Rear Panel; Model 1485 2~1 - Keithley 181 Service Manual

MODEL 181
THEORY OF OPERATION
Section 4. Theory of Operation
4-l. GENERAL
This section contains ln~depth discussions of the maj"r circuit
blocks, which are listed below:
A. Power Supply (included on MotherboardI PC~531. page
B IEEE Standard lnrerface iinr:ludcd on MotherboardI
PC ~531, page 6~27
C DigItal Section (included on Mofhcrboradi PC~531. page
D Dispiay [Display Board1 PC~530, page 6,25.
E D/A Converter iMotherboard PC~531. page
6~27,
F Nanovolt Preamp lNanovol~ Preamp Board1 PC~526. page
G A/D Corrvcrter (Analog Board) PC~529, page 6~23,
There are several figures and tables [provided wilt1 each major
circuit block that will aid in their oxplanation~
4-z. Power supply
While studying the theory of operation for the Power Supply.
it will be helpful 1" refer t" the Schematic Diagram 30583D.
Sheet 1 of 2.
4-3.
The AC power is brought int" the 161 by a recessed line
plug JlOll. Fuse FlOl and line plug J,O,l are located on the
rear panels Switch SlOl is the main power swtch and ii is
located on the Front Panel. Switch S102 is the 115V 1" 230V
configuration switch and is located internally.
4~4
The regulated portion of the 181 power supply is sect
tioned into six different supplies. There are three supplies
(+15V, -15V, +5V) ior the analog section of the 181. The
other three supplies (+ 1%'. -15V. -+ 5VI are for the digital sec-
tion of the 181.
The key to the Analog/Digital isolation is the split bobbin
transformer TlOl. The digital supply winding, along with tbe
primary windings, are at the bottom haIf of th~ bobbin, and
the analog supply windings am located al the lw of the
bobbin.
4~5. The six supplies "perate from four separate windings 01
transformer TlOl. The four windings are fed info four full
wave bridae rectifiers: Two recfifiers for the analog section
CR105 ani CR106 and two rectifiers for the digital sect~"n
CR104 and CR107. Capacitors C128, C123. and Cl27 bypass
regulators U12B l5VI, U129 (+ 15Vl and U130 I-15Vl respec-
tively. These are the analog supplies.
Capacitors Cl34 and Cl35 filter the input voltage to U136 and
U137 respectively. Capacitors Cl32 and Cl33 bypass
regulators U136 and Ul37 respectively. Capacitor Cl30 filters
the "utput of CRlC4 Regulator U135 receives its input from
the regulated "utput of Ul36. Q103 and Q105 are configured
as a high current gain voltage regulator. 0104 completes the
unify gain loop by compensating the VeE drop of Q103. CR101
provides short circuit limiting t" the circuit.
4-6. IEEE Standard Interface
While studying the theory of operation oi the IEEE Standard
Interface, it will be helpful 1" refer to the Schematic Diagram
30583,
sheet 1 of 2.