on one therefore implicates the other: this is not two possible faults, it is one source.
The SSIO sound stage¶
The sound does not live on the main board: it has its own dedicated SSIO board.
An I/O map partly deduced
The driver records that the upper part of the I/O map is determined by a PAL, and that only the SSIO ports are verified from the schematics.
The rest of the map is therefore reconstructed, not documented. On this platform an I/O anomaly cannot be compared against a sure reference.
Discs of Tron — one manual, two cabinets, and an optical assembly ✅¶
What the manual covers, and what it does not
The Discs of Tron Parts and Operating Manual (Bally Midway, © 1983, 36 pp.) documents two cabinets in one document:
| Model | Number | Breakdown sections |
|---|---|---|
| Upright | No. 696 | front · rear access |
| Environmental ("enclosed") | No. 383 | front · rear access · ⚠️ control area |
ⓘ The environmental cabinet has a section the upright does not — the Control Area, with its own parts list.
⚠️ But the operating text names a third cabinet the breakdowns do not cover: the Cocktail Table, cited for its two-player rules and its safety interlock. This manual gives no parts for it. That is a limit of the document, not of the model.
⚠️ And a difference in play, not just in joinery: "The Upright and Cocktail Table models all have two player operation (the ENCLOSED Upright model only has single player operation)." The enclosed cabinet is one player only.
⚠️ \"Must be grounded\" — for the components, not for you
The warning at the head of the manual is not the one you expect:
THIS GAME MUST BE GROUNDED. FAILURE TO DO SO MAY RESULT IN DESTRUCTION TO ELECTRONIC COMPONENTS.
ⓘ Most earthing warnings speak of electrocution; this one speaks of breakage. Both reasons exist, and this manual picks the second.
ⓘ The second American manual on this wiki to carry a bilingual French warning — after the Wells-Gardner 19″: under the CSA C22.2 No. 1 reference, the text is repeated in French for the Canadian market, "pour prévenir chocs électriques ne pas utiliser cette fiche polarisée avec un prolongateur […] sauf si les lames peuvent être insérées à fond sans en laisser aucune partie à découvert".
Rack advance — skipping levels to test, in order ✅
The manual describes a service feature that saves playing for an hour to see a late level: "Your NEW game is equipped with a RACK ADVANCE feature that can allow you to view and/or play the higher racks for test purposes."
- switch the game on;
- open the coin door. ⚠️ ⓘ "COCKTAIL TABLE MODELS ONLY — gently pull out the plunger on the Safety Interlock Switch located just inside the coin door";
- with the Test Credit Button, put one credit on;
- press the "1 PLAYER" start button;
- ⚠️ IMMEDIATELY set the Self-Test switch to ON;
- then press either one or both "1 PLAYER" / "2 PLAYER" buttons to advance one rack — repeat for each rack you want;
- set the Self-Test switch back to OFF, close and lock the door.
⚠️ The order matters: credit and start before the self-test, and the word immediately is the manual's.
The cabinet: a mirror, two \"not seen\" scenery panels, and a black light
ⓘ The parts list describes an optical arrangement, without ever naming the principle:
| Part | Number | Printed dimensions |
|---|---|---|
| MAIN VIEWING GLASS | 0696-00917-00XF | 23″ × 16-1/4″ × 3/16″ |
| ⚠️ MIRROPANE | 0383-00913-0000 | 23-1/32″ × 19-11/16″ × 1/4″ |
| ⚠️ SCENERY — RED (NOT SEEN) | 0383-00901-00XF | 23-1/16″ × 17-1/2″ × 1/8″ |
| ⚠️ SCENERY — BLUE (NOT SEEN) | 0383-00902-00XF | 23-1/16″ × 17″ × 1/8″ |
| SCENERY — REAR | 0383-00904-0000 | — |
⚠️ The "(NOT SEEN)" is in the part description, not a comment: those two painted panels are not looked at directly. With the MIRROPANE between them and the main glass, the cabinet composes an image the screen alone does not give. ⓘ This wiki describes what the parts list lists; the manual does not name the optical process, and nothing is added here.
ⓘ And the lighting takes part: an ARENA BLACK LIGHT W/O FILTER ASSY (0696-00048-0000), ⚠️ "LOCATED BEHIND SPEAKERS", with its 18″ black light lamp, starter, 12″-lead socket, ballast and a line filter. A second black-light assembly lights the header. Looking for a dead lamp behind the speakers is not a far-fetched idea on this cabinet: that is where the manual puts it.
ⓘ The controls: the parts list names an OPTICAL ENCODER DISC ASSY, a KNOB & SHAFT ASSY and an optical encoder knob assembly — the target controller is optically encoded, with its own breakdowns (control grip, grip parts, encoder knob).
⚠️ Tamper-resistant hardware: the header is held by #8 × 5/8″ Torx tamper-resistant screws, eight of them, with their T-20 Torx key listed just below. Have the tool before you open it.
Notable games¶
Solar Fox (1981) · Kick (1981) · Satan's Hollow (1981) · Tron (1982) · Kozmik Krooz'r (1982) · Domino Man (1982) · Wacko (1982) · Two Tigers (1984) · Journey (1983).
Tron, and its four revisions
MAME records four distinct sets for Tron. A board bought second-hand is not necessarily the revision you know.
Common faults¶
| Symptom | Probable cause | Next step |
|---|---|---|
| Nothing starts | 19.968 MHz crystal | CPU and CTC derive from it together |
| Z80 “at 2.5 MHz” | nothing — that is the normal value | See the box |
| No sound | SSIO board | A stage separate from the main board |
| Inconsistent controls or I/O | I/O map handled by a PAL | ⚠️ partly undocumented |
| No picture | Z80 or the video stage | PCB diagnosis |
Repair (technician)¶
- Do not look for a 4 MHz Z80. This platform runs at 2.496 MHz by design.
- Separate the SSIO board before diagnosing a sound fault: it is self-contained.
- The I/O PAL is a grey area: an input anomaly cannot be checked against complete documentation.
→ Methods: PCB diagnosis · Recap
Procedure — a dead board, step by step ✅¶
What this adds
The general method lives in PCB diagnosis. This is its application to this platform: the same order, but with the values actually expected here, so you do not have to go back and forth.
Tools: multimeter, oscilloscope (or frequency counter), current-limited bench supply, desoldering braid. → Tooling
If the board is still in a cabinet
A CRT holds its charge with the machine unplugged. → CRT safety
1. Inspection, power off. Corrosion, cut tracks, oxidised EPROM legs, leaking SMD capacitors, dubious earlier repairs.
2. The connector, before anything else. A large share of "dead boards" are a contact problem, not a fault. Clean the contacts with isopropyl alcohol and reseat — not with an abrasive eraser on a gold edge, which takes the plating off and brings the problem back worse. It costs two minutes and settles the question.
3. Power, measured on the board. Aim for +5.0 V measured as close as possible to where power reaches the board — not at the PSU output: the drop is in the loom, and that is exactly what makes a board boot on the bench and not in the cabinet. Where you probe depends on this board's connector — JAMMA edge, JVS loom or a proprietary harness — plenty of makers kept one long after JAMMA arrived — see Identification above. → Wiring Check −5 V and +12 V too if the sound stage uses them. Suspected short: bring it up current-limited and find the hot spot.
4. /RESET. It must release after power-up. Held low = the CPU never starts. A 1–2 Hz loop is not the reset circuit, it is the watchdog: the CPU is crashing at boot, so look at the bus, not at the reset.
5. Clocks — the values expected on this platform.
- Z80 → 2,496 MHz (Main CPU)
- Z80 CTC → 2,496 MHz (timer — même horloge que le CPU)
No clock on the Z80 (2,496 MHz) and there is no point measuring anything else: the fault is the crystal, the divider, or the chip itself. That single measurement splits a dead board in two.
6. Bus. Activity on the main CPU's address lines tells you whether it is running or frozen. Frozen with good clocks and good reset → work RAM, bus buffers, ROMs.
7. It boots but the picture or the sound is wrong — that is another job: see Common faults on this page, then signal tracing.
The Standardized General Instructions — the platform manual, and its reference trap ✅¶
Bally/Midway did not publish only a manual per game. It also published a platform manual, the Standardized General Instructions, which the game manual then completed. Two of them are archived here, both © 1983:
| Document | Pages | Address printed | Cover reference |
|---|---|---|---|
| MCR System Games | 105 | 10601 W. Belmont Avenue, Franklin Park, Illinois 60131 · (312) 451-9200 | M051-00986-A025 |
| MCR II System Games | 65 | 10750 W. Grand Avenue, Franklin Park, Illinois 60131 · (312) 451-1360 | M051-00986-A025 |
⚠️ The same reference on two different manuals
M051-00986-A025 is printed at the foot of both covers. Yet they are not the same document: different titles, 105 pages against 65, and two different factory addresses. A third harvested copy, titled MCR II System Games and 60 pages long, carries that reference too.
⚠️ So a Bally/Midway manual cannot be identified by its form number. You need the title and the pagination. ⓘ Two other manuals from the same batch — Standardized General Instructions (99 pp.) and Licensed Games Standardized General Instructions (55 pp.) — share the reference M051-00132-0000 in the same way.
ⓘ And the address moves inside a single document: the MCR System Games manual carries 10601 W. Belmont Avenue on its cover and 10750 W. Grand Ave. in its copyright notice, three pages apart.
What the platform manual gives that the game manual does not
ⓘ A numeric mains threshold, repeated word for word in all five manuals of this family:
"If the line voltage drops too low, a circuit in the computer will disable game credit. The point at which the computer will fail to work will vary some from game to game, but no game will work on line voltage that drops below 105 VAC."
⚠️ The symptom is not the one you expect: a cabinet on low mains does not crash, it refuses the coin. A coin mech that stops crediting is therefore not necessarily a coin mech problem → mains chain and thresholds.
ⓘ Its monitor chapter compares the three makes Midway fitted — Wells-Gardner, Electrohome and Zenith — and publishes the picture tube interchangeability chart → Other CRT chassis.
Modernisation & mods¶
- No suicide: no battery-backed protection.
- ⚠️ Do not host ROMs — method only.
Emulation & FPGA¶
MAME covers the family (bally/mcr.cpp, driver by Christopher Kirmse and Aaron
Giles).
Photos / assets¶
(To take: the 19.968 MHz crystal — its value deserves a photograph —, the separate SSIO board, and the I/O map's PAL.)
The Midway Service Notes index — 92 pages made navigable ✅¶
The Midway Service Notes archived below have no text layer: without their table of contents you have to leaf through them. It is therefore read off the images and transcribed here, page by page.
⚠️ This document is not limited to the MCR. It covers Bally/Midway's range of the period, including licensed games — Pac-Man and Galaxian (Namco), Space Invaders (Taito) — and several earlier platforms. Cited here because this is the Midway page, not because everything in it is MCR.
General and power — Self-testing RAMs & ROMs (p. 1) · ROMs & PROM chart (2) · Game numbers (3-6) · Different power supplies (7) · Anti-cheat devices (8-10) · Anti-wire coin switch (11) · New medium power supply (12) · Special Allen wrench (50) · Free games hitting coin door (51) · Anti-static cleaner (52) · New back door key (53) · Credit bypass jack (54) · Parts & operating manual location (40) · Custom chips (86) · PROM strapping single supply (87) · Glossary of terms (91)
By game — Lazarian lock-up (13) · Tron: line fuse change (14), control assembly (15), Kickman hum in sound (16) · Ms. Pac-Man & Galaga wavy video (17) · Ms. Pac-Man & Pac-Man loss of sound (18) · Coin meter with clamping diode (19) · Ms. Pac-Man main logic board (20) · Kick / Kickman: new features (21), name change (22), button light bulbs (23), capacitor change I/O board (24), RAM/ROM test location (25), EPROM identification (26), PROM location (27), speakers (28), logic board interchanging (29-30), video lock-up (31) · Galaga: lock-up (32), loss of base ship (33), PROM identification (34) · Logic board IC replacement (35) · Omega Race: new features (36), DIP switch setting (37), door keys (38), ROM & RAM code (39) · Wizard of Wor: features (41), ROMs & PROMs (42) · Rally-X code (43-44) · Gorf: card rack system (45), mission self test (46), program change (47-49) · Pac-Man: DIP switches (56), ROM & RAM test code (57) · Card rack system test cards (58) · Space Encounter: strobe unit fix (59-60), game boards (61), 25K pots (62-63), power supply (64) · Galaxian: static kit with Pac-Man (65-66), RAM & ROM test (67), programs #1 & #2 (68), program #3 (69) · Space Invaders: Deluxe PROM & ROMs (83), lines in video (84), DIP switch setting (85)
Twenty-one pages of monitor — and they reach beyond Midway
Pages 70 to 90 stop talking about game boards and talk about displays, and they are the most transferable part of the document:
| Subject | Pages |
|---|---|
| Video game monitors | 70 |
| 19″ colour monitor CRT replacement | 71 |
| ⚠️ Electrohome — colour adjustment | 72-73 |
| Galaxian interface wire sequence | 74-78 |
| ⚠️ Wells-Gardner interface wire sequence | 79 |
| ⚠️ Electrohome interface wire sequence | 80 |
| Galaxian / Wells-Gardner 19K4500 | 81 |
| Wells-Gardner 19K4900 — vertical deflection | 82 |
| Yoke wiring | 88-89 |
| List of monitor manufacturers | 90 |
✅ Three of these entries touch chassis this wiki documents elsewhere: Electrohome G07 — which entered the wiki the day before by an entirely different route — and Wells-Gardner. A game troubleshooting document turns out to be a monitor source as well.
The Service Bulletin volume 2 — a second compilation, an older one ✅¶
The Service Bulletin archived below is a different compilation from the Service Notes above, and an older one: its range stops at Gorf, Wizard of Wor, Rally-X and Pac-Man (1980-81) where the first goes on to Tron and Ms. Pac-Man (1982). ⓘ It carries its own table of contents, with a text layer — volume 1's had to be read off the images.
⚠️ The scan is incomplete, and we can say where
The contents list announces 87 pages, the PDF holds 86 plus the contents page itself: two pages are missing. ⓘ They can be located by comparing the printed pagination with the PDF's — the offset goes from +1 to 0 and then to −1:
- one between pages 29 and 31 — Space Encounter Trigger Kit (29-30) and Game Boards (31);
- one between pages 37 and 40 — the Galaxian run (end of the static kit, RAM & ROM test, programs #1 to #3).
✅ The rest is there, and the list below is the document's own, not a reconstruction.
General, power, tooling — Self testing RAMs & ROMs (1) · RAMs & PROM chart (2) · Game numbers (3-6) · Different power supplies (7) · Anti-penny kit (8-9) · Anti-string kit (10) · Special Allen wrench (18) · Anti-static cleaner (19) · New back door key (20) · Credit bypass jack (21) · Card rack system test cards (26) · PROM & ROM identification (62) · Custom chips (73) · Strapping 2716 & 9316 (80) · PROM strapping single supply (81) · Power supply check list (82) · Mother board reset fix (83) · Glossary of terms (87)
Per game — Wizard of Wor features (11) · Rally-X RAM code (12-13) · Gorf: card rack system (14), program change (15-17) · Pac-Man: custom chip (23), DIP switches (24), ROM & RAM test code (25) · Space Encounters: strobe unit fix (27-28), trigger kit (29-30), game boards (31), 25K pots (32-33), power supply (34) · Galaxian: coin switch capacitor (35), static kit with Pac-Man (36-37), RAM & ROM test (38), programs #1 & #2 (39), program #3 (40), alternate P.C. board (41), bad RAM #2 (42), black & white modification (56) · Extra Bases: ball sensor modification (57), RAM cards (58) · Space Invaders: Deluxe name features (59), Deluxe custom I.C. (60), Deluxe PROM & ROMs (61), random garbage (63-64), lines in video (65), DIP switch setting (66), 25S10 & 74151 chips (67) · Submarine Zero switches (68) · 18 Wheeler: premature game over (69), back door venting (70-71) · Rotation VIII: power reset (75-76), "F" & "G" board (77) · RAM & ROM self test — Bowling Alley (78-79)
Twelve monitor pages, and they cut across three pages of this wiki
| Subject | Pages |
|---|---|
| Color monitor brightness adjustment | 22 |
| Video game monitors | 43 |
| ⚠️ Electrohome — colour adjustment | 44-45 |
| Galaxian interface wire sequence | 46-50 |
| ⚠️ Wells-Gardner interface wire sequence | 51 |
| ⚠️ Electrohome interface wire sequence | 52 |
| Galaxian / Wells-Gardner 19K4500 | 53 |
| Galaxian — 25-inch monitor | 54-55 |
| ⚠️ Electrohome "G0-2" colour | 72 |
| Sea Wolf II — Electrohome monitor | 74 |
| Monitor yoke wiring | 84-85 |
| ⚠️ List of monitor manufacturers | 86 |
✅ The "G0-2" on page 72 is the G02 whose manufacturer's manual this wiki archives. ⚠️ And the page-86 list changed between the two volumes — that is handled on the Other CRT chassis page.
Self-test, game by game — 26 rows, 30 titles ✅¶
Page 1 of the compilation is a table nothing else on this wiki replaces: which switch puts which game into self-test. ⓘ These are platforms earlier than the MCR, cited here for want of a page of their own.
| Game | Self-test switch | Game | Self-test switch |
|---|---|---|---|
| ⚠️ Gun Fight | ⚠️ None | Sea Wolf II | #8 off |
| ⚠️ Sea Wolf | ⚠️ Roms only — 6, 7 and 8 off | Space Walk | #8 off |
| Tornado Baseball | #6 off | Dog Patch | #6 on |
| Maze | #4 off | Shuffleboard | #8 on |
| 280 ZZap | #3 off & #4 on | Bowling Alley | #8 on |
| Check Mate | #8 off | 18 Wheeler | #8 on |
| Boot Hill | #5 off | Phantom II | #6 off |
| Double Play | #7 on | Super Speed Race | #8 off |
| Road Runner | #7 & #8 off | Space Invaders II | #8 off |
| Guided Missile | #8 on | Galaxian, Pac-Man & Rally X | Slide Switch on |
| Laguna Racer | #7 off & #8 on | Space Encounters | #5 off & #6 on |
| M-4 | #5 on | Space Zap, Gorf & Wizard Of WOR | Slide Switch on |
| Clowns | #8 off | ||
| Extra Inning | #7 on |
✅ And two notes worth the table:
"Activate coin door tilt switch to start self test." "Invaders & Blue Shark has no Ram and Rom Test."
⚠️ What this table explains about the document absorbed this morning
The Standard Test Procedure for Midway's Processor Boards (MM 1700-1, 55 pp.) covers exactly Gun Fight and Sea Wolf. ⓘ And those are the only two rows of the table with no usable self-test: None for one, ROMs only for the other.
ⓘ The table does not say so — this is a link between two documents, not a claim by either. But it makes legible why those two boards earned 55 pages of separate procedure. ⚠️ This wiki still covers the 1975-76 8080 hardware on no page; the gap stands.
Mortal Kombat II — section 3 is nothing but wiring ✅¶
The Mortal Kombat II operations manual archived below (Midway, October 1993, ref. 16-40029-101) runs to 7 pages, but they are the 7 pages of its Section three — Schematics and Wiring Diagrams. No text layer: the values below were read off the images at high magnification.
The boards, by their Midway part numbers — CPU board A-17266-40029 · sound board A-17319-40029 · switch-mode power supply A-15202-1 · transformer XFMR 5610-13326-00. A ribbon cable joins the CPU's P12 to the sound board's P1, and the manual warns that the ribbon's red line must go to pin 1 on both boards — "ribbon cable may need to be twisted to accomplish this".
Converting a US MK II to 230 V — what the manual requires
The power diagram is explicit, and it is not just a matter of moving jumpers:
"NOTE: FOR 230VAC OPERATION: 1. FUSE IS 2.0A SLOW BLOW. 2. VARISTOR IS 250V."
At 115 V the machine carries a 3 A slow-blow fuse and a 130 V varistor. Both must change. Moving only the jumpers leaves a 130 V varistor on 230 V — that is, a component that conducts continuously.
Voltage selection table (jumpers on the transformer primary):
| Voltage | Jumper pins |
|---|---|
| 103.5 V | 5-6, 7-8, 10-9 |
| 115 V | 5-6, 7-11, 10-12 |
| 132 V | 5-6, 7-14, 10-15 |
| 207 V | 5-9, 10-8 |
| 218.5 V | 5-9, 10-11 |
| 230 V | 5-12, 10-11 |
| 247 V | 5-12, 10-14 |
⚠️ The manual notes the diagram is drawn jumpered for 115 V: on an imported machine, that is the configuration you will find. The secondary gives 120 V isolated for the monitor and the power supply, plus 5.9 V / 6 A for the coin door lights.
A JAMMA chart with wire colours (page 3-2) gives the full pinout and, on the controls side, Mortal Kombat's own layout: High Punch, Block and High Kick on top of the directions and start, for both players.
Two further plates: the interboard wiring (CPU ↔ sound, every pin named) and the cabinet wiring — control panel, coin door, meters, speakers, shielded video, and the provision for an optional electronic coin acceptor or bill acceptor.
Archived original documents ✅¶
Documents archived locally, found on Internet Archive on 13 August 2026 and identified from their title page. ⚠️ The items declare no licence.
- PDF — bally-midway-discs-of-tron-parts-operating-manual.pdf — DISCS OF TRON — Parts and Operating Manual, Bally Midway, August 1983, Game No's. 696-383, 36 pp. Via Internet Archive
-
PDF — bally-midway-service-notes.pdf — Midway Service Notes, 92 pp. — the most useful of the lot: a fault index covering the whole range. Its contents run from self-testing RAMs & ROMs to the glossary of terms, by way of different power supplies, Tron line fuse change, Ms. Pac-Man & Galaxian wavy video, color monitor brightness adjustment, Electrohome color adjustment, Wells-Gardner interface wire sequence and monitor manufacturers. Via Internet Archive
-
PDF — bally-midway-mcr-system-games-standardized-general-instructions.pdf — Bally/Midway's MCR System Games — Standardized General Instructions, © MCMLXXXIII Bally/Midway, 105 pp., FORM NO. M051-00986-A025, MIDWAY MFG. CO., 10601 W. Belmont Avenue, Franklin Park, Illinois 60131. ⓘ A platform manual, not a game one: installation, maintenance, technical troubleshooting, self-test, microprocessor glossary. Harvested from Internet Archive's arcademanuals collection.
-
PDF — bally-midway-mcr-ii-system-games-standardized-general-instructions.pdf — Bally/Midway's MCR II System Games — Standardized General Instructions, © MCMLXXXIII Bally/Midway, 65 pp., FORM NO. M051-00986-A025 — ⚠️ the same reference as the previous one, on a different document. Address printed: 10750 W. Grand Avenue. ⓘ Its section VI is a monitor chapter in its own right: a Wells-Gardner / Electrohome / Zenith comparison, a picture tube interchangeability chart, and schematics for all three makes → Other CRT chassis. Harvested from Internet Archive's arcademanuals collection.
-
PDF — bally-midway-service-bulletin-volume-2.pdf — SERVICE BULLETIN — a Midway Mfg. Co. compilation, a Bally company, 10750 W. Grand Avenue, Franklin Park, Ill. 60131, 63 entries, paginated 1-87. ⓘ A second compilation, older than the Service Notes above: its range stops at Gorf / Rally-X (1980-81). ⚠️ The scan is incomplete — two pages are missing (see above for where). It carries its own table of contents, with a text layer. Via Internet Archive
Sources & attribution¶
Text (synthesised here; sources listed for attribution):
- MAME —
src/mame/bally/mcr.cppandbally/mcr.h(BSD-3-Clause / GPL, driver by Christopher Kirmse and Aaron Giles):MAIN_OSC_MCR_Idefined as 19.968 MHz, the Z80 and Z80 CTC at that clock ÷ 8 with the note that the CTC is “same as maincpu”; the SSIO sound stage; and the note that the upper I/O map is determined by a PAL, only the SSIO ports being verified from the schematics — https://github.com/mamedev/mame/blob/master/src/mame/bally/mcr.cpp
Review & corrections
How this space is used
Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.
Reading is open to everyone; posting requires signing in with Discord. Reports from the wiki's declared reviewers are handled first; anyone else's are read too, but go through a human before anything is changed.