Sharp MZ-80K Troubleshooting Guide

Sharp's fault chart for the UK power supply DBOXD0005PAZZ, from the MZ-80K service manual

Fault-finding on the Sharp MZ-80K follows Sharp's own order: power supply first, then the display, the cassette recorder, the keyboard and the CPU board. Component references, test voltages and check points are from Sharp's service manual for the European 220 V and 240 V machines. Owners' reports are cited where they are used.

Warning: mains voltage is present on the power switch, fuse F101, C101 and the transformer, and the CRT anode runs at about 11 kV. Unplug the machine before opening it, and follow the CRT Discharge Procedure before working near the tube.

Quick reference

MZ-80K symptoms

Symptom	Probable cause	Section
Dead, smoke or burning smell from the supply	Shorted tantalum capacitor in the power supply	Power supply
Dead, F101 blown	Short on a secondary, often a tantalum capacitor	Power supply
One rail missing	Fuse F202, F401 or F501, regulator IC1, IC2 or IC3, rectifiers	Power supply
Garbage on screen at switch-on	RAM, ROM, CPU, character generator, bus buffers	Garbage on screen
Returns to the monitor, or errors after running a while	RAM	Memory faults
No picture, no raster	+12 V, CRT heater, high voltage, horizontal circuit	Display
Tape will not load	Heads, azimuth, IC3001, Q3003	Cassette recorder
Key does not register	Key switch, soldering, keyboard track, IC5, IC6	Keyboard

Power supply

The supply has three outputs: +12 V and +5 V from two SG3524 switching regulators (IC2 and IC3) fed by about 23 V DC from the bridge D301 and D302 and the capacitors C301 to C304, and −5 V from the regulator IC1 (µA78MGU) on a separate winding. Sharp's fault charts, for the 220 V unit (DBOXD0004PAZZ) and the UK unit (DBOXD0005PAZZ), work through it like this:

No output at all

1. Check for mains (220 V or 240 V AC) across C101. If it is not there, check the mains lead, the socket and the switch.
2. If it is there, check the fuse F101 and the transformer T101.

F101 is a T500 mA fuse. Owners on the nIGHTFALL blog report it blowing as soon as the supply is switched on, and smoke from the supply, and trace both to the supply's tantalum capacitors shorting. Sharp's parts list and schematic give four 10 µF 35 V tantalums: C401 on the input of the +12 V regulator, and C501, C502 and C509 on the input side of the +5 V regulator. Check them with the supply unplugged before replacing the fuse; see the Sharp MZ-80K Capacitor Replacement Guide.

−5 V missing

−5 V checks (Sharp fault chart)

Check	220 V unit	UK unit	If wrong
AC on the orange lead	9 V	10 V	Transformer T101
DC across C201	10 V	11 V	F201, D201 and C201
DC across C202	−5 V	−5 V	IC1, R201, VR201 and C202
Output terminal	−5 V	−5 V	L201 and F202

+12 V or +5 V missing

1. Check for 20 V AC on the yellow lead. If it is missing, check T101.
2. Check for 23 V DC across C301. If it is missing, check the rectifiers D301 and D302 for a short.
3. Check for 12 V across C404 (for +12 V) or 5 V across C507 (for +5 V). If it is there but the output is not, check the filter coil and fuse after it: L402 and F401 for +12 V, L503 and F501 for +5 V.
4. If the voltage across C404 or C507 is missing, lift one end of R403 (for +12 V) or R503 (for +5 V) and look for a 0.3 V peak-to-peak pulse on pin 12 of IC2 or IC3. With no pulse, suspect the SG3524 and its parts; with the pulse present, suspect the output switching circuit (Q401 and Q402, or Q501 and Q502, and their diodes).

Sharp's waveforms for a working supply show a 3 V sawtooth with a 45 µs period at pin 7 of each SG3524, and 23 V pulses repeating every 45 µs at pin 12 of IC2 and IC3 and at the collectors of Q401 and Q501.

Garbage on screen

Sharp calls a screen of garbage at switch-on "panic". The service manual's checks for it are the monitor ROM, the CPU (IC46), the character generator and IC43; the address bus buffers IC44 and IC45; the data bus buffers IC36, IC37 and IC50; the control line buffer IC35; the RAM; and IC52, IC53 and IC56. All the major chips are in sockets, so reseat them first. Sharp's general advice for the CPU board is to feel for an IC running hot (except the ROM and the video RAM), to look for a fault that changes when the board is touched (poor soldering or a bad socket contact), and to check TTL levels with an oscilloscope: above 2.4 V for high and below 0.5 V for low.

Memory faults

Sharp lists the RAM as the cause when a program gives errors or misbehaves, when the machine returns to the "MONITOR SP-1002" prompt, and when errors appear after a long run. A disappearing cursor points to IC46 or IC1.

Sharp's service tool was the RAM checker, which plugs into the monitor ROM socket (marked M-ROM) in place of the monitor ROM. At switch-on it runs two tests from address $1000 to the top of memory ($5FFF on a standard 20 KB machine) and reports the failing address with the data written and read back. The service manual explains how to work out the failing bank and chip from that: bank I is the 16 KB of 4116s and bank II the 4 KB of 4027s. The RAM checker is not generally available today. Adam Wilson tested every 4116 from his machine out of circuit on a DRAM tester and found nine faulty. The links on the headers CS1 and CS2 must match the RAM fitted. SharpMZ.net lists the settings for 20, 24, 36 and 48 KB.

Display

CPU board video

Video faults on the CPU board (Sharp)

Symptom	Check
No sync	Vertical: IC15 and IC16. Horizontal: IC10.
No video	V-GATE at pin 1 of IC24 (present: IC24; absent: IC5). V-BLANK at pin 2 of IC24 (present: IC24; absent: IC20). H-BLANK at pin 13 of IC17 (present: IC17 and IC3; absent: IC15). Output at pin 2 of IC17 (present: IC17 and IC3; absent: IC29).
Characters partly missing	IC29 and the character generator
Picture displaced	Sync circuit

Monitor board

Sharp's display chart starts by checking the +12 V supply and then splits by symptom:

• Raster but no picture: check for a video input signal (if missing, check the CPU board), then the video circuit, the contrast control R2002, Q2001 and the 40 V video supply.
• No raster and no picture: check the CRT heater. If it is not lit, check the CRT plug; with the plug good, the tube is faulty. If it is lit, check the voltages on the cathode and on G1, G2 and G3; if they are correct, the tube is faulty. If they are wrong, check the flyback circuit. With the flyback circuit working, check the middle and high voltage rectification (D2006, D2009 and the flyback transformer); otherwise check the AFC, horizontal oscillator and horizontal drive and output circuit.
• No sync at all: if there is no sync signal from the CPU board, check the CPU board; if there is, check the sync separator (D2001 and Q2002).
• Vertical sync only lost: check the integrating circuit R2012, R2013 and C2006 to C2008.
• Horizontal sync only lost: check the AFC circuit (the flyback output at check point 7, R2026, C2018 and R2027).
• Picture too narrow vertically: check the vertical oscillator waveform at pin 4 of IC2001 (µPC1031H). If it is wrong, check the V-HOLD control, C2009 and R2014. If it is right, check the input at pin 7 (if wrong, R2018 and C2012) and then the output of IC2001 (if wrong, IC2001 is faulty; if right, check the deflection coil and C2016).
• No sound: check the input at pin 3 of IC2002 (if wrong, check the volume control, R2046 and C2038) and its output at pin 7. If both are good, check C2045 and the speaker.

The electrolytic capacitors named in these checks (C2009, C2012, C2016, C2038 and C2045) are listed in the Sharp MZ-80K Capacitor Replacement Guide.

Cassette recorder

Sharp's cassette charts:

• Motor does not run when PLAY is pressed: check for +12 V between M+ and M−. If it is there, the governed motor is faulty. If not, check that the switch linked to the PLAY button goes high (if not, the switch is faulty), then the SENSE line at check point 4, which must be 0 V with the switch on and +5 V with it off. If SENSE is wrong, check the +5 V input and R3024; if it is right, check the motor's +12 V supply and the CPU board.
• Will not record: check that SW3001 operates when RECORD is pressed (+5 V at check point 2). Then look for a signal at check point 6 (if none, check the CPU board), at the base of Q3001 (if none, check SW3001 and R3002) and at the record/playback head (if none, check SW3001's contacts, Q3001 and Q3002, and the voltage on the erase head).
• Will not load, or loads with errors: check the tape is at the start of the program and look for a clean waveform at check point 5. Check the CPU board's +5 V, the 6-pin plug and cable, IC3001, the resistors and SW3001. If the waveform is noisy, check whether SW3001 leaves the erase head running; if the level is low, check around Q3003 and the head azimuth.

On the CPU board, Sharp's checks for LOAD are the output at pin 4 of IC4 (present: IC5; absent: IC4), for SAVE the output at pin 15 of IC5 (present: IC4; absent: IC5), and for a motor that will not start or stop the level at pin 6 of IC2 (correct: IC3, Q1 and Q2; wrong: IC2, IC24, IC4 and IC5). Clean the heads, capstan and pinch roller before anything else, as described in Sharp MZ-80K General Maintenance. Maurice Hawes found that the PRESS PLAY contact needed occasional cleaning.

Keyboard

Sharp's causes for a key that does nothing are poor soldering, a faulty key switch or a broken track; after that, check IC5 (8255) and IC6 (74LS145) on the CPU board.

Sound and clock

With no sound, check for an output at pin 10 of IC18 (8253): present, check IC12 and IC3; absent, check IC18. For distorted sound check IC12 and IC18, for wrong tempo IC13, and for a faulty clock IC18. The speaker itself is driven from the monitor board; see Monitor board.

Related pages

• Sharp MZ-80K
• Sharp MZ-80K General Maintenance
• Sharp MZ-80K Capacitor Replacement Guide
• Sharp MZ-80K Service Manual
• Capacitor Failure Symptoms