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Atari TT Capacitor Replacement Guide

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TT030 main board

The Atari TT030 main board has few electrolytic capacitors: most of its capacitors are ceramic decoupling parts. Atari's service manual contains no component-level parts list for the main board or for the power supply, but the main board schematic printed in the manual gives a value at every capacitor position, and the parts list for the ST RAM daughterboard lists its two electrolytics with their rating.[1][2]

Safety

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Warning: the power supply works from mains voltage and its primary-side capacitors can hold a dangerous charge after the machine is unplugged. Measure across the bulk capacitor before touching the primary side.

Main board electrolytic and tantalum capacitors

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The table lists every polarised capacitor on the twelve sheets of the July 1991 schematic in the service manual, which is for the 32 MHz main board, assembly CA400770. The schematic prints values but no voltage ratings.[1]

TT030 main board (C301762-001, July 1991): polarised capacitors
Ref Value Sheet Position on the schematic
C105 470 µF 1 +5 V at power connector J100
C120 10 µF 1 −5 V at J100. The schematic says "C120 should be a tantalum cap"
C106 10 µF 1 Power-on reset timer (LM556, U103A)
C108 10 µF 1 Reset button timer (U103B, switch S100)
C402 10 µF 4 Real-time clock supply
C608 100 µF 6 Cartridge port +5 V, after fuse F61 (5.0 A) and ferrite bead L601
C846 100 µF 8 Sound section +5 V, after R842
C848 100 µF 8 Sound section −5 V, after inductor LD01 (10 µH) and R843
C850 100 µF 8 Output of the 8.2 V regulator Q803 (LM78L82)
C852, C853 47 µF 8 DMA sound DAC outputs, left and right
C809, C823 47 µF 8 DMA sound filter outputs, left and right
C833 47 µF 8 LMC1992 (U816) bias network
C837 47 µF 8 Sound chip (YM2149) input to the LMC1992
C840, C841 47 µF 8 GIL and GIR inputs to the LMC1992
C838 10 µF 8 LMC1992 pin 27
C828, C835 47 µF 8 Left and right line outputs to the phono sockets J801 and J802
C830, C839 47 µF 8 LMC1992 outputs to the buffer amplifiers U803B and U803C
C845 3.3 µF 8 Internal speaker driver
C854 47 µF 8 Internal speaker driver
CD20 10 µF 12 Supply of the monochrome video serialiser UD01 (DP8516). The schematic says "CD20 should be a tantalum"

That is 25 parts, 18 of them in the sound section on sheet 8. The other sheets (MCU and video, MIDI and keyboard, ST RAM, SCU and ROMs, VMEbus, SCC, SCSI and TT RAM connectors) carry only ceramic capacitors.[1]

C120 and C848 are on the −5 V rail, so their positive leads go to ground. Note the orientation of every part before removing it.

The earlier schematic

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The separately hosted Atari TT030 Schematic is an earlier issue of the same drawing (sheet 8 dated 7 May 1990). Its sound sheet has no inductor LD01 on the −5 V feed and no ferrite beads FB5 and FB6 on the outputs, and its power connector sheet has no ferrite beads on the J100 rails.[3][1] Check the silkscreen on your own board against whichever drawing matches it.

ST RAM daughterboard

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Atari's 2/8 MB ST RAM board, CA401059, has two electrolytics, C23 and C24, each 100 µF 16 V axial (Atari part C070499-004).[2]

Power supply

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Atari's parts list gives the supply only as a complete unit: Phihong PSM 5341, Atari part C302074-001.[2] Atari's documents contain no schematic or parts list for its inside, so take the values and ratings from the parts fitted, and photograph the board before removing anything. The same Phihong PSM-5341 is fitted to some Mega STEs; a community list of its capacitors, from Console5 and exxos, is at Atari Mega STE Capacitor Replacement Guide#Power supply. exxos lists recapping the supply as a mandatory fix for every ST-family machine.[4] The supply's rating is 64 W from 100–240 V AC, with +5 V at 7 A, +12 V at 2 A, −5 V at 0.4 A and −12 V at 0.3 A.[5]

Procedure

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  1. Remove the top cover and take out the power supply and main board as described on Atari TT General Maintenance#Disassembly.
  2. Photograph the board and note the polarity and the voltage rating printed on each capacitor in the table.
  3. Replace one capacitor at a time with a part of the same value and the same or higher voltage rating. Fit tantalum parts at C120 and CD20, as the schematic asks.
  4. Clean off the flux and inspect the joints under magnification.
  5. When refitting the supply, check that fuse F61 and ferrite bead L601 are still covered with insulation; Atari added it (ECO 1323) so that they cannot short against the supply's mounting foot.[6]

See Capacitor Failure Symptoms and Recommended Tools.

After the work

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  1. Before connecting the supply, check each rail at J100 for a short to ground.
  2. Power up and measure +5 V, +12 V, −5 V and −12 V at J100 (pins 2 and 10–12, 3, 9 and 4).[1]
  3. Boot to the desktop, then check sound from both the internal speaker and the two phono outputs. Atari's diagnostic cartridge has an audio test (A) for this.[5]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Atari Corporation, TT030/32 schematic, drawing C301762-001, twelve sheets dated 1 July 1991 (revision block up to ECO 1323), reproduced in the Atari TT030 Computer Field Service Manual, C302483-001, August 1991, section 8 (PDF pp. 114–125). Sheet 1 (PDF p. 114): J100, C105, C120, C106, C108; sheet 4 (p. 117): real-time clock, C402; sheet 6 (p. 119): cartridge port, F61, L601, C608; sheet 8 (p. 121): sound, C809–C854; sheet 12 (p. 125): monochrome video, CD20. Hosted as Atari TT030 Computer Field Service Manual (PDF).
  2. ↑ 2.0 2.1 2.2 Atari Corporation, Atari TT030 Computer Field Service Manual, C302483-001, section 7, parts list: PDF p. 92, Assy ST RAM 2/8 MB CA401059 Rev C (16 July 1991), C23 and C24, C070499-004, cap electrolytic axial 100 µF 16 V; PDF pp. 99–101, main assembly CA400416 with the Phihong PSM 5341 supply (C302074-001) and ECO 3005.
  3. ↑ Atari Corporation, TT030/32 schematic, drawing C301762-001, earlier issue (sheet 8 dated 7 May 1990). Hosted as Atari TT030 Schematic (PDF).
  4. ↑ exxos, "MANDATORY FIXES", exxos's Atari, Amiga & retro forum, 31 January 2019, item 1. Community source.
  5. ↑ 5.0 5.1 Atari Corporation, Atari TT030 Computer Field Service Manual, C302483-001, section 1.4.1, power supply rating, p. 3 (PDF p. 10); section 3.4.3, diagnostic cartridge test menu, p. 35 (PDF p. 42).
  6. ↑ Atari Microsystems Corporation, Assy. Drawing, TT030 32 MHz, drawing CA400961, 16 May 1991, in the Atari TT030 Computer Field Service Manual, section 8 (PDF p. 112): ECO 1323 steps 10–12 and note 5.


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