155-0076-00: Difference between revisions

Line 86:

----

Q36, Q37, Q38, and Q39 form an RS flip-flop.

When input overload is detected, Q32 turns on, supplying current to the base of Q36.

The collector voltage of Q36 and Q37 goes to around 0 V.

The pin 13 "MODE INPUT" pin is normally grounded (e.g., in the [[485]].

In this state, Q38 and Q39 are cut off and their collectors are pulled up by R38.

That causes a current to flow through R53, into the base of Q37,

which keeps the collectors of Q36 and Q37 close to ground potential.

So, once an overload condition is detected, the flip flop stays in the

left-side-active state even after the input is removed.

[[File:Tek m94 output logic.png|600px]]

When the excessive input signal has been removed and the operator wants to re-enable the 50 Ω input,

pin 13 of the M94 is momentarily allowed to float high.

In the 485, this is done by the operator pressing the impedance selector button on the front panel, S125B.

Letting pin 13 float allows current to flow through R45, into the base of Q39.

The resulting collector current in Q39 pulls the collector of Q38/Q39 low,

which stops the current in R53, thus turning off Q37.

Q36 is also off because the excessive input has been removed.

So the collector of Q36/Q37 is pulled high by R36, which also turns on R38.

This leaves the flip-flop in the stable right-side-active state.

----

@@ Line 86: / Line 86: @@
 ----
 Q36, Q37, Q38, and Q39 form an RS flip-flop.
 When input overload is detected, Q32 turns on, supplying current to the base of Q36.
 The collector voltage of Q36 and Q37 goes to around 0 V.
 The pin 13 "MODE INPUT" pin is normally grounded (e.g., in the [[485]].
 In this state, Q38 and Q39 are cut off and their collectors are pulled up by R38.
 That causes a current to flow through R53, into the base of Q37,
 which keeps the collectors of Q36 and Q37 close to ground potential.
 So, once an overload condition is detected, the flip flop stays in the
 left-side-active state even after the input is removed.
 [[File:Tek m94 output logic.png|600px]]
+When the excessive input signal has been removed and the operator wants to re-enable the 50 Ω input,
+pin 13 of the M94 is momentarily allowed to float high.
+In the 485, this is done by the operator pressing the impedance selector button on the front panel, S125B.
+Letting pin 13 float allows current to flow through R45, into the base of Q39.
+The resulting collector current in Q39 pulls the collector of Q38/Q39 low,
+which stops the current in R53, thus turning off Q37.
+Q36 is also off because the excessive input has been removed.
+So the collector of Q36/Q37 is pulled high by R36, which also turns on R38.
+This leaves the flip-flop in the stable right-side-active state.
 ----

Navigation menu