Classic Mac Tech Info

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…