Amiga 1200 Accelerator Timing Fix

The clock outputs of U20 (Budgie) and U2 (Alice) on Commodore's A1200 schematic. Each line has a series resistor and a capacitor position; E123 is on the 7MHZ line and E125 on _CDAC.

Many Amiga 1200 main boards left the factory with small capacitors fitted on the clock lines that Commodore had said should be removed. Individual Computers says that Commodore's internal memos call for E123C and E125C to be removed with any version of the Budgie chip, and that some boards also have E121C and E122C fitted; these parts spoil the board's 14 MHz clock enough to make most trapdoor accelerators unstable. Amiga-wiki lists the changes for each common board and Budgie revision.

Symptoms

• A trapdoor accelerator that runs unreliably. Individual Computers says the capacitors make most accelerators unstable, including other makers' cards.
• Individual Computers' later cards, the ACA1233n, ACA1221ec and ACA1221lc, have logic that copes with most of these timing errors, so they may run reliably on an unmodified board; the company still recommends removing the capacitors because they have other bad effects.

How the circuit works

The Budgie, U20, on an A1200 board. This one is marked CBM 391425-01.

On Commodore's schematic each clock leaving U20 (Budgie) and U2 (Alice) passes through a series resistor with a 22 pF capacitor position to ground:

Position	Signal	From
E121, E122 (27 Ω)	CPUCLK, CPUCLK_A	Budgie pin 90
E123 (27 Ω)	7MHZ	Alice pin 38
E125 (27 Ω)	_CDAC	Alice pin 37

The capacitors are the "C" parts of these positions, so E123C is the capacitor on the 7MHZ line and E125C the one on _CDAC. Several of the capacitor positions are marked with an asterisk on the schematic. Individual Computers' board photograph shows the four capacitor positions on the solder side of the board.

Which changes apply

Amiga-wiki gives these changes for -01 and -02 Budgie chips. The Budgie is U20, Commodore part 391425; the one in the photograph above is marked CBM 391425-01.

A1200 timing changes (Amiga-wiki)

Board	Budgie	Changes
Rev. 1D.4	-01	Remove E123C and E125C.
Rev. 1D.4	-02	Remove E123C and E125C. Remove XR358, a 470 Ω pull-up resistor. Add a 470 Ω pull-up resistor at DRA0, pin 43 of U2.
Rev. 2B	-01	Remove E123C and E125C.
Rev. 2B	-02	Remove E123C and E125C. Add a 470 Ω pull-up resistor at DRA0, pin 43 of U2.

Individual Computers adds that the board revision and date do not tell you whether the capacitors are fitted: practically every revision was built with every combination, so each board has to be checked.

Tools and parts

• Soldering iron with a fine tip, flux and desoldering braid.
• Magnifier.
• For a -02 Budgie: a 470 Ω resistor and a short length of fine wire.
• The main board schematic, to follow DRA0 to U2 pin 43.

Procedure

Prepare

1. Remove the accelerator and take the main board out of the case; see the Amiga 1200 Maintenance Guide.
2. Note the board revision printed on the board, and read the part number on U20 to find whether the Budgie is a -01 or a -02.

Remove the capacitors

1. Turn the board over and find the capacitor positions E123C and E125C, and E121C and E122C, on the solder side.
2. Remove E123C and E125C if they are fitted.
3. If E121C or E122C is fitted, remove it as well; Individual Computers names these as harmful when fitted.
4. Remove only the capacitors. The parts named for removal are the capacitors E123C and E125C; E123 and E125 themselves are the 27 Ω series resistors on the same lines.

-02 Budgie only

Alice, U2, on an A1200 board. The DRA0 pull-up for a -02 Budgie goes to its pin 43.

1. On a Rev. 1D.4 board, remove XR358, the 470 Ω pull-up resistor.
2. On Rev. 1D.4 and Rev. 2B boards, fit a 470 Ω pull-up resistor at DRA0, pin 43 of U2 (Alice).

Reassemble

1. Check the area for solder splashes and bridges, refit the board and the accelerator, and close the case.

Testing

1. Run the machine with the accelerator fitted under the software and loads that were unreliable before.
2. If the accelerator is still unstable, check the solder side again for any capacitor left fitted at E121C, E122C, E123C or E125C.

Board revisions

Commodore's schematic covers Rev. 1 to 1D. Pure Amiga describes Rev. 1D.4 and Rev. 2B as the most common A1200 boards: the 1D.4 uses four 256K × 16 RAM chips and the 2B four 512K × 8 chips. Amiga-wiki's list covers only these two revisions; for other boards, check the capacitor positions against Individual Computers' advice.

Related pages

• Amiga 1200
• Amiga 1200 Troubleshooting Guide
• Amiga 1200 Maintenance Guide
• Amiga 1200 Schematics