diff --git a/src/mame/sinclair/zx.h b/src/mame/sinclair/zx.h
index 2993bab875e..932cb485d89 100644
--- a/src/mame/sinclair/zx.h
+++ b/src/mame/sinclair/zx.h
@@ -98,6 +98,7 @@ class zx_state : public driver_device
 	bool m_nmi_on = false, m_nmi_generator_active = false;
 	uint64_t m_base_vsync_clock = 0, m_vsync_start_time = 0;
 	uint32_t m_ypos = 0;
+	uint8_t m_const = 207;
 
 	uint8_t m_prev_refresh = 0;
 	uint8_t m_speaker_state = 0;
diff --git a/src/mame/sinclair/zx_m.cpp b/src/mame/sinclair/zx_m.cpp
index dd4b74e8001..847171f5c3d 100644
--- a/src/mame/sinclair/zx_m.cpp
+++ b/src/mame/sinclair/zx_m.cpp
@@ -22,6 +22,10 @@ void zx_state::init_zx()
 		m_program->unmap_readwrite(0x8000, 0xffff);
 	else if(m_ram->size() < 16384)
 		m_program->unmap_readwrite(0x4000 + m_ram->size(), 0xffff);
+
+	m_const = 207;
+	if (m_region_gfx1)
+		m_const = 208;
 }
 
 void zx_state::machine_reset()
@@ -53,10 +57,10 @@ void zx_state::drop_sync()
 		m_vsync_active = false;
 		m_cassette->output(-1.0);
 
-		int xs = 2*((m_vsync_start_time - m_base_vsync_clock) % 207);
-		int ys = (m_vsync_start_time - m_base_vsync_clock) / 207;
-		int xe = 2*((time - m_base_vsync_clock) % 207);
-		int ye = (time - m_base_vsync_clock) / 207;
+		int xs = 2*((m_vsync_start_time - m_base_vsync_clock) % m_const);
+		int ys = (m_vsync_start_time - m_base_vsync_clock) / m_const;
+		int xe = 2*((time - m_base_vsync_clock) % m_const);
+		int ye = (time - m_base_vsync_clock) / m_const;
 		if(xs >= 384) {
 			xs = 0;
 			ys++;
@@ -106,7 +110,7 @@ void zx_state::drop_sync()
 			m_nmi_on = m_hsync_active = false;
 			recalc_hsync();
 		} else
-			m_ypos = ((time-m_base_vsync_clock)%207) < 192 ? 0 : -1;
+			m_ypos = ((time-m_base_vsync_clock)%m_const) < 192 ? 0 : -1;
 	}
 }
 
@@ -198,6 +202,11 @@ uint8_t zx_state::pc8300_io_r(offs_t offset)
 		m_cassette->output(+1.0);
 		if(m_cassette_cur_level <= 0)
 			data &= 0x7f;
+
+		if (!m_vsync_active && !m_nmi_generator_active) {
+			m_vsync_active = true;
+			m_vsync_start_time = m_maincpu->total_cycles();
+		}
 	}
 
 	return data;
@@ -235,6 +244,11 @@ uint8_t zx_state::pow3000_io_r(offs_t offset)
 		m_cassette->output(+1.0);
 		if(m_cassette_cur_level <= 0)
 			data &= 0x7f;
+
+		if (!m_vsync_active && !m_nmi_generator_active) {
+			m_vsync_active = true;
+			m_vsync_start_time = m_maincpu->total_cycles();
+		}
 	}
 
 	return data;
diff --git a/src/mame/sinclair/zx_v.cpp b/src/mame/sinclair/zx_v.cpp
index 0aea732383f..9347dc7eb2f 100644
--- a/src/mame/sinclair/zx_v.cpp
+++ b/src/mame/sinclair/zx_v.cpp
@@ -41,8 +41,8 @@ void zx_state::refresh_w(offs_t offset, uint8_t data)
 	m_prev_refresh = offset;
 	if(m_ula_char_buffer != 0xffff) {
 		uint64_t time = m_maincpu->total_cycles();
-		int x = 2*((time-m_base_vsync_clock) % 207);
-		int y = (time-m_base_vsync_clock) / 207;
+		int x = 2*((time-m_base_vsync_clock) % m_const);
+		int y = (time-m_base_vsync_clock) / m_const;
 		uint8_t pixels;
 		if(m_region_gfx1)
 			pixels = m_region_gfx1->base()[((m_ula_char_buffer & 0x3f) << 3) | (m_ypos & 7)];
@@ -62,10 +62,10 @@ void zx_state::refresh_w(offs_t offset, uint8_t data)
 void zx_state::recalc_hsync()
 {
 	uint64_t time = machine().time().as_ticks(m_maincpu->clock());
-	uint32_t step = (time - m_base_vsync_clock) % 207;
+	uint32_t step = (time - m_base_vsync_clock) % m_const;
 	uint32_t delta;
 	if (m_hsync_active)
-		delta = 207 - step;
+		delta = m_const - step;
 	else {
 		if(step < 192)
 			delta = 192 - step;
@@ -87,9 +87,9 @@ uint8_t zx_state::ula_low_r(offs_t offset)
 
 	if(m_nmi_on) {
 		uint64_t time = m_maincpu->total_cycles();
-		int pos = (time-m_base_vsync_clock) % 207;
+		int pos = (time-m_base_vsync_clock) % m_const;
 		if(pos >= 192)
-			m_maincpu->adjust_icount(pos - 207);
+			m_maincpu->adjust_icount(pos - m_const);
 	}
 	return cdata;
 }
@@ -106,9 +106,9 @@ uint8_t zx_state::ula_high_r(offs_t offset)
 
 	if(m_nmi_on) {
 		uint64_t time = m_maincpu->total_cycles();
-		int pos = (time-m_base_vsync_clock) % 207;
+		int pos = (time-m_base_vsync_clock) % m_const;
 		if(pos >= 192)
-			m_maincpu->adjust_icount(pos - 207);
+			m_maincpu->adjust_icount(pos - m_const);
 	}
 
 	if(cdata & 0x40)