Sharp MZ-3500 Troubleshooting Guide
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Fault-finding on the Sharp MZ-3500 starts with the power supply and its alarm signal, then uses the self-check programs built into the machine. Sharp's service manual, code 00ZMZ3500SM/E, describes a main CPU test that reports errors by flashing the front-panel LED, and a sub-CPU test that shows its results on the screen.[1][2]
Warning: the power supply unit is connected to the mains. Unplug the mains cable before opening the case, and take care when measuring with the machine running.
Quick reference
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| Dead, no LED, no fan | Mains supply, fuse, supply unit | No power |
| +5 V and +12 V missing, −12 V present | Overcurrent protection has stopped the oscillator, or a fault in the switching section | +5 V and +12 V missing |
| Supply rails correct, nothing happens | ALARM line, reset network | Rails present but the machine does not start |
| LED flashes in groups after a self-check | Main CPU test error | Main CPU self-check |
| No picture or the wrong picture | Display type switch, CRT socket, video RAM | Display faults |
| Disk errors | Drive switch settings, drives, MFD interface | Floppy disk faults |
| Clock loses the time when switched off | Clock battery | Clock |
No power
[edit | edit source]- Check the mains cable and the fuse in the supply's input. Sharp's block diagram shows the fuse ahead of the noise filter and the power transformer.[3]
- Check that the supply unit matches the mains voltage. Sharp made separate units for the 100 V and 200 V series.[4]
- With the machine unplugged and opened, look over the supply for burnt parts and leaking or swollen capacitors. The capacitors are listed in the Sharp MZ-3500 Capacitor Replacement Guide.
- Plug in, switch on and measure the rails at the CPU board's test pins or the PS connector: VCC (pins 3 and 4) +5 V, VDD (pin 2) +12 V, VGG (pin 1) −12 V, against ground on pins 6 to 8.[5][6] The rail capacitors on the main CPU diagram show that VDD is the positive and VGG the negative 12 V rail.[6]
- The front-panel LED is fed from VCC through 680 Ω, so an LED that never lights points to a missing +5 V or to the LED board and its connector.[6]
+5 V and +12 V missing
[edit | edit source]The +5 V and +12 V switching circuits share one oscillator, and an overcurrent on either output fires the protection circuit, which stops the oscillator and shuts off both supplies. The −12 V supply comes through a dropper circuit with its own rectifier and smoothing circuit.[3] So:
- If −12 V is present but +5 V and +12 V are both missing, look for a short on the +5 V or +12 V lines (the CPU board, the drives and any option boards) before suspecting the supply. Unplug the loads one at a time and measure again.
- If the loads are clear, the fault is in the switching section, the oscillator or the rectifier that feeds them. The surviving scan of the service manual has no power supply schematic. Its parts list gives its semiconductors by reference number, among them the µPC78M12H M001, the switching transistors Q001 to Q018, the S10SC4M DU001, the 30DF-1 diodes D010 and D011, the 10DF-1 D013, the S10VB10 and 1B4B1 bridge rectifiers RC001 and RC002, and the 03P05M thyristor TH001.[7]
- If one of +5 V and +12 V is present and the other is low, check that rail's output capacitors and filter, and only then its adjusting trimmer. The supply has two 2 kΩ trimmers, RV001 and RV002; the manual does not say which rail each sets.[7]
−12 V missing
[edit | edit source]The −12 V rail comes from its own rectifier and smoothing circuit through the dropper circuit, separate from the switching supplies.[3] If it is missing while +5 V and +12 V are correct, the fault lies in that part of the supply or in a short on the VGG line. The manual does not give the dropper's parts separately from the rest of the supply.[7]
Rails present but the machine does not start
[edit | edit source]The MZ-3500 has no reset key.[8] The system reset comes from a resistor-capacitor network and from the power supply's alarm output.[9]
- Measure ALARM at pin 5 of the PS connector. It should go high just after +5 V comes up.[3][5]
- If ALARM stays low, the CPUs stay in reset. The alarm circuit in the supply runs from its own winding and holds its output for a moment after switch-off from the charge on its capacitors.[3]
- If ALARM is high, check the reset network on the CPU board: the 100 Ω resistor and 1 µF 50 V capacitor on ALARM, the 33 kΩ resistor, diode and 1 µF 50 V capacitor of the power-on network, and the gate and 7404 inverter that drive the Z80A's RESET pin.[6]
- After reset, the main CPU loads the IPL, starts the sub-CPU and tries to load the system from the floppy disk.[9] A machine that gets this far but cannot load the system points to the drives or the system disk; see Floppy disk faults.
Main CPU self-check
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The main CPU test checks the MFD interface, 128 KB of RAM and, on ROM-based machines, the 16 KB ROM. Sharp's procedure:[2]
- Turn on all four switches of the 4-bit service switch (A), in the middle of the front edge of the CPU board, and both switches of the 2-bit switch (B).
- Put a floppy disk in drive unit 2, the third drive. Without a disk there, or without an MFD interface, the MFD interface is not tested.
- Switch on. The LED flickers for a moment, then stays dark while the test runs.
- After about four seconds the result shows on the LED: steady on for a pass, flashing for a failure.
The number of one-second flashes in each group, separated by four seconds off, identifies the error:[2]
| Flashes | Error |
|---|---|
| 1 | MFD interface error |
| 2 | SD0 read/write error |
| 3 | SD0 bank alternation error |
| 4 | AD2 bank alternation error |
| 5 | AD3 bank alternation error |
| 6 | ROM sum-check error |
| 7 | Option RAM read/write error (shown even when no option RAM is fitted) |
| 8 | Option RAM bank alternation error |
[2] Sharp also describes a loading check, in which a test program is loaded from a set track and sector of a disk in an FD-55B drive and run, with the service switches set as shown in the manual.[2] Put the service switches back to their factory settings afterwards; the settings are in the General Maintenance guide.[10]
Sub-CPU self-check
[edit | edit source]The sub-CPU test covers its memory, the video RAM, the display controller, the clock, the speaker and the printer, light pen and RS-232C interfaces. Results appear on the screen, except for the display and speaker tests, which are judged by eye and ear.[2]
- Turn off all switches of (A) and (B).
- Set the ten-way system switch on the underside of the board, reached through the bottom of the cabinet, to: 1 OFF, 2 ON, 3 ON, 4 ON, 5 ON, 6 OFF, 7 OFF, 8 ON, 9 ON, 10 ON.
- Hold down the HALT switch and switch on. Press HALT to move from one test to the next.
| Test | What it checks | Result |
|---|---|---|
| Memory | Sub-IOCS RAM (4000H to 5FFFH) and the shared RAM (2003H to 23FFH and 2440H to 27FFH) | RA OK on three lines, or RA ER |
| Video RAM | Character and attribute RAM, filling the screen with test characters | VR OK or VR ER |
| CRT interface | Tests 1 to 8 for a 400-line display and 9 to 16 for a 200-line display: character sets at 40 × 20 and 80 × 25, attributes, and coloured characters on coloured backgrounds | Judged on the screen |
| Speaker | Speaker and volume control | Judged by ear; set the volume |
| Printer interface | Printer signal lines and the 8255 | PR OK or PR ER |
| Light pen interface | Light pen lines and the GDC | LP OK or LP ER |
| RS-232C interface | RS-232C signal lines and the 8251, with a loopback connection wired on the edge connector as drawn in the manual | RS OK or RS ER |
Display faults
[edit | edit source]- Check user switch 3 on the underside: ON for a high-resolution (400-line) display, OFF for a medium-resolution (200-line) one. Switch 9 chooses the character set used on a 200-line display.[10]
- The two display sockets, CRT1 and CRT2, are 8-pin DIN sockets with ground, blue, green and red, vertical and horizontal sync, video and composite sync.[5] Check the cable and the socket before the board.
- Run the sub-CPU video RAM and CRT interface tests.[2]
- Graphics need the MZ-1R03 board in its front slot.[11]
Floppy disk faults
[edit | edit source]- The service switches (A) and (B) must match the drives fitted: Sharp set them differently for single-sided and double-sided machines and for each kind of optional drive.[10]
- The main CPU self-check tests the MFD interface when there is a disk in drive unit 2.[2]
- The MZ-3500's CP/M disks have track 0 in single density, which Ohishi Nobuaki reports stops other machines' CP/M from reading them and the MZ-3500 from reading theirs. A disk from another machine that will not read is not necessarily a fault.[12] The service manual's media format also gives track 0 in single density.[13]
- For drive cleaning and mechanical faults, see Floppy Drive Repair.
Clock
[edit | edit source]The µPD1990AC clock runs from a battery through a 1.5 kΩ resistor when the machine is off.[14] If the time is lost at every switch-off, check the battery and the diode that feeds the clock from VCC. The parts list names the battery only by its Sharp code, UBATN1001ACZZ.[15] Check the board around an old battery for leakage; see Battery Explosion, Capacitor or Corrosion Damage.
Not faults
[edit | edit source]- There is no reset key. A crashed machine has to be switched off and on again. HALT works only in FDOS and BASIC.[8]
- LED code 7 appears even when no option RAM is fitted.[2]
Related pages
[edit | edit source]- Sharp MZ-3500
- Sharp MZ-3500 General Maintenance
- Sharp MZ-3500 Capacitor Replacement Guide
- Sharp MZ-3500 Service Manual
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ Sharp, Service Manual, Personal Computer Model MZ-3500, code 00ZMZ3500SM/E (File:Sharp MZ-3500 Service Manual.pdf). Hosted with a description at Sharp MZ-3500 Service Manual.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), section 11, self check functions, printed pages 94 to 98 (PDF pages 87 to 91), with flow charts on printed pages 99 to 102 (PDF pages 92 to 95).
- ↑ 3.0 3.1 3.2 3.3 3.4 Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), section 9, power circuit description, printed pages 87 to 89 (PDF pages 80 to 82): mains input with fuse, noise filter and power transformer; three rectifier and smoothing circuits, one feeding the +12 V and +5 V switching circuits and the others the oscillator, the alarm circuit and the −12 V dropper; an overcurrent detector resistor in each switching supply raises the current in the protection transistor, fires a thyristor and pulls the oscillator to ground, which stops both switching circuits and shuts off +5 V and +12 V; the oscillator is a two-transistor circuit running at about 25 kHz; each switching supply has an adjusting variable resistor; the alarm circuit holds the PFD (alarm) output active for a short time after power-off from the charge on its own capacitors, and the timing chart shows PFD rising after +5 V at switch-on and falling before it at switch-off.
- ↑ Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), parts guide, exteriors, page 2 (PDF page 110): power supply unit DUNT-1018ACZZ for the 200 V series and DUNT-1035ACZZ for the 100 V series; fan motor; LED board.
- ↑ 5.0 5.1 5.2 Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), 13-15, CPU P.W.B. connector terminal table (PDF page 104): PS connector pin 1 VGG, pin 2 VDD, pins 3 and 4 VCC, pin 5 ALARM, pins 6 to 8 GND; LED connector; HALT-VOL-SP connector (GND, SP, GND, VOICE GND, VOICE, VOL, HLT); CRT1 and CRT2 sockets.
- ↑ 6.0 6.1 6.2 6.3 Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), 13-1, main CPU and memory circuit diagram (PDF page 99): test pins for GND, VCC, VDD and VGG; rail capacitors with the positive end of a 220 µF 25 V capacitor on VDD and the negative end of a 100 µF 25 V capacitor on VGG; the reset gate takes ALARM through 100 Ω with 1 µF 50 V to ground and a 10 kΩ pull-up to VCC, together with a power-on network of 33 kΩ, a diode and a second 1 µF 50 V capacitor, and drives the Z80A RESET through a 7404; front-panel LED fed from VCC through 680 Ω on the LED board.
- ↑ 7.0 7.1 7.2 Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), parts guide, power supply unit, pages 7 and 8 (PDF pages 115 and 116): M001 µPC78M12H; Q001 to Q018 (2SC1815-Y, 2SA733-Q, 2SC2750-L, 2SA965-Y, 2SC1627-Y, 2SA1015-Y, 2SA1020-Y, 2SC2334-L, 2SC2655-Y); DU001 S10SC4M; D001 to D014 (HZ9L-A1, 1S2348H, HZ11L-B1, HZ7L-C2, 1S2076A-FEC, HZ7L-B2, 30DF-1, 10DF-1); RC001 S10VB10; RC002 1B4B1; TH001 03P05M; C001 to C031; RV001 and RV002 2 kΩ trimmers; R001 to R035. The list stops at R035 at the foot of page 8, and parts-guide pages 9 to 12 are not in the scan.
- ↑ 8.0 8.1 A Brief History of the MZ-3500– Maurice Hawes, SharpMZ.net (from SUC magazine, November 1999)—link(accessed 2026-10-21) The MZ-3500 has no RESET key, so a crashed machine has to be switched off and on again; the HALT key works only in FDOS and BASIC.
- ↑ 9.0 9.1 Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), section 2, printed pages 7 and 8 (PDF pages 8 and 9): after power-on reset the main CPU loads the IPL into RAM at 4000H with the sub-CPU held in reset, then releases the sub-CPU, assigns the ROM IPL to it, sets state SD1 and loads DOS from the system floppy disk; the reset signal is generated from the CR network and the power supply.
- ↑ 10.0 10.1 10.2 Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), section 8-4, printed pages 85 and 86 (PDF pages 78 and 79): system SW1 and the service DIP switches (A) and (B), with factory settings for single-sided and double-sided drives, the optional MFD settings, test modes 1 and 2 and the individual CPU board test.
- ↑ Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), section 1-4, MZ-1R03, printed page 3 (PDF page 4): graphic board with 32 KB of RAM and a µPD7220, inserted through the slot on the front panel.
- ↑ MZ-3500– Ohishi Nobuaki, Nibbles lab. Sharp museum—link(accessed 2026-10-21) CP/M disks with track 0 in single density, apparently for compatibility with the PC-3200, could not be read by other machines' CP/M, nor theirs by the MZ-3500.
- ↑ Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), track 1 sector 1 information for CP/M disks (PDF page 63) and sector 1 track 0 information for the test disk in section 11 (PDF page 89): N = 0 for single density on track 0, N = 1 for double density elsewhere.
- ↑ Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), section 8-1, clock circuit, printed page 81 (PDF page 74), and 13-2, sub-CPU circuit diagram (PDF page 100): µPD1990AC on port B of the µPD8255 with a 32 kHz crystal; its VDD pin is fed from VCC through a diode and from the battery through 1.5 kΩ, with 33 µF 16 V and 0.01 µF to ground.
- ↑ Sharp, MZ-3500 service manual (File:Sharp MZ-3500 Service Manual.pdf), parts guide, CPU PWB, pages 6 and 7 (PDF pages 114 and 115): battery UBATN1001ACZZ.