Atari 850 Troubleshooting

The 850 power supply with test points A to J, from figure A3-1 of the service manual

Fault-finding on the Atari 850 Interface Module follows Atari's 850 Interface Module Service Manual (CS 400/800-S004-B), which splits faults by whether the module still boots when the computer is switched on. Routine care, the cable check and the port test are on the Atari 850 Maintenance Guide.

Set-up

Atari's test bench is a 400 or 800 with BASIC, a TV and the 850, with known-good cables. Switch the 850 on before the computer. When the computer is switched on, the 850 should boot: it answers the computer's first disk request and loads its R: handler. The module waits in reset until the computer is on, because it watches the +5 V/READY line (pin 10 of the SIO connector).

User-level faults

Operator's manual troubleshooting (C017651)

Symptom	Possible cause
No information sent or received	830 modem or 850 not switched on; loose cables
"RS 232 ERROR: 130" when talking to a non-Atari device	850 not switched on before the computer; switch both off, then the 850 on, then the computer
ERROR 138 or ERROR 139 when printing on an 825	Printer or 850 not switched on; loose cable; printer switched to local instead of online

Atari also suggests using GET and PUT in place of INPUT and PRINT while tracing a problem, since single characters give clearer clues.

When the 850 boots

A successful boot shows that the power-on reset, the +5 V supply, the clock, the 6507, most of the ROM and the system I/O buffers work.

Faults when the 850 boots (table 3-1)

Observed fault	Action or probable fault
No I/O instructions executed	Check the +10 V and -8 V supplies
Errors on all serial ports	Check the power supply
Errors on some serial ports only	Buffers of the affected ports, A105 to A108
Printer prints during boot	Local RAM; replace A103 or A104
Printer prints erratically	Printer port buffers
Printer does not print	FAULT line not high, BUSY line not low; bad cable

+10 V and -8 V

These rails feed only the serial port line drivers and, for +10 V, the clock circuit, so a module can boot with them out of specification. Low or noisy rails show as serial ports that work with some devices and not others, or give intermittent errors. Output from the 850 uses these rails; input does not, so a fault confined to output points to them. Atari's tolerance is ±5 %.

If the +10 V regulator A113 runs hot and its output is low, a line driver is loading it. Switch off, remove the I/O cables, pull A105 and recheck; if the voltage is still low, pull A106 as well. If it stays low with both out, the supply itself is at fault.

Local RAM

The 850 tests the RAM in its two 6532s during the boot and reports the result on the printer port. 00 means a pass; AA hex means the RAM in A104 failed and 2A hex the RAM in A103. No strobe is sent, so a printer usually shows nothing; watch printer port pin 3, 5 or 7 with a scope instead. If the printer does print during the boot, the RAM has failed.

Serial ports

Replace A105, A106, A107 and A108 in turn and run the port test on the faulty port. If a fault remains, look for broken traces or traces touching because the board has warped. A port that drops characters at higher speeds needs its buffer chips replaced.

Printer port

• The printer must hold FAULT (pin 12 of J101) high at about 5 V; if it does not, suspect the printer or the cable. If FAULT is high but not recognised, suspect A110 or A104.
• BUSY (pin 13 of J101) must be high while the printer prints. If not, data gets garbled or lost; check the printer's BUSY signal, then that it reaches A110 pin 8, then replace A104.
• Characters printed wrongly, such as lower case appearing as capitals, mean dropped bits: replace A110 and A111, then A103. A remaining fault may be in the ROM.

When the 850 does not boot

Atari's order of checks is: power LED and +5 V, clock, power-on reset, 6507, ROM, local RAM, system I/O, then noisy signals that suggest a bad ground.

Power supply

If the power LED does not light, the supply is probably faulty, though a lit LED does not prove the +5 V is good. Remove the top shell and the top of the RF shield and measure the test points.

Power supply test points (appendix 3, figures A3-1 and A3-2)

Point	Expected	If wrong
A-B (adaptor input)	9-10 V AC RMS, about 20-25 V peak to peak, with 120 V AC mains	Adaptor
C (bridge output)	Full-wave rectified, with no significant loss of peak against A-B	Bridge diodes CR114 to CR117
D (+5 V)	4.75-5.25 V DC, ripple no more than 0.1 V	Regulator A112 or its loose heatsink; zero volts suggests a broken trace, low volts a load
E (A113 input)	About 18.5 V DC; not below 18 V	C142, CR118, CR119
F (+10 V)	9.5-10.5 V DC	Regulator A113
G (-8 V input)	Half-wave rectified 9 V; not below 8.8 V	C136, C139, CR112, CR113
H (-8 V)	At least 8.25 V negative	C147, CR111, Q102
J (zener)	8.7-9.1 V, the voltage of zener CR111	CR111 if the waveform at J matches G

Loading the -8 V rail by running all the serial ports and the 20 mA current loop at once can induce a little ripple; Atari does not count small loss of regulation under those conditions as a defect.

Clock

A 4.433618 MHz crystal (X101) and transistor Q101 drive two D flip-flops in Z102 that divide by four, giving 1.1084045 MHz to pin 27 of the 6507. If the frequency is far off this figure, the 850 makes transmission errors and may not boot.

1. Check for about 1 MHz at pin 13 of Z102 (unloaded), then compare pin 12 (loaded).
2. A clean signal at pin 13 and a weak or distorted one at pin 12 means a load. Switch off, remove A101 and recheck. If pin 12 is still loaded, check C146, R161 and CR121, then replace Z102. If the load goes with A101 removed, replace A101.
3. If the flip-flop outputs do not follow the input at pin 3, and pin 3 is not at the full supply of about 10 V, replace X101 and check C144 and C145.

Use high-impedance probes; the probe alone can distort the waveform or stop the oscillator.

Power-on reset

First check that +5 V/READY arrives from the computer. Then monitor pin 1 of A101 (or pin 10 of Z101, or R149) while switching the computer on: it should stay low for about 200 ms, then go high and stay high. If it does not, remove Z101, hold A101 pin 1 low, switch on and pull pin 1 high; a boot then proves the reset circuit is at fault. Replace Z101, then A109, then check CR110, CR123, C129, C135, C153 and R150, R151, R166, R167 and R168.

Processor, ROM and RAM

With power, clock and reset good, A0 (pin 5 of A101) should pulse irregularly at about 500 kHz; if not, replace A101. Pin 28 (phase 2 out to the 6532s) should pulse at the clock rate of about 1.1 MHz. Check the ROM (A102) by substitution. Pulses on pins 3, 5 or 7 of J101 during an attempted boot mean defective local RAM; replace A103 or A104.

System I/O

Check with a scope that the command frame reaches pin 8 of A104 through transistor A109A, and that the acknowledgement from pin 13 of A104 gets out through A109B and Q103. Z101D buffers the +5 V/READY input.

Grounds and address lines

Very noisy signals suggest a poor ground connection. Broken or shorted address lines can cause almost any fault; Atari checks them last, with the method in appendix 9.

Related pages

• Atari 850
• Atari 850 Maintenance Guide
• Atari 850 Capacitor Replacement Guide
• Atari 850 Documentation
• Capacitor Failure Symptoms