HP 214B - External Trigger Mode

Model 214B
The action of the MAN /NEG /POS switch can best be explained by considering the current node at
CR5 / CR6 anode. With NEG selected, diodes CR4 and CR5 are reverse biased and the following
equations relate the differ~nt transistor currents :
(i)
(ii)
14 = 12 + 13
15=16 +
1
3
(where 14 = 04 current; 12 = 02 current ; 13 = 03 current)
(where 15 = 05 current ; 16 = 06 current; 13 = 03 current)
Substituting (i) in (ii) the following equation results:
Since
15 and 14 are constant currents then:
So any change in the transistor 02 current causes a corresponding change in the 06 current . As a result,
when 01 a gate voltage is high, TP1 is also high (02 and 06 are conducting max imum current), therefore
the rate Schmitt trigger is disabled by the CR14 cathode voltage (09 is switched off; 010 switched off ;
and
011 is switched on) . When the signal applied to the 01a gate goes negative, a negative-going edge is
generated at TP1, which removes the disable voltage at CR 14 cathode (09 is switched on; 010 is
swi tched on; and 011 is switched off) and allows the rate Schmitt trigger to switch. With POS selected
at the MAN/ NEG/POS switch, CR3 and CR6 are reverse biased, and the following equ a tions relate
the different transistor currents :
(v)
(vi)
1
4
= 12 +
1
3
15 = 12+16
(where 14 = 04 current ; 12 = 02 current; 13 = 03 current)
(where 15 = 05 current; 12 = 02 current; 16 = 06 current)
From equation (vi), it can be deduced that ...
.
.. because 15 is constant. An increase, therefore, in 02 current is followed by a decrease in 06 current .
In voltage terms, a positive·going voltage at 01a gate generates a negative·going edge at TP1, which in
turn removes the rate Schmitt trigger disable voltage at CR 14 cathode .
Normal Mode
In NOR MAL mode , the timing erro r circuit is enabled via U20E and 060. I n addition, current
source 022 is enabled (via U20A) which in turn cuts off 010. With 010 dis abled, any ex ternal
trigger signal is prevented from reaching the internal rate circuit. Current source 025 is also
disabled
and CR14 reverse biased.
External Trigger Mode
With EXT TRIG mode selected:
current sources 022 and 024 are disabled
current source 025 is switched on
rate Schmitt trigger 034 / 036 is disabled via 011 and CR14
(approx. +4 Vat CR14 cathode)
cur r ent source 023 is switched on, L 1 acting as an inductive load for 09.
Service
8-
23
Scans by Artekmedia => 2009