Sencore FE149 - Page 14

The range switch selects the resistors R35 to R42. Тһе voltage drop across this
resistor is coupled to the input transistor ТКІ. The range resistors are selected
so that а .3 volt RMS signal will cause full scale meter deflection. The current
from the circuit will cause a voltage drop and the resultant drop is then measured
and converted into current on the meter scale. When measuring DC current, the
FE149 canbe turned off as the FET's are notused. The meter is placed in the cir-
cuit with the proper shunt resistor R35 to R42 selected by the range switch. The
meter is protected from overload by the diodes CR7 to CRlland CR13, With exces-
sive current, the voltage drop across the meter increases, and the diodes conduct,
shunting the excessive current around the meter. If the meter is left in the circuit
too long, or the excessive current is extreme, the fuse F1 will blow, opening the
circuit. When БІ blows, both the ohms and current functions will be inoperative.
The power supplyfor the FE149 circuits canbe either a rechargeable battery or the
AC line. The power supply for the AC line andalso for recharging of the recharge-
able battery consists of a half wave rectifier CR5 and a power transformer to iso-
late the FE149 from the AC line. Transformer ТІ can be used to operate from 115
VAC or 230 VAC by simply changing the internal connections. The pulsating DC į
filtered by С10. The zener diode CR4 provides further filtering and regulates t
voltage to approximately 12 volts so that regardless of line voltage changes, the
supply voltage to the FET circuit is constant and also prevents overcharging of the .
rechargeable battery. Diode CR3 prevents the battery from discharging back through
the zener diode when the FE149 is not plugged into an AC outlet. The AC line cord
is athree conductor type and puts the case of the FE149 at ground potential as rec-
ommended by UL. No fuse is used in the primary of the power transformer as the
impedance of the circuit is high and the maximum current, even from a shorted
component will not damage the unit.
TROUBLE CHART
SYMPTOM
PROBABLE
CAUSE
CORRECTIVE
MEASURE
Unit dead
on
ohms
Blown
fuse
Replace
fuse
- Be sure to
use
and
current
functions
low resistance
type specified
a
л
т
тт
ee
eee
eee
eee
Cannot
adjust
ohms
Poor
contact
on
1.5
volt
Check
contacts,
replace
ADJ to full scale on volt battery, 1.5 volt battery
RXI range battery weak
a ——— ы те ыле.
Measurements
in-
Some
leakage
in
circuits,
Adjust
meter
zero
оп
КХІ
accurate on RX10 wiring or components meg range then depress RX10
meg range meg. If meter reads slightly
up scale 1/8 to 1/4 inch it is
an indication of normal circuit
leakage. More than this is
abnormal and circuit should be
checked out.
Incorrect readings on Diode CR12 or CR14 leaky Replace diode
different ranges BET ERI leaky: Replace FET (specify color е
when ordering)
DC voltage readings Unit improperly calibrated Check and recalibrate per
inaccurate calibration instructions in
manual,
14