Troubleshoot an Alvin BSMT audio board
Replacement of the PAL
Repair of an Alvin BSMT audio board and replacement of the defective PAL (protected).
Create a substitute GAL by reverse-engeenering.
This board ship a Mystery Castle pinball and is located at top right of the pediment.
Built on the basis of a CPU 6809 (68B09) and an audio circuit BSMT (derived from the Texas Instruments TMS 320), this card uses a PAL 16L8 (labeled ECP-025) to reduce the number of components. As can be seen in the diagram, this circuit is used only for the address decoding.
Diagnosis and solution
The board that we own wasn't work at all and the PAL was abnormally heat. Being mounted on support, we have done a cross-test with another functional card.
After exchange of components, it is proves that it was the PAL in question. Unfortunately, the replacement of this programmable chip is problematic because its content is protected and it is not possible to duplicate it.
There is however a solution because looking at the schema, it can be seen that the realized decoding is not so complex. There is 8 output used, but mostly only 8 entries:
- A11 to A15
What is total 256 possibilities, can therefore be considered a manual scan of the operation (of the functional PAL). This number can also be logically reduced, since it is known for example that the address $FFFF will be necessarily of the ROM.
Will replace the PAL 16L8 by a GAL 16V8, a pin to pin compatible component and especially still available.
The 256 patterns analysis is facilitated by the study of the datagram of the 6809. It shows for example that the addresses are valid only when E is "1".
It only remains to try all possibilities and establish a truth table, operation facilitated by the use of our test bench. It visualizes the result output by connecting directly with leds on the outputs.
Programming the GAL
Once the truth table is known, we must create a JEDEC file for the programming of the GAL. We use WINCUPL (available from MICROCHIP/ATMEL) to write the logical equations. This software has a simulator to test the logic of operation expected.
It remains to program the file generated by a programmer (we use the TOP 3100 or the TL866CS).
It can then install the new GAL instead of the defective PAL.
This achievement is available in our shop.