Sharp MZ-80B Troubleshooting Guide

MZ-80B power supply boards: primary board with F101, F102, C101, C102 and L101, and the secondary board with the three regulators (Sharp service manual)

Fault-finding on the Sharp MZ-80B follows Sharp's order: power supply, monitor, CPU board, then cassette recorder. Component references and check points are from Sharp's service manual for the 240 V machine; owners' reports are cited where used.

Warning: mains voltage is present on the switch, fuses F101 and F102, the line filter and the transformer, and the CRT 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-80B symptoms

Symptom	Probable cause	Section
Bang and burning smell at switch-on	Mains filter capacitor C101 or C102	Power supply
One output missing	Fuse, rectifier, reservoir capacitors, SG3524N or FS7905, switching transistors	Power supply
Picture squashed into a line	Vertical circuit; C2010	Monitor
No picture or no raster	+12 V, CRT heater, high voltage, horizontal circuit	Monitor
No boot, garbage, memory errors	IPL ROM, RAM, address and data lines	CPU board
Tape will not move, record or load	Motors, solenoids, relays, heads, belt	Cassette deck

Power supply

The power supply (DBOXD0028PAZZ) makes −5 V with an FS7905 regulator (IC201), +12 V with an SG3524N (IC301) and the transistors Q301 and Q302, and +5 V with an SG3524N (IC401) and Q401 and Q402. Sharp's charts:

• No output at all: check the primary circuit, including the transformer. The primary fuses are F101 and F102 (T630 mA).
• No −5 V: if the other outputs are also wrong, check the primary circuit and T101. If they are right, check for about 11 V across C201. With 11 V present, check IC201, C203 and R201; without it, check T101, F201, D201, C201 and C202.
• No +12 V: check for about 24 V at pin 15 of IC301. Without it, check F301, T101, D301, C309, C301, C302 and C304. With it, open R309 and look for 0.2 V peak-to-peak at pin 12 of IC301. If the signal is there, check Q301, Q302 and the smoothing circuit; if not, check IC301 and its circuit.
• No +5 V: check for about 14 V at pin 15 of IC401. Without it, check F401, T101, D401, C412, C401 to C404 and C407. With it, open R405 and look for 0.2 V peak-to-peak at pin 12 of IC401. If the signal is there, check Q401, Q402 and the smoothing circuit; if not, check IC401 and its circuit.
• An output too high: −5 V, check IC201; +12 V, check Q301, Q302 and IC301; +5 V, check Q401, Q402 and IC401.

Sharp's waveforms were taken with loads of 2.5 A on +5 V, 1.25 A on +12 V and 10 mA on −5 V.

On the machine restored by Retro.Engineer, one of the two mains filter capacitors on the primary board exploded with a loud bang about ten seconds after the first power-on; both were replaced. Their rails had been checked with the logic board disconnected beforehand. Sharp lists these as C101 and C102, 0.047 µF 250 V.

Monitor

Sharp's monitor charts:

• No picture: check the +12 V supply first. With a raster but no picture, check for a video input signal (if missing, check the CPU board), then the video circuit, VR2002, Q2001, Q2002 and the 40 V video supply (D2009, D2010 and Q2007).
• No raster and no picture: check the CRT heater (if it is not lit, check the CRT plug). With the heater lit, check the voltages on the cathode and G1 to G3; if they are correct the tube is faulty, otherwise check the flyback circuit, then the middle and high voltage rectification (D2006, D2009 and the flyback transformer) or the AFC, horizontal oscillator and drive circuit.
• Sync: if there is no sync signal from the CPU board, check the CPU board. With neither horizontal nor vertical lock, check the sync separator (D2001 and Q2003). With only vertical lock lost, check the integrating circuit: C2011 to C2013 and the resistors around them, which the chart prints as "R2030 to R2023". With only horizontal lock lost, check the AFC circuit (the flyback output at check point 7, R2030, C2021, R2033, R2055 and C2055).
• Raster too narrow (vertical collapse): check the vertical oscillator waveform at pin 4 of IC2001 (µPC1031H). If it is wrong, check the V-HOLD control, C2010 and R2014. If it is right, check the input at pin 7 (R2016 and C2007), then the output of IC2001, then the deflection coil and C2016.
• No sound: check the input at pin 3 of IC2002 (LA4200); if it is wrong, check the volume control, R2050 and C2043. Then check the output at pin 7; if both are right, check C2053 and the speaker.

Retro.Engineer's MZ-80B showed vertical collapse on its second start, with the picture squashed into a single line. They worked through the test points to point 7 and found C2010, a 1 µF 35 V tantalum, short-circuited.

CPU board

Sharp's checks by block:

MZ-80B CPU board checks (Sharp)

Symptom	Check
No sync	Signals at pins 12 and 13 of IC36: present, check IC36; absent, check IC42 and IC43 and their 8 MHz and 16 MHz clocks.
No video	Pin 16 of IC42 high? If not, check IC23. If so, look for video at pin 8 of IC46: present, check IC31, IC33 and IC36; absent, check IC42, IC43 and the clocks.
Characters in the wrong place	Signals at pins 3, 6, 10 and 13 of IC37, IC38 and IC39, and the address lines of IC41.
Tape will not load	Signal at pin 6 of IC26: present, check IC23; absent, check IC26.
Tape will not save	Signal at pin 10 of IC23: present, check IC26; absent, check IC23.
Motor will not start or stop	IC23 (8255).
Clock wrong	31.25 kHz at pin 9 of IC19: present, check IC19 (8253); absent, check IC42.
No sound	Output at pin 4 of IC31: present, check the amplifier on the monitor board; absent, check IC23 and IC31.
Errors from programs in RAM	RAM

Sharp's RAM checker plugs into the boot ROM socket and tests from $0000 to $FFFF, reporting the failing address and the data written and read; the address identifies the bank and the data bits identify the chip. Sharp's general advice also applies: feel for a chip running hot (except the ROM and the video RAM), look for faults that change when the board is touched, and check TTL levels with an oscilloscope.

Cassette deck

Sharp's cassette charts:

• Tape does not move on FF or REW, motor not turning: check for +12 V between pins 7 and 9 of J3005. If it is there, the reel motor is faulty. If not, check the +5 V, regulated +12 V and unregulated +12 V supplies, the FF, REW and STOP button switches and the micro-switch, and the level at pin 1 of J3004, which should be 0 V with a tape inserted and 5 V without. Then check the Hall IC (the level at pin 3 of CN3002 should change between 0 V and 5 V when the counter linkage magnet is turned by hand), pin 11 of J3003, IC3011 and the CPU board.
• Solenoid not working: check for +12 V and then +4 V between pins 3 and 4 of J3005; if it is there, check the brake solenoid.
• Will not record: check for a signal from pin 1 of J3003 and for 0 V at pin 5 (if not, check the CPU board), then for a signal at the recording head (if none, check relay RY1; if present, the head).
• Will not load or gives errors: check for a clean waveform from pin 2 of J3003; if it is clean, check the start position of the tape. Otherwise check the output of IC3102, whether the erase head works because of a bad relay RY1, and the head azimuth.

Clean the heads, capstan and pinch roller and check the drive belt first; the procedures are in Sharp MZ-80B General Maintenance.

Related pages

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