Keithley 2015, Service Manual, Page: 74 / 158

Troubleshooting 4-11
Analog circuitry
Refer to Figure 4-3 for the following discussion on analog circuitry.
Figure 4-3
Analog circuitry block diagram
INPUT HI
INPUT HI protection is provided by the SSP (solid state protection) circuit. The SSP is pri-
marily made up of Q101 and Q102. An overload condition opens Q101 and Q102, which dis-
connects the analog input signal from the rest of the analog circuit.
Note that for the 100VDC and 1000VDC ranges, Q101 and Q102 of the SSP are open. The
DC voltage signal is routed through the DCV Divider (Q114 and Q136 on) to the DCV switch-
ing circuit.
Distortion
Analog
Circuitry
(See Figure 4-5)
Digital
Circuitry
(See Figure 4-2)
ADC
U165
A/D
MUX &
Gain
U163, U166
U129, U132 BUFCOM
X1
Buffer
U113
X1 Buffer
Q121, U126
Scanner
Option
Scanner Control
Scanner
Inputs
Scanner
Output
DCV & Ohms
Switching
K101, Q104,
Q105, Q108,
Q113, U115
AC Switching
&
Gain
K102, U102, U103, U105,
U112, U118, U111, U110
Current
Shunts
K103, R158, R205
SSP*
Q101,
Q102
* Solid State Protection
DCV
Divider
R117, Q109,
Q114, Q136
Ohms I-Source
U133, Q123,
Q125, Q124,
Q126, Q119,
Q120, U123
ACV,
FREQ
DCV
OHMS
INPUT
HI
AMPS
ACA
DCA
SENSE
HI
SENSE
LO
DCV/100