Atari 835 Troubleshooting
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Fault-finding on the Atari 835 Direct Connect Modem follows Atari's 835 Direct Connect Modem Field Service Manual (FD100268 Rev. 01, March 1983): a power-up check, a test program run from Atari's 835 test diskette, and a set of diagnostic flowcharts that end in a part to replace.[1]
Before you start
[edit | edit source]- Atari's service centres asked for the modem, its adaptor and the TeleLink II cartridge together, because all three are needed for a complete test.[1]
- Test equipment: an Atari 800, an Atari 810 disk drive, the BASIC cartridge, the 835 test diskette, an oscilloscope and, optionally, a digital voltmeter.[1]
- Do not connect the 835 to a Bell "Horizon" four-wire system; Atari warns that it damages the telephone system.[1]
- Most of the tests work without a telephone line. The autodial test can also be run on a real line, dialling the local time service.[1]
Power-up check
[edit | edit source]Switch on the 810 and insert the test diskette, fit the BASIC cartridge, and leave the computer off. Connect and switch on the 835. The power LED should light and stay lit, the ON-LINE light should flash once and stay off, and the hook relay K1 should click once. Then switch on the computer, which boots the test diskette and loads the 835's device handler.[1] Atari's set-up leaflet for owners describes the same single flash as the end of the modem's self-test.[2]
| Symptom | Check | Action |
|---|---|---|
| Power LED off | Swap in a known-good adaptor | Replace the adaptor if that cures it |
| Power LED off with a good adaptor | About 19 V DC at the cathode of CR3 | If not, replace the bridge diodes CR1 to CR4; if so, replace the LED CR12 |
| ON-LINE light keeps flashing at a slow rate | Self-test failure | Replace the controller M5 |
| ON-LINE light flashing, but not slowly | Self-test failure | Replace the modem chip M4 |
| Relay does not click at switch-on | Load and run the 835 test program |
Test program
[edit | edit source]At READY, type RUN"D1:TEST835". Press any key at the title to reach the option menu.[1]
| Test | Keys | Correct result |
|---|---|---|
| TV sound | 2, then 1, then 1, then 2 | Key 2 turns the TV sound on; key 1 gives a high tone (answer), pressed again a low tone (originate); key 2 turns the sound off |
| Off-hook | 3, then 3 again | The relay clicks each time; the menu shows OFF, then ON |
| Carrier wait | 9 | The relay clicks on and WAITING FOR CARRIER appears for 30 seconds; then the relay clicks off and the menu returns |
| Autodial | 7, then RETURN, then 1 | With no line connected, choose option 1: the relay clicks on, waits about three seconds for a dial tone, then pulses short for 1 and progressively longer up to 0. The menu returns after 30 seconds |
| Analog loopback | 8 | The screen turns grey and shows CARRIER (ACQUIRED), the ON-LINE light comes on, and the data sent and received appear on screen and match |
Hook relay
[edit | edit source]Atari's off-hook flowchart sorts relay faults three ways:[1]
- The relay does not click on with key 3: follow the autodial checks below.
- It clicks on but does not click off when key 3 is pressed again: replace K1.
- It does not click off at the end of the 30-second carrier wait after key 9: put a scope on test point TP1 at 1 V per division. It should read about 4.8 V, and after key 9 it should dip slightly (about 25 mV) for 30 seconds and then return. If either check fails, replace C9, R30 and M4, in that order. If both pass, tap the relay with the handle of a screwdriver: if it then clicks off, replace K1; if not, follow the autodial checks.
Autodial
[edit | edit source]If the relay does not click on and off with key 3, probe jumper W14; it should switch high (+5 V) and low (0 V) as key 3 is toggled. If it does not, replace M5. If it does, probe pin 14 of M3, which should switch high (about 4 V) and low (-5 V); if not, replace M3, then Q1. Finally, the collector of Q1 should go high (about 19 V) and low (-5 V); if it does not, replace Q1, and if it does, replace K1.[1]
If the relay works but does not pulse long and short during the autodial test, probe W14 for negative pulses at about 10 Hz matching the digits the computer sends. If they are missing, replace M5; if they are present, carry on with the M3 and Q1 checks above.[1]
Audio and transmit
[edit | edit source]When the TV tones are missing or the modem does not transmit, Atari checks along the signal path:[1]
- Test point TP2: the signal on this line should be low (0.1 V). If not, replace M5, then M3.
- Test point TP3 (the line amplifier output): there should be a sine wave of 2,225 Hz in answer mode, or of the originate tone in originate mode. If not, probe pin 1 of M2: no sine wave there means replacing M4, then M2; a sine wave there means replacing R19, then R20.
- Pin 15 of M3 should carry the same tone; if it does not, replace M3.
- If the tone reaches M3 but not the TV, replace the I/O cable, then C14, then R5.
The flowchart gives the originate tone as 1,225 Hz, while the theory of operation gives 1,270 Hz for a mark and 1,070 Hz for a space in originate mode.[1]
Analog loopback
[edit | edit source]If any part of the loopback result is missing, put the scope on pin 1 of the modem chip M4 while key 8 is pressed. It should be high (+5 V). If it is, replace M4; if not, replace the controller M5.[1]
Related pages
[edit | edit source]References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 Atari, Inc., 835 Direct Connect Modem Field Service Manual (FD100268, Rev. 01, March 1983): introduction p. v; theory of operation pp. 1-1 to 1-5; schematic (oversize sheet, page 2); test procedures pp. 3-1 to 3-3; diagnostic flowcharts pp. 4-2 to 4-13; parts list p. 5-1. Hosted here as File:Atari 835 Direct Connect Modem Field Service Manual FD100268 Rev 01.pdf.
- ↑ Atari, Inc., The Communicator II: Getting Started (C024438-001 Rev. A), step 9. Hosted here as File:The Communicator II Getting Started C024438-001.pdf.