Atari Mega STE Troubleshooting Guide

Mega STE main board C301564, labelled in German (atari-computermuseum.de): cache (Cache) and CPU at the bottom left, SIMM sockets (Arbeitsspeicher) at the right, ROM jumpers W201–W203 near the TOS ROMs (Betriebssystem).

Fault-finding on the Atari Mega STE draws on Atari's main-board schematic (revision 2, April 1990), its 1.44 MB floppy upgrade manual and its coprocessor kit manual. Atari's service manuals for the Mega ST and STE cover the parts of the design the Mega STE shares with them.

Safety

Warning: the internal power supply's primary side runs at rectified mains voltage and its reservoir capacitor holds a dangerous charge after the machine is unplugged. Unplug the machine before working inside it, and confirm the primary capacitor has discharged with a meter before touching the supply board.

Power supply

Rails at the main board

The power connector J201 is a 12-way header carrying four rails and Power Good:

J201 main-board power connector

Pin	Signal
1	Power Good
2, 10, 11, 12	+5 V
3	+12 V
4	−12 V
5, 6, 7, 8	Ground
9	−5 V

Check all four rails. The −5 V rail feeds the DMA sound converters and −12 V the serial line drivers, so a machine with either missing can still start. On the board, the incoming rails are decoupled by C223–C226 and C228 (0.1 µF) and by C227, 470 µF on +5 V at the connector. The same 12-way arrangement, with the same rail order, appears on the TT030 main board at J100. Atari's documents for the Mega STE give no rail tolerances. The supply is a Phihong PSM-5341 or PSM-8012-2; Atari rates the PSM-5341, in the TT030, at +5 V 7 A, +12 V 2 A, −5 V 0.4 A and −12 V 0.3 A.

Fuses and headers

• F21, 0.5 A, feeds +5 V to the cartridge port through ferrite L204. A dead cartridge port on a working machine points here first.
• F31, 0.5 A, feeds the keyboard socket. A keyboard that is completely dead, mouse included, points here.
• J290 is the power-LED header (through a 120 Ω resistor); J291 is a three-pin header for a second fan, on +12 V.

Supply faults

Symptom	Possible cause	Action
Completely dead, no LED, no fan	Mains lead, switch, or a failed supply	Check the lead and switch with the machine unplugged, then test the supply on its own.
Supply ticks and will not start	Overcurrent protection tripping, or a shorted output capacitor	Unplug J201 and test the supply into dummy loads. Check the secondary-side capacitors.
+5 V low, or the picture dims when the drives start	Aged secondary electrolytics	Recap the supply. See Atari Mega STE Capacitor Replacement Guide.
+5 V high	Failed feedback loop	Do not run the machine until the supply is repaired.
−12 V or −5 V missing	Rectifier or filter on that rail	Check J201 pins 4 and 9. Serial ports fail without −12 V; DMA sound without −5 V.

To test the supply out of the machine, load +5 V with a 2.2 Ω 17 W resistor and +12 V with a 10 Ω 17 W resistor and run it for less than 30 seconds at a time. The output leads are red +5 V, yellow +12 V, blue −12 V, white −5 V, orange Power Good (about +5 V when healthy) and black ground.

Clocks

Clock sources (Rev 2 schematic)

Location	Frequency	Notes
U204	32.215905 MHz (NTSC) or 32.084988 MHz (Peritel)	Master oscillator module, fitted to non-PAL machines only
Y201, Y202	4.433618 MHz; about 32.085 MHz	Discrete oscillator and colour crystal, fitted to PAL machines only
U408	16.021226 MHz	Oscillator module for the VME clock
Y301	2.4576 MHz	MC68901 MFP
Y401	32.768 kHz	Real-time clock, trimmed by C403 (5–30 pF)
UA12	3.672 MHz	Oscillator module for the Z85C30 serial controller

The CPU and the MC68881 socket both take CLK16. Because the master oscillator is not exactly 32 MHz, the CPU clock measures slightly above or below 16 MHz; that is not a fault.

No boot

1. Check all four rails at J201 and the Power Good line.
2. Check CLK16 at the CPU. Without it nothing runs.
3. Check that reset pulses low at power-on and then goes high. The power-on and reset-button timers are the two halves of an LM556 (U201), timed by C208 and C218 (10 µF each).
4. Check HALT. If it stays low, the CPU has halted, typically on a double bus error.
5. Reseat the ROMs and the socketed chips.

If the keyboard is not connected, the machine clicks continuously at power-on. That is not a fault.

TOS ROMs

The Mega STE shipped with TOS 2.05 and, from November 1991, TOS 2.06. The Rev 2 board has two 32-pin ROM sockets, U206 and U207, and three jumpers, W201–W203, that set them for the type of device fitted. The factory setting is for 1 Mbit ROMs.

ROM jumper settings (Rev 2 schematic, sheet 2)

Device	W201	W202	W203
27256	Either	1–2	Not connected
27512	2–3	2–3	Not connected
571001, 27C1000, 531000	2–3	2–3	2–3
27010, 571000, 27C1001	1–2	2–3	1–2

A machine that shows nothing after a ROM change is more often a jumper problem than a bad EPROM.

Symptom	Possible cause	Action
White screen, no activity	ROMs not being read: socket contact, or jumpers wrong for the devices fitted	Reseat the ROMs. Check W201–W203.
Corrupted desktop or garbled text	Partly failed ROM	Test with a known-good set.
TOS 2.06 needed for a 1.44 MB drive	TOS 2.05 fitted	Fit TOS 2.06; see the floppy section.

Memory

The four 30-pin SIMM sockets form two banks: U701 and U703 share RAS0, and U702 and U704 share RAS1. Each bank takes one module on the high byte and one on the low byte. Modules go in matched pairs, and a single pair goes in U701 and U703. exxos's list of recommended fixes rules out parity SIMMs in STE-family machines unless the parity chip is removed.

Symptom	Possible cause	Action
No boot, no video	SIMMs missing, unseated or failed	Reseat. Try one known-good pair in U701 and U703 only.
Random crashes and data corruption	Marginal or parity SIMM; SIMMs too slow for 16 MHz	Test each pair. If the machine is stable at 8 MHz but not at 16 MHz, suspect module speed.
Wrong memory size shown	Mismatched pair, or a pair in the wrong sockets	Match the modules in each bank. On the STE, Atari requires every SIMM in the machine to be the same size; start by matching all four.

CPU speed and cache

The machine starts at 8 MHz with the cache off. Software switches it by writing to $FFFF8E21: the MSTE_CC utilities write $FF for 16 MHz with the cache on, $FE for 16 MHz with the cache off and $F4 for 8 MHz. Software written for an 8 MHz ST can fail at 16 MHz for timing reasons; that is not a hardware fault.

The cache is two 8K × 8 data SRAMs (U004, U005) and two 8K × 8 tag RAMs (U008, U009) with PAL logic. A machine that is stable at 16 MHz with the cache off but crashes with it on points at this section. In one repair documented on the exxos forum, replacing the cache SRAMs cured random crashes at 8 MHz, but the machine still crashed at 16 MHz with the cache enabled. If the machine is unstable at 16 MHz whatever the cache setting, check SIMM speed and the +5 V rail under load before suspecting the CPU.

Floppy drive

The floppy controller sits in a socket at U405. Machines built with a 720 KB drive have a WD1772 there; Atari's kit CA401229-001 converts them to a 1.44 MB drive, and its steps are the list of what must be right for high-density disks to work:

1. Remove the controller at U405 and fit the Ajax chip, C302096-001.
2. Remove the PAL at UA2.
3. If the main board is below Rev B, add a jumper wire on the solder side from U7 pin 32 to UA2 pin 19.
4. Fit the replacement PAL, C301901-002, in UA2.
5. Fit TOS 2.06 (CA401187-001) if the ROMs are older.
6. Turn segment 7 of switch S701 on. The schematic annotates it "7 = Enable HD-drive (1.44MB)".
7. Set the drive's rear jumpers to 1-2, 5-13 and 14-15.
8. Fit the 1.44 MB drive (C301371-001) in the top cover. Pin 1 of the interface cable is at the opposite end to the 720 KB drive's. Plug the interface cable into J2 and the power cable into J1 of the drive.

Symptom	Possible cause	Action
Cannot read or write HD disks	720 KB drive fitted; Ajax not fitted; S701 segment 7 off; TOS below 2.06; pre-Rev B jumper missing	Work through the upgrade list in order.
Drive not detected	Controller at U405, cable orientation, drive select	Check the cable; pin 1 differs between the 720 KB and 1.44 MB drives.
Intermittent read errors	Dirty heads, or a supply sagging under motor load	Clean the heads with isopropyl alcohol. Watch +12 V during access. See Floppy Drive Repair.
Errors with a replacement drive	Drive has no pull-up resistors on its outputs	Add pull-ups on the board, as exxos describes for the STE and Mega ST.

Hard disk

The internal hard disk was optional. It is a SCSI drive on a host adapter that plugs into the main board.

Symptom	Possible cause	Action
Not detected at boot	No host adapter fitted, ID conflict, or no driver installed	Confirm the adapter is present, the SCSI ID is unique and the driver is installed (owner's manual, appendix C).
Spins but is not recognised	Termination or driver	Terminate the last device on the bus; reinstall the driver.
Data corruption	Cable, or DMA timing at 16 MHz	Try a short cable, and 8 MHz, to see whether the fault follows the speed.
Clicking or grinding	Mechanical failure	Back up at once. SCSI2SD or BlueSCSI can replace the drive.

See Hard Drive Maintenance and Repair.

Serial ports and LAN

The MFP's serial port and the Z85C30's two channels all use MC1488 drivers and 1489 receivers, which depend on ±12 V. The LAN port uses RS-422 buffers (26LS30 driver, DS34F86 receivers) on an 8-pin mini-DIN, and shares its SCC channel with Serial 2.

Symptom	Possible cause	Action
No output on any serial port	−12 V missing	Check J201 pin 4 first.
One port dead, the others working	The 1488 or 1489 for that port	Identify the driver for that channel and replace it.
Serial 2 dead	LAN port in use	Serial 2 is disabled while the LAN port is used.

VMEbus

The VME slot's interrupts go through the system control unit (U801), which also takes the SYSFAIL, ACFAIL and Power Good lines. A card that asserts SYSFAIL or ACFAIL therefore affects the whole machine. If the machine is unstable with a card fitted, measure +5 V with the card in and out, reseat the card, and check that its address range does not clash with the system. Sheet 9 notes that on the Rev 1 board, J901 pin 34 is open and should be jumpered to U904 pin 19.

Real-time clock

The RP5C15 (U402) runs from its own 32.768 kHz crystal and is backed by a battery on connector J401 through diode D401. The original battery is a non-rechargeable 3.6 V lithium cell on a lead. A clock that loses the date when the machine is off has a flat cell; check for corrosion around the cell at the same time. See Atari Mega STE General Maintenance.

Coprocessor

The socket takes an MC68881 or MC68882 in a PLCC package at UB02, clocked from CLK16 with the CPU; the PAL at UB01 decodes it, and the schematic notes that UB01 need not be fitted if UB02 is empty. Atari's kit fits the PAL C301903-001 in UB1 and then either parks jumper W3 on one pin or, on boards without W3, cuts out the resistor under the UB1 socket.

• Coprocessor not detected: check its orientation and seating, that the PAL is fitted in UB1, and that W3 has been moved or the resistor removed.
• Crashes in floating-point software: try a known-good coprocessor rated for at least 16 MHz.

DMA sound and video

DMA sound uses two DAC0802 converters and MF4 filters, and the LMC1992 feeds the RCA outputs and a mono mix to the monitor socket. The section runs from its own 8.2 V regulator (U609, LM78L82, from +12 V) and the DACs also take −5 V. If PSG sound works but DMA sound does not, check 8.2 V at U609 and −5 V at the DACs before suspecting the Shifter.

The STE and Mega STE show faint ghost images beside bright objects on screen; exxos lists a video ghosting fix for both.

Related pages

• Atari Mega STE
• Atari Mega STE General Maintenance
• Atari Mega STE Capacitor Replacement Guide
• Atari Mega STE Schematic
• Atari Mega ST Troubleshooting Guide
• Atari STE Troubleshooting Guide
• Atari TT Troubleshooting Guide
• Floppy Drive Repair
• Hard Drive Maintenance and Repair