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Macintosh Plus Vertical Deflection Repair

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Macintosh analog board vertical output stage (Macintosh Repair and Upgrade Secrets Fig 5-21)
The vertical output stage: Q1 and Q2 drive the vertical yoke through R3 and R2, with R4 to the +5 V side of the yoke at J1 (Pina, Figure 5-21; International board Q4 in place of Q2).

When the picture on a Macintosh Plus collapses to a horizontal line, half of it disappears, or its height cannot be set, the fault is in the vertical sweep. On the 128K to Plus analog board this is built from discrete parts and an LM324 quad op-amp, not the TDA1170 IC of later compact Macs.[1] Pina finds the vertical circuit far less prone to cracked joints than the horizontal one, because it runs at 60 Hz, not 22.25 kHz.[2]

The Macintosh 128K, Macintosh 512K and Macintosh 512Ke use the same circuit. Designators are for the U.S. board; on the International board Q2 is numbered Q4.[2]

Symptoms

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Macintosh display bottom half missing (Macintosh Repair and Upgrade Secrets Fig 5-19)
Top half squashed above a bright line, bottom half black: R3 or Q1 (Pina, Figure 5-19).
Vertical sweep faults
Symptom Cause Source
Picture collapses to a horizontal line after a while, or rolls and then collapses; tapping the cabinet brings it back Cracked joints at J1 pin 1, J4 pin 5 or C5 (47 µF 10 V) [3]
Horizontal line; the green vertical sync wire of the logic board harness open Harness [2]
Bright horizontal line at start-up, then smoke Q4 (2N4401) burned, Q2 (MPSU51) shorted, R3 open, zener CR15 and other burned parts in the vertical circuit [4]
Small grey area, a flashing bright horizontal line and three dotted lines; cursor just visible Field-effect transistor Q5 (2N5485) [4]
Top half squashed, bright white line in the middle, bottom half black R3 (1.5 Ω) and Q1 (MPSU01) [4][2]
Bottom half squashed, bright line in the middle, top half black R2 (1.5 Ω) and Q2 (MPSU51) [4][2]
One half stretched off the screen, and the Height control affects only the other half LM324 at U1 [4]
Picture lacks height, out of the blue Zener CR15 (1N5234B) [5]
Picture lacks height after a board swap Adjust R55 (Height) to 4.75 in [5]
Bottom of the picture shakes during disk activity or when a window opens Zener CR15 [5]

A vertical line is the horizontal sweep, a different circuit; see Macintosh Plus Horizontal Sweep Repair. A bright dot in the centre means both sweeps are missing, usually because the yoke cable was left off J1.[2]

Safety

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Warning: the analog board carries mains voltage and the CRT anode about 13 kV. Unplug the power cord before every resistance check and all soldering, and discharge the CRT before working near the yoke connector; see CRT Discharge Procedure.[2][6]

Tools and parts

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  • T15 Torx driver, case-cracking tool, multimeter, inspection lamp, soldering iron and desoldering pump.[7]
  • Parts for the U.S. board, from Pina's parts list:[8]
Vertical circuit parts (Pina, Appendix C)[8]
Part Type
Q1 MPSU01, NPN, 2 A 10 W
Q2 MPSU51, PNP, 2 A 10 W
Q4 2N4401, NPN
Q5 2N5485, N-channel JFET
U1 LM324N quad op-amp, 14-pin DIP
R2, R3 1.5 Ω ¼ W carbon film
CR15 1N5234B, 6.2 V ½ W zener
C5 47 µF 10 V electrolytic
R55 500 kΩ Height trimmer

Procedure

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Opening up

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  1. Switch off, unplug the cord and remove the rear housing. Discharge the CRT.[6]
  2. Remove the vinyl jacket from the solder side of the analog board; see Macintosh Plus Horizontal Sweep Repair#Removing the vinyl jacket.[7]

1. Joints and the harness

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  1. Light the board from the component side and resolder any cracked joint at pin 5 of J4, at pin 1 of J1 and at C5.[2][3]
  2. Check the green wire (vertical sync, J4 pin 5) of the analog-to-logic-board harness for continuity from end to end.[2]

2. R2 and R3

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  1. With the cord unplugged, measure R2 and R3 in circuit. Each is 1.5 Ω, so a good one reads almost a dead short.[2]
  2. Replace either if it reads infinity or over-range. Pina warns that these resistors usually look perfect even when they have burned open.[2]
  3. If the bottom half of the picture was missing, replace Q1 with R3; if the top half was missing, replace Q2 with R2.[4]

3. Other parts

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  1. If the picture went to a bright horizontal line and smoked, check Q4, Q2, R3 and CR15, and replace anything visibly burned in the vertical circuit.[4]
  2. For one half stretched off screen with the height control affecting only the other half, replace the LM324 at U1.[4]
  3. For a picture lacking height that R55 will not correct, or a bottom edge that shakes with disk activity, replace CR15.[5]

4. Yoke check

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  1. With the cord unplugged and the CRT discharged, unplug J1 and measure the yoke. Lee gives about 8 Ω for the vertical winding and about 0.2 Ω for the horizontal winding, with no continuity between the two. Only a large departure from these figures means a fault.[1]
J1 yoke connector (Pina, Table 5-2)[2]
Pin Wire Function
4 Green Horizontal yoke
3 Yellow Ground
2 Red +5.00 V
1 Blue Vertical yoke

Replacing parts

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Replace parts with the general procedure in Macintosh Plus Low Voltage Repair#Replacing a zener. Pina's rule for orientation: if in doubt which way a part goes, check the marking on the component side of the board.[7]

Reassembly

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  1. Refit J1, the vinyl jacket and the rear housing.[7]

Testing

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Macintosh display top half missing (Macintosh Repair and Upgrade Secrets Fig 5-20)
Bottom half squashed below a bright line, top half black: R2 or Q2 (Pina, Figure 5-20).
  • Switch on. The picture should fill the screen top to bottom and stay steady when the cabinet is tapped and when the disk drive runs.[3][5]
  • Set the height to 4.75 in (Pina) or 4.7 in (Apple) with R55, then check the voltage and width again; see Macintosh Plus Video Adjustment.[5][9]
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References

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  1. ↑ 1.0 1.1 Thomas H. Lee, Classic Mac Repair Notes, revision of 31 May 2007 (©2000 Thomas H. Lee), archive.org. Section 7.2, p. 17 (horizontal yoke about 0.2 Ω) and section 7.3, pp. 17-19 (discrete vertical circuit with LM324 U1, Q1, Q2, Q4, Q5; the TDA1170 used after the Plus; vertical yoke about 8 Ω).
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 Pina, Macintosh Repair & Upgrade Secrets, chapter 5 "Video Problems", pp. 81-85 (symptom table; scan rates; why vertical joints crack less), p. 83 Table 5-2 (J1 yoke cable), p. 93 (13 kV anode), pp. 104-107 (Table 5-7, horizontal line, R2 and R3, Figures 5-19 to 5-21, pinprick).
  3. ↑ 3.0 3.1 3.2 Larry Pina, The Dead Mac Scrolls (Peachpit Press, 1992), "Early Macs", p. 49. Hosted as File:The Dead Mac Scrolls 1992.pdf; see The Dead Mac Scrolls.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Pina, The Dead Mac Scrolls, pp. 50-51 and 55-58.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Pina, The Dead Mac Scrolls, pp. 32-33 and 36.
  6. ↑ 6.0 6.1 Apple Computer, Macintosh Plus Service Source, hosted as File:Macintosh Plus.pdf (see Macintosh Plus Service Source). Take Apart, Cover pp. 1-2 and Power/Sweep Board p. 3 (PDF pp. 27-29).
  7. ↑ 7.0 7.1 7.2 7.3 Pina, Macintosh Repair & Upgrade Secrets, chapter 4, pp. 59-69: the vinyl jacket and the general replacement procedure for analog board parts.
  8. ↑ 8.0 8.1 Larry Pina, Macintosh Repair & Upgrade Secrets (Hayden Books, 1990), Appendix C "Parts List - Macintosh Analog Board - U.S. Version", pp. 327-332. Hosted as File:Macintosh Repair & Upgrade Secrets.pdf; see Macintosh Repair & Upgrade Secrets.
  9. ↑ Apple Computer, Macintosh Plus Service Source, Adjustments, Video p. 16 (PDF p. 66): raster 4.7 in (119.4 mm) high.


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