Atari XF551 Capacitor Replacement Guide

XF551 power supply on Atari's schematic C100727: C102, C103 and C104 are the three 4700 µF capacitors

The Atari XF551 controller board rectifies the adaptor's 9 V AC itself, so its power supply carries three large electrolytic capacitors. Atari's schematic C100727 gives six electrolytics on sheet 1: the three 4700 µF parts, two 47 µF output capacitors and the 1 µF reset capacitor of the microcontroller. The scan of the schematic has no sheet 2, and the drive mechanism's own board is not covered by it, so check the board in front of you for any electrolytic not in the table.

Signs of failure

• +12 V missing at TP14 or J5 pin 4 while +5 V is present. The +12 V rail depends on the voltage doubler, C102 with D2 and D3, and on C104.
• Bulging tops on the three large capacitors. The white material round their bases is adhesive against vibration, not leaked electrolyte.
• A drive that runs hot for long periods: the capacitors sit near the regulators' heatsink and the rectifier, and Gramblička adds heatsinking partly to keep them cooler.

See Capacitor Failure Symptoms for general signs.

Capacitor list

Atari XF551 electrolytic capacitors (schematic C100727, sheet 1)

Ref	Value	Voltage	Where it sits	Notes
C102	4,700 µF	35 V	Voltage doubler, between the bridge D1 and the junction of D2 and D3
C103	4,700 µF	35 V	Input of Q5, the +5 V regulator	About 14 V DC in use (Gramblička)
C104	4,700 µF	35 V	Input of Q6 (7812), after the doubler	About 23 to 26 V DC in use (Gramblička)
C109	47 µF	16 V	Output of Q5, +5 V
C110	47 µF	16 V	Output of Q6, +12 V, at TP14
C113	1 µF	Not marked	Reset input of U1 (pin 4) to ground

Values, ratings and positions are from Atari's schematic; the working voltages are Gramblička's figures. The other capacitors in the power supply are not electrolytics: C101, C111 and C112 are 0.033 µF film capacitors at the AC input, and C105 to C108 are 0.1 µF decoupling capacitors at the regulators.

Gramblička notes two board versions, of 1987 and 1988, and says the later one drops some of the extra DC filtering, and that boards exist with duplicated component designators. Atari's drawing is dated November 1987. Identify each capacitor on the board by its position in the circuit as well as by its printed designator.

In his own XF551 design Gramblička uses a 1 µF 50 V reset capacitor and 47 µF 35 V output capacitors, having moved the outputs up from 16 V parts because he prefers the higher rating.

Procedure

1. Unplug the adaptor and the SIO cables, and wait for the large capacitors to discharge. Measure across C102, C103 and C104 before touching them.
2. Open the case. The drive mechanism sits over the controller board and has to be moved out of the way to reach it.
3. Note the polarity of each capacitor before removing it, and replace them one at a time.
4. Use parts of the same capacitance and at least the same voltage rating, and check that the larger replacements clear the heatsink.
5. Refit the mechanism and close the case.

Checks after recapping

Voltages to check

Point	Expected	Source
J5 pin 1	+5 V	Atari schematic
J5 pin 4, TP14	+12 V	Atari schematic
Across C103	About 14 V DC	Gramblička
Across C104	About 23 to 26 V DC	Gramblička

Then boot a known-good disk and format a spare one.

Switching regulators

Gramblička replaces the 7805 and 7812 with Traco TSR 1-2450 and TSR 1-24120 switching modules, which fit the same positions and run almost cold. Writing about the 1050, whose supply works on the same principle, he also warns that if a switching DC adaptor is used instead, with the bridge and doubler removed, 4700 µF capacitors are too large for the adaptor's output and 470 to 1000 µF is enough.

Related pages

• Atari XF551
• Atari XF551 Maintenance Guide
• Atari XF551 Drive Troubleshooting
• Atari XF551 Documentation
• Atari 1050 Capacitor Replacement Guide
• Capacitor Failure Symptoms