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Old 13 November 2019, 20:31   #262
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Join Date: Dec 2016
Location: East Kilbride, Scotland
Posts: 235
If it helps, this is the fix posted for the Checkmate Zorro adaptor:

Checkmate 1500 Plus ACA500+ Issue

An issue has been reported with certain ACA500+ accelerators refusing to boot, or only booting after several retries. In these cases, once the machine has booted it is usually stable. It appears to primarily affect later revision ACA500+ boards with a different type of 68000 CPU fitted. A potential fix has been identified by Edu Arana that eliminates the problems starting up the system with the ACA attached. This fix is described below.

Important: Modifying hardware like this always carries risks. Please do not attempt this unless you have the necessary tools and equipment to successfully carry out a job like this!

The nature of this fix is to improve the grounding of the ACA500+ board. While the Zorro-II / PSU control board has all the ground terminals connected to the system ground, it appears that this is not sufficient to protect the tight signal tolerances required by the later ACA500+ models. The ground plane of the Zorro riser is traversed by two signals used for the PSU control circuitry, which interrupts the continuous plane and results in a longer path for the ground between the Amiga and the ACA slot.
Fitting six short wires between the ground connections on the Amiga motherboard connector and those on the ACA connector provides a more direct ground path to the ACA500+, thus improving the signal integrity enough to allow booting of the ACA500+.
The ground connections in question are pins 13, 25, 37, 49, 61 and 73 on both the 86-pin connectors. Attention should be paid especially to the ACA connector, as the connector is a mirrorimage of the motherboard connector and therefore the odd-numbered pins are on the lower side, not the upper side as on the motherboard connector. Using six wires approximately 70mm in length, connect pin 13 of the motherboard connector to pin 13 of the ACA connector, pin 25 to pin 25 and so on, on the rear of the PCB. Take care while soldering so as not to short these connections to any other lines as doing so and powering it up could damage your hardware.

Rob Cranley, 02 November
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