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Amiga 1200 Chip RAM Repair

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Amiga 1200 chip RAM and Budgie (Commodore schematic 364717)
The chip RAM on Commodore's A1200 schematic for Rev. 1 to 1D: U16 and U17, with U18 and U19 marked optional, all driven by U20 (Budgie).

The Amiga 1200 has 2 MB of chip RAM in four surface-mount DRAM chips, U16 to U19, controlled through U20, the Budgie.[1][2] A fault in the chip RAM stops the machine at a green screen during start-up. The steps below follow a repair by modrobert, who replaced chips and then had to track down bad solder joints, including an open UCAS pin, with DiagROM and a multimeter.[3][4]

Symptoms

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  • A solid green screen at power-on instead of the Workbench. Commodore's start-up colour sequence uses green for a memory problem, and modrobert's machine showed green with bad chip RAM.[4][3]
  • DiagROM reports chip RAM errors, or address line errors after chips have been changed.[3]
  • Only 1 MB of chip RAM shown although 2 MB is fitted. modrobert notes that the A1200's 3.1 ROM still boots with only U16 and U17 working, because Commodore's schematic marks U18 and U19 as optional.[3][1]

How the circuit works

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Amiga 1200 Budgie and chip RAM (MOS6502)
The Budgie, U20, and the chip RAM ICs on an A1200 board.

Commodore's schematic shows the chip RAM as four 256K × 16 DRAMs. U16 and U17 make up the first megabyte; U18 and U19 are marked "U18-19 optional". The address lines from U20 pass through 68 Ω resistors R151A-R151I, and the row and column strobes and write enable through R152A-R152G. Each DRAM has separate upper and lower column address strobes (UCAS and LCAS).[1] Pure Amiga notes that Rev. 1D.4 boards use four 256K × 16 chips while Rev. 2B boards use four 512K × 8 chips, both giving 2 MB.[2]

Chip RAM positions (Commodore schematic, Rev. 1 to 1D)[1]
Ref Part Notes
U16, U17 DRAM 256K × 16 First 1 MB
U18, U19 DRAM 256K × 16 Marked optional; second 1 MB
U20 Budgie, 391425 Bridge and DRAM control

Tools and parts

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  • An EPROM programmer and EPROMs for DiagROM, or a ready-made DiagROM. modrobert burned DiagROM 1.3 with a MiniPro TL866A.[3]
  • A serial cable and a terminal at 9600 baud, 8N1; DiagROM reports through the serial port on the A1200.[3]
  • Hot-air rework station, flux and a fine soldering iron. modrobert's joints improved after raising the hot-air temperature to 380 °C.[3]
  • Replacement DRAMs of the type fitted; modrobert's board used KM416C256BJ-7 chips in SOJ packages.[3]
  • Multimeter and the schematic, to check every DRAM pin to its neighbours.[3][1]

Procedure

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Confirm the fault

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  1. Note the start-up colour. Green points to memory.[4]
  2. Fit DiagROM in the Kickstart sockets, connect the serial port to a terminal at 9600 baud 8N1, and run its chip RAM test.[3]

Replace a suspect chip

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  1. Remove the main board; see the Amiga 1200 Maintenance Guide.
  2. Desolder the suspect DRAM with hot air, clean the pads and tin them.[3]
  3. Solder in the new chip and re-run the DiagROM test before changing anything else. modrobert changed three chips one after another without re-checking each, and ended with address line errors that came from his own joints, not from the chips.[3]

Check the solder joints

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  1. Using the schematic, check continuity with a multimeter on each data and address line between the DRAMs that share it. A joint that does not connect shows up as an open circuit between chips.[3][1]
  2. Reflow any joint that fails, with fresh flux.[3]
  3. Then check every other DRAM pin as well. modrobert's last fault was pin 28 of U19, the UCAS line, which was not connected to the board; soldering it with an iron restored the full 2 MB.[3]

Reassemble

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  1. Refit the original Kickstart ROMs and reassemble the machine.[3]

Testing

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  1. Run the DiagROM chip RAM test until it passes.[3]
  2. Boot the original ROMs and check that Workbench reports the full 2 MB of chip RAM; a board that boots with only 1 MB still has a fault in U18 or U19 or their connections.[3]

Board revisions

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Commodore's schematic, for Rev. 1 to 1D, shows four 256K × 16 DRAMs.[1] Pure Amiga says Rev. 1D.4 boards use four 256K × 16 chips and Rev. 2B boards four 512K × 8 chips, so check the chips on the board before ordering replacements.[2]

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Commodore, Amiga 1200 main board schematic 364717, "A1200 Rev 1->1D PCB" (1992), DRAM sheet: U16-U19 (256K × 16), "U18-19 optional", U20 Budgie bridge and DRAM part, R151A-R151I and R152A-R152G, PDF p. 7. Hosted at Amiga 1200 Schematics.
  2. ↑ 2.0 2.1 2.2 Chris, Repair Notes: A1200 distorted audio, 12 March 2022—link(accessed 2026-10-25)
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 Amiga 1200 chip RAM repair, 29 October 2025—link(accessed 2026-10-25)
  4. ↑ 4.0 4.1 4.2 Commodore Business Machines, Service Manual A500, PN-314981-04, October 1990, start-up sequence, PDF p. 33, hosted at Amiga 500 Service Manual (314981-04). Commodore's description is for Kickstart 1.3 on the A500.
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