diff --git a/hardware/tektronix_base.py b/hardware/tektronix_base.py
index 54f8f3a..54c946f 100755
--- a/hardware/tektronix_base.py
+++ b/hardware/tektronix_base.py
@@ -292,7 +292,7 @@ class TektronixOscilloscopeBase:
 
         # Validate coupling
         valid = False
-        for variants in self.CHANNEL_COUPLING.items():
+        for variants in self.CHANNEL_COUPLING.values():
             if coupling_upper in [v.upper() for v in variants]:
                 valid = True
                 break
diff --git a/tests/test_tektronix_base.py b/tests/test_tektronix_base.py
new file mode 100644
index 0000000..16ba18d
--- /dev/null
+++ b/tests/test_tektronix_base.py
@@ -0,0 +1,71 @@
+"""TektronixOscilloscopeBase: the SCPI setters that validate their input.
+
+These run against the real class with only the socket replaced.  The scan
+tests use FakeScope, which stubs the setters out entirely — so a setter
+could raise on every call and nothing in the suite would notice, which is
+what happened: set_channel_coupling() iterated CHANNEL_COUPLING.items()
+instead of .values(), called .upper() on the list half of each pair, and
+raised AttributeError for any coupling at all.  configure_channels() runs
+on connect, so the scope could not be connected to.
+"""
+import pytest
+
+from core.scope_sras import SRAS_CHANNELS, configure_channels
+from hardware.tektronix_base import TektronixOscilloscopeBase
+
+
+class RecordingScope(TektronixOscilloscopeBase):
+    """The real instrument class with the wire replaced by a list."""
+
+    def __init__(self):
+        super().__init__(resource_name="192.0.2.1")
+        self._connected = True
+        self.written: list[str] = []
+
+    def write(self, command):
+        self.written.append(command)
+
+
+@pytest.fixture
+def scope():
+    return RecordingScope()
+
+
+def test_connect_time_channel_setup_reaches_the_wire(scope):
+    """The regression: every command configure_channels() sends must go out."""
+    configure_channels(scope)
+
+    for ch, profile in SRAS_CHANNELS.items():
+        assert f"SELect:CH{ch} ON" in scope.written
+        assert f"CH{ch}:COUPling {profile.coupling}" in scope.written
+        assert f"CH{ch}:TERmination {profile.termination_ohm}" in scope.written
+        assert f"CH{ch}:SCAle {profile.scale_v_div}" in scope.written
+
+
+@pytest.mark.parametrize("coupling", ["DC", "AC", "dc", "ac"])
+def test_channel_coupling_accepts_both_modes_in_any_case(scope, coupling):
+    scope.set_channel_coupling(1, coupling)
+    assert scope.written == [f"CH1:COUPling {coupling}"]
+
+
+def test_channel_coupling_rejects_an_unknown_mode(scope):
+    """Rejection has to be the documented ValueError, not an AttributeError
+    raised while building the list of valid options."""
+    with pytest.raises(ValueError, match="Invalid coupling mode"):
+        scope.set_channel_coupling(1, "GND")
+    assert scope.written == []
+
+
+# The other setters share this shape; a table keeps them honest together.
+@pytest.mark.parametrize("setter,good,bad,sent", [
+    ("set_acquire_mode", "AVERAGE", "SMOOTH", "ACQuire:MODe AVERAGE"),
+    ("set_acquire_mode", "SAM", "SMOOTH", "ACQuire:MODe SAM"),
+    ("set_trigger_slope", "RISE", "SIDEWAYS", "TRIGger:A:EDGE:SLOpe RISE"),
+    ("set_trigger_mode", "NORMAL", "SOMETIMES", "TRIGger:A:MODe NORMAL"),
+])
+def test_validated_setters_take_valid_values_and_reject_the_rest(
+        scope, setter, good, bad, sent):
+    getattr(scope, setter)(good)
+    assert scope.written == [sent]
+    with pytest.raises(ValueError):
+        getattr(scope, setter)(bad)