diff --git a/src/mame/drivers/fidel_sc9.cpp b/src/mame/drivers/fidel_sc9.cpp
index 9b7a0bb3199..e61bdec68ad 100644
--- a/src/mame/drivers/fidel_sc9.cpp
+++ b/src/mame/drivers/fidel_sc9.cpp
@@ -3,6 +3,15 @@
 // thanks-to:Berger, yoyo_chessboard
 /******************************************************************************
 
+Fidelity SC9, Fidelity Playmatic "S"
+
+TODO:
+- fscc9ps module switch and led
+- verify fscc9ps XTAL (checked against sound recording, 99.97% similarity)
+
+
+Hardware notes:
+
 Fidelity Sensory Chess Challenger "9" (SC9) overview:
 - 8*(8+1) buttons, 8*8+1 LEDs
 - 36-pin edge connector, assume same as SC12
@@ -19,12 +28,13 @@ I/O is via TTL, not further documented here
 The Playmatic S was only released in Germany, it's basically a 'deluxe' version of SC9
 with magnet sensors and came with CB9 and CB16.
 
+
 Starting with SC9, Fidelity added a cartridge slot to their chess computers, meant for
 extra book opening databases and recorded games.
 
 Known modules (*denotes undumped):
 - CB9: Challenger Book Openings 1 - 8KB (label not known)
-- CB16: Challenger Book Openings 2 - 8+8KB 101-1042A01,02
+- CB16: Challenger Book Openings 2 - 2*8KB 101-1042A01,101-1042A02
 - *CG64: 64 Greatest Games
 - *EOA-EOE: Challenger Book Openings: Chess Encyclopedia Volume A-E (5 modules)
 - *TDF: Challenger Book Openings: Tarrasch Defense to the Queen's Gambit
@@ -256,7 +266,7 @@ INPUT_PORTS_END
 void sc9_state::sc9d(machine_config &config)
 {
 	/* basic machine hardware */
-	M6502(config, m_maincpu, 3.9_MHz_XTAL/2); // R6502AP, 3.9MHz resonator
+	M6502(config, m_maincpu, 3.9_MHz_XTAL / 2); // R6502AP, 3.9MHz resonator
 	m_maincpu->set_addrmap(AS_PROGRAM, &sc9_state::sc9d_map);
 
 	const attotime irq_period = attotime::from_hz(600); // from 555 timer (22nF, 102K, 2.7K), ideal frequency is 600Hz
@@ -295,7 +305,7 @@ void sc9_state::playmatic(machine_config &config)
 	sc9b(config);
 
 	/* basic machine hardware */
-	m_maincpu->set_clock(1500000 * 2); // advertised as double the speed of SC9
+	m_maincpu->set_clock(5.626_MHz_XTAL / 2); // advertised as double the speed of SC9
 	m_board->set_type(sensorboard_device::MAGNETS);
 
 	config.set_default_layout(layout_fidel_playmatic);
diff --git a/src/mame/drivers/mephisto_modular.cpp b/src/mame/drivers/mephisto_modular.cpp
index 4d7ae582cc0..f789c92b489 100644
--- a/src/mame/drivers/mephisto_modular.cpp
+++ b/src/mame/drivers/mephisto_modular.cpp
@@ -44,8 +44,8 @@ Undocumented buttons:
 
 TODO:
 - match I/S= diag speed test with real hardware (good test for proper waitstates)
-- gen32 waitstates emulation is preliminary (without it, sound pitch is too high
-  and lcd write speed too fast)
+- gen32 waitstates emulation is preliminary (without it, sound pitch is way too high
+  and lcd write speed too fast). Real gen32 sound is a bit lower pitched than MAME.
 
 ===============================================================================
 
diff --git a/src/mame/drivers/mephisto_risc.cpp b/src/mame/drivers/mephisto_risc.cpp
index b4df69bf1e7..a81e5a6fd8d 100644
--- a/src/mame/drivers/mephisto_risc.cpp
+++ b/src/mame/drivers/mephisto_risc.cpp
@@ -5,7 +5,8 @@
 Mephisto Risc 1MB/II (stylized "risc")
 
 The chess engine in Mephisto Risc is also compatible with Tasc's The ChessMachine,
-see ROM defs for details.
+see ROM defs for details. "Main" CPU is slow, but all the chess calculations are
+done with the ARM.
 
 Hardware notes:
 - G65SC02P-4 @ 2.5MHz