Troubleshoot an MA-1739 video board for Dot Matrix

Pinup
GAL1739

Replacement of the GAL

Repair of an MA-1739 video board and replacement of the defective GAL (protected).

Create a substitute GAL by reverse-engeenering.

Presentation

Video cards MA-1739 failures often are caused by a defective GAL, indeed frequently, the U8 circuit is sometimes grilled.

This occurs when we unplug the connectors the card while pinball is still powered.

The replacement of this programmable circuit is problematic because its content is protected and it is not possible to duplicate it. We conducted its analysis by reverse-engineering, in order to produce a replacement GAL.

The principle is similar to the one we used for the lag of the BSMT cards.

MA-1739 board GAL decoding schematic

Method

There are 9 entries (i.e. 512 possibilities) and 8 outputs used on the GAL. To simplify the work of analysis we used a PIC 16F886.

This circuit will sweep all entries and store the status of the outputs (i.e. Byte) in its internal EEPROM. The memory of the PIC is only 256 bytes, we proceed in twice, the entry of manually fixed levels 0 and 1.

This method allows to obtain a table of truth for each output of the GAL.

GAL16V8 GAL16V8

  • 1 - Mounting plate for experimentation.
  • 2 - The wiring detail.
  • 3 - Reading of the memory of the PIC
  • 4 - Verification OK.

Once the table of truth is known, must be creating a JEDEC file for the programming of the GAL. We use WINCUPL (available from ATMEL software) to capture the logical equations.

It remains then to program the generated file.

Distribution

This achievement is not publicly available, in any form whatsoever (free or paid).

Private

Last update of this page: August 7th, 2018

French Tech

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