Classic Mac Tech Docs, v1.1: No warranties expressed or implied. Use at your own risk! Classic Mac Tech Info 1.0 Introduction Except for minor variations, the analog board for the Mac Plus described in this document is the same as that used in earlier models (the 128K and 512K/512Ke), and contains the horizontal, vertical, video and power supply circuits. The horizontal and video circuits are also virtually identical to those used in the SE and SE/30, but the vertical and power supply circuits are completely different. As will be obvious from the schematics, the vertical circuit in the classic machines makes sparse use of ICs, so it is a bit “partsy.” However, it is a reliable design with no history of unusual problems. The power supply, on the other hand, is notoriously prone to failure, as there is no fan inside the unit (Steve Jobs objected to the noise, and preferred to risk overheating the machine instead). 2.0 Video Circuit The video amplifier is a simple shunt-peaked common-emitter amplifier: FIGURE 1. Video amplifier +30V +12-15kV From horizontal circuit 9” CRT +5V L4 27µH R25 470 CR13 9 U2 J4-1 10 8 CR14 CR12 R14 1k 2W R20 220 NE2 Q7 2N3904 R27 220 CR12-14: 1N4150 (similar to 1N914) J2-9 7 6 1 2 +12F 3 L5 27µH J2-8 J2-3 J2-1 J2-5 C15 C17 4 J2-6,7 NE1 C16 Note: C15-17 are 0.01µF capacitors with 1.5kV spark gaps IC U2 is a quad open-collector NAND, one gate of which is connected as a simple inverter to present the video signal to Q7 with the proper polarity (a low at J4-1 corresponds to white). Resistor R25 is the NAND gate’s pullup resistor. Diodes CR13 and CR14 form a Baker clamp to prevent saturation of Q7 when driven hard, thereby speeding up response. Inductor L4 is a shunt peaking element for boosting the high frequency response of the video amplifier. Diode CR12 is a protection device, clamping the collector of Q7 to a max- Classic Mac Tech Info ©2000 Thomas H. Lee, rev. April 17, 2002; All rights reserved Page 1 of 18 Classic Mac Tech Docs, v1.1: No warranties expressed or implied. Use at your own risk! imum of a diode drop above 30V, despite inductive kickback from L4, and even if some CRT fault occurs. Resistor R20 limits the current under the latter fault condition. The two neon bulbs act as voltage clamping devices to prevent serious overvoltages on the filament and cathode terminals of the CRT. Similarly, the 1.5kV spark gaps integral with C15-C17 act to protect the CRT under transient fault conditions. Finally, inductor L5 provides ac isolation between the CRT and the 12V filament supply. 3.0 Horizontal Circuit The high voltages in this circuit are all byproducts of horizontal deflection. The TTL-level horizontal drive signal from the main logic board is inverted by open-collector NAND U2 (here connected as a simple inverter, with pullup resistor R26). The inverted signal then feeds a small driver transistor Q6, whose output couples to the horizontal output transistor Q3 through isolation tran…

Macintosh Video Circuit +30V J4! PIN1 VIDEO L1 +12V! ! FROM POWER! SUPPLY L4! 27 +5 +12V TO SWEEP CKTS C2 4700/16V CR12 +5 U2 PIN3 R35! 1K R25! 470 Q6! 2N4401 11 13 R20 220 9 12 HOR! DRIVE R14 CR13 8 10 Q7! 2N3904 CR14 YEL NE2 2 BLK 3 BRN 4 L5 +12V NE1 T2 Q3! BU406 C12! .001 C7! .01 R8! 100 R12! 100 R17 1M R13! 100K C12! .01 C17! .01 C13! 33/16V BRIGHTNESS! R57 1M C15! .01 SPKR! 63Ω CR11 PIN7 C1! 3.9/25V R1! 220 WIDTH L2 PIN9 1 C4! .033 L3 HORIZ! 4! TO! J1! YOKE 3 R15! 100K T1 2 3 C3! 220/16V C8 .01 CR10 7 CR1 +30V 8 4 C14! 22/50V 5 +12V Q1! MPSU01 +5V 1 PIN5 2 R24! 10K Q5 2N5485 C10 .22 S 3 1/4 U1 - 13 4 12 6 R22! 330K CR4 C5! 47/10V D G 1/4 U1! LM324 3 + 2 - + 1 CR3 14 R4! R2! 100 1.5 R6! 2.2K CR2 1! VERT! J1! TO! 2 YOKE Q2! MPSU51 R10! 10K +5V R3! 1.5 C6! 22/50V 5 R11 1.5 R21! 1K HEIGHT! R55! 500K R30! 10K CR7 C11! 33/16V CR15! 1N5234! 6.2V R31! 10K -12V R16! 100K CR5 +12V VERT! DRIVE 7 BLU R19! 470K R7! 39 5W J3 6 RED R18! 100K C9! 10/160V CR4 +12V PIN4 1 GRN CR28 Q4! 2N4401 CR6 C19! .01 R28 10K R29! 1M 10 + 9 1/4 U1! LM324 8 C20! .01 -12V CUTOFF! R53 2M R5! 470K R9! 1M FOCUS! R54 2M SW1 14 49 CPU 68000 23 IPL2 R2 3.3K 24 IPL1 R30 3.3K 13 BR R20 3.3K 22 BERR 25 IPL0 19 VMA 18 RESET 17 HALT 6 AS 7 UDS 8 LDS 9 R/W 10 DTACK 15 CLK 20 E 1 C5 D0-D15 D0-D15 A1-A23 CONTROL 4 SPKR CONTROL 5 +5V 4 3 OSC 1 -12V 15.6672 MHZ R8 3.3K 7 20 4 11 19 1 18 BMU1 R/W A1 A21 9 30 A2 A22 8 31 A3 A23 7 13 SCC:RD 32 A4 VA7! 3 12 IWM:CS 33 A5 VA8! 2 34 A6 VA9! 1 35 A7 VA14 5 36 A8 A10 39 9 L1 10 BAT 11 29 38 CLK 2 OVERLAY 6 A9 R31 47 L2 8 +12V VPA 37 L3 6 7 U4D LS04 2 AS 12 X 2.2K 33MFD 16V 3 A1-A23 21 17 RAM.E 16L8 9 RAM.E 2 15 VPA 342-0250 3 15 BMU0 3 16 VIDEO-OUT 1 CLK 19 DTACK 16R4 20 19 DTACK TSM 2 12 CAS.LO 16R4 A19 1 CLK 20 3 TSG 13 CAS.HI VPA 2 14 RAS IRQ 9 INT 8 7.8336MHZ 18 3 19 IPL0 17 16R6 20 2 CB1 CLK 1 14 RTxC VA1! 9 VA2! 8 VA3! 7 17 CPU-VID VA4 4 19 RESYNB AS 7 18 VIDEO-IN UDS 8 VA10! 13 RAM.WR LDS 9 VA11! 14 AX13 RAM.E 5 18 VA12! 17 AX14 ROM.E 6 17 5 5 18 RESLN S1 4 15 .9792MHZ 4 6 15 S1 342-0255 VA13! VA14 OE RP 10 342-0252 KBRD CLK 7 S0 E 12 6 16 OE 11 10 342-0254 11 R5 47 D0 10 10 10 8 2 311 3 7 D8 11 8 40 SCC TRxCA 39 A11 D11 2 40 A12 D12 38 41 A13 D13 3 42 A14 D14 37 43 A15 D15 4 44 A16 A1 34 SYNCA 45 A17 A2 32 RTSA 46 A18 R/W 35 DTRA 47 A19 SCC.RD 36 TxDA 48 A20 SCC.WR 33 50 A21 SCC.INT 5 SYNCB 51 A22 SCC.CLK 20 RTSB 52 A23 R6 47 A1 A2 9 A3 8 ROM.LO A4 7 23256 A5 6 4 D1 A6 5 3 D2 A7 2 D3 A8 D4 28 10 D0 D2 W/REQB 3 15 D3 17 D5 D7 A12 19 D7 61 D8 A13 60 D9 A14 59 D10 A15 58 D11 A16 57 D12 A20 56 …