diff --git a/src/mame/drivers/midxunit.cpp b/src/mame/drivers/midxunit.cpp
index 34d4dc0f281..466d50cfcdc 100644
--- a/src/mame/drivers/midxunit.cpp
+++ b/src/mame/drivers/midxunit.cpp
@@ -77,7 +77,38 @@ Notes:                                                         |
       P10      - 10 pin connector
       P12      - 4 pin connector
 
-There's a separate sound board also, but it wasn't available so is not documented here.
+There's a separate sound board also.
+
+SOUND BOARD - 5766-13959-01 - Williams Electronic Games Inc.
+________________________________________________________________
+|                   __________   __________  ________________   |
+|                   PC74HCT541P  |74HC541N_| |U9 M27C4001    |  |
+|                    __________  __________  |_______________|  |
+|                  CY7C128A-35PC |74HC541N_|   ________________ |
+|                                             |U8 M27C4001     ||
+|                    __________  __________   |________________||
+|                  CY7C128A-35PC SN74HC245N    ________________ |
+|                    __________  __________   |U7 M27C4001     ||
+|                  CY7C128A-35PC |74LS174N_|  |________________||
+|                                              ________________ |
+|                                __________   |U6 M27C4001     ||
+|                                |74HC174N_|  |________________||
+|                   __________                 ________________ |
+|                   |ADSP-2105|  __________   |U5 M27C4001     ||
+|             ____  |         |  M74HC138B1   |________________||
+|             XTAL  |         |  __________    ________________ |
+|        10.0000MHz |_________|  |_74F00PC_|  |U4 M27C4001     ||
+|                   __________   __________   |________________||
+|   __________ U17->|GAL16V8A_|  |_M7406N__|   ________________ |
+|SCC2691AC1N24 _________                      |U3 27C040-10    ||
+|    ________  |AD1851N_|                     |________________||
+|    M74HC14B1                                 ________________ |
+|    ________  _________                      |U2 M27C4001     ||
+|   AM26LS31CN |TL084CN_|                     |________________||
+|    ________  AUX-IN AUX-OUT                          FUSE     |
+|   AM26LS31CN                    _________                     |
+| :::::::::: <-SERIAL  PWR/PPKR-> ···· ····              ·· ··  |
+|_______________________________________________________________|
 
 **************************************************************************/
 
@@ -417,9 +448,10 @@ ROM_START( revxp5 )
 	ROM_LOAD32_BYTE( "revx_p5.53",  0xe00002, 0x80000, CRC(fcfcf72a) SHA1(b471afb416e3d348b046b0b40f497d27b0afa470) )
 	ROM_LOAD32_BYTE( "revx_p5.54",  0xe00003, 0x80000, CRC(fd684c31) SHA1(db3453792e4d9fc375297d030f0b3f9cc3cad925) )
 
-	ROM_REGION( 0x400, "plds", 0 )
-	ROM_LOAD( "a-17722.u1",   0x000, 0x117, CRC(054de7a3) SHA1(bb7abaec50ed704c03b44d5d54296898f7c80d38) )
-	ROM_LOAD( "a-17721.u955", 0x200, 0x117, CRC(033fe902) SHA1(6efb4e519ed3c9d49fff046a679762b506b3a75b) )
+	ROM_REGION( 0x600, "plds", 0 )
+	ROM_LOAD( "a-17722.u1",       0x000, 0x117, CRC(054de7a3) SHA1(bb7abaec50ed704c03b44d5d54296898f7c80d38) )
+	ROM_LOAD( "a-17721.u955",     0x200, 0x117, CRC(033fe902) SHA1(6efb4e519ed3c9d49fff046a679762b506b3a75b) )
+	ROM_LOAD( "snd-gal16v8a.u17", 0x400, 0x117, NO_DUMP ) // Protected
 ROM_END