Model 2750 Multimeter/Switch System Service Manual Troubleshooting 4-11
Troubleshooting
INPUT HI
INPUT HI protection is provided by the SSP (Solid State Protection) circuit. The SSP is
primarily made up of Q101 and Q102. An overload condition opens Q101 and Q102,
which disconnects 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 switching circuit.
AMPS input
The ACA or DCA input signal is applied to the Current Shunt circuit, which is made up of
K103, R158, R205, and R338. For the 20mA DC range, 5.1 (R205R338 + R158) is
shunted across the input. Relay K103 is energized (set state) to select the shunts. For all
other DCA ranges, and all ACA ranges, 0.1 (R158) is shunted across the input (K103
reset).
The ACA signal is then sent to the AC Switching & Gain circuit, while the DCA signal is
routed directly to the A/D MUX & Gain circuit.
Signal switching
Signal switching for DCV and OHMS is done by the DCV & Ohms Switching circuit.
FETs Q113, Q104, and Q108 connect the DCV or ohms signal to the X1 buffer (U113).
For offset-compensated ohms and all dry-circuit ohms measurements, the signal is routed
directly to the A/D mux by Q106, bypassing U113.
Note that the reference current for OHMS is generated by the Ohms I-Source circuit. For
4-wire ohms measurements, SENSE LO is connected to U163.
Signal switching and gain for ACV, FREQ and ACA is done by the AC Switching & Gain
circuit, which is primarily made up of K102, U102, U103, U105, U112, U118, U111,
U110, and U117. Note that U111 is used for frequency adjustment. The states of these ana-
log switches vary from unit to unit.
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