Atari 835 Capacitor Replacement Guide

The Atari 835 Direct Connect Modem has six or seven aluminium electrolytic capacitors, depending on how the main filter is fitted. Atari's 835 Direct Connect Modem Field Service Manual (FD100268 Rev. 01) lists no capacitors in its parts list, but its schematic marks the electrolytics "ELO" and gives a value and rating for each.^[1]

Signs of failure

Power LED off with a good adaptor. Atari's flowchart checks for about 19 V DC at the cathode of CR3, at the bridge rectifier output.^[1]
A hook relay that fails the 30-second carrier wait. Atari's flowchart names C9 first, but C9 is a 0.047 µF Mylar part, not an electrolytic.^[1]
Bulged, leaking or crusted capacitors.

See Capacitor Failure Symptoms for general signs.

Capacitor list

**Atari 835 electrolytic capacitors (FD100268 schematic)**
Ref	Value	Rating	Tolerance	Where it sits
C7	220 µF	35 V	20 %	+V supply (about 18 V), after the bridge rectifier
C8	220 µF	35 V	20 %	+V supply, beside C7
C13	470 µF	35 V		+V supply; drawn dashed (optional) on the schematic
C1	10 µF	35 V	20 %	+5 V output of the 7805C regulator M1
C3	10 µF	35 V	20 %	+12 V supply
C2	10 µF	35 V	20 %	-5 V supply, across zener CR5
C12	1.0 µF	50 V	+80/-20 %	Reset pin of the 8048 controller M5

The values, ratings and tolerances are as printed on page 2 of the schematic, and the polarity marks on the silkscreen agree for C1, C2, C3, C7, C8 and C12.^[1] The silkscreen draws C13 as a dashed outline across the positions of C7 and C8, so check which parts a given board carries before ordering.^[1] The other capacitors on the board are ceramic or Mylar types: C4, C5, C6 and C14 (0.1 µF), C10 and C11 (20 pF), C9 (0.047 µF Mylar), and C15 and C16 (0.01 µF and 0.0022 µF, rated 1 kV, on the telephone line side).^[1]

The supply rails are +V (about 18 V) for the hook relay and carrier LED drivers, +12 V for the modem chip, +5 V for the rest of the board, and -5 V for the hook relay driver and the modem chip.^[1]

Procedure

Unplug the adaptor, the SIO cables and both telephone cords.
Replace the capacitors one at a time, matching polarity to the board markings and noting the orientation of each before removing it.
Keep to at least the original voltage ratings.

Checks after recapping

Switch on with the computer off: the power LED should light, the ON-LINE light flash once and stay off, and the relay click once.^[1]
With the power LED lit, the bridge output at the cathode of CR3 should be about 19 V DC.^[1]
Run Atari's 835 test program, as described in the Atari 835 Troubleshooting guide.

Related pages

References

↑ ^1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 ^1.6 ^1.7 ^1.8 Atari, Inc., 835 Direct Connect Modem Field Service Manual (FD100268, Rev. 01, March 1983): power supply p. 1-5; parts list p. 5-1; silkscreen (oversize sheet, page 1) and schematic (oversize sheet, page 2); flowcharts pp. 4-3 and 4-7. Hosted here as File:Atari 835 Direct Connect Modem Field Service Manual FD100268 Rev 01.pdf.

[fsm-1] 1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 ^1.6 ^1.7 ^1.8 Atari, Inc., 835 Direct Connect Modem Field Service Manual (FD100268, Rev. 01, March 1983): power supply p. 1-5; parts list p. 5-1; silkscreen (oversize sheet, page 1) and schematic (oversize sheet, page 2); flowcharts pp. 4-3 and 4-7. Hosted here as File:Atari 835 Direct Connect Modem Field Service Manual FD100268 Rev 01.pdf.

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