I Section V
Model 185B
Paragraphs 5-29 to 5-35
Table 5-3. Resistance to Ground (values may vary 25%)
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supply Color Code 412A Range 412A Reading 410B Range 410B Reading
-100 volt violet RXlK 750 ohms RXlOO 750 ohms
12.6 volt vio/wht RXlOO 50 ohms * RXlO 150 ohms *
+ 6.3 volt brn/wht RX1 0.5 ohm RX1 0.5 ohm
+ 12.6 volt brown RX1 1.0 ohm RX1 1.0 ohm
+ 100 volt red/wht RXlK 1600 ohms RXlOO 1600 ohms
+ 250 volt red RXlOK 5000 ohms RXlK 5000 ohms
* Note: Readings differ due to diode, transistor action and applied voltage polarity from ohmmeter. .
Note (a) Check to see if excessive ripple is cominfg
from -100 volt, -12.6 volt, or +lo0 volt sup-
Do not reset High Voltage Adj R306unless plies. +250 volts is referenced to +lo0 volts
such adjustment is clearly indicated, The
and -100 volts. +lo0 volts is referenced to
adjustment of high voltages affects the sen- -100 ;rolts and -12.6 volts. +12.6 volts and
sitivity of the crt and makes necessary the
-12.6 volts are referenced to -100 volts.
readjustment of vertical and horizontal gain,
frequency response and time scales.
(b) Check for weak or defective transistors.
5-29. LOW-VOLTAGE POWER SUPPLIES. When
(c) Check capacitors throughout supply. Check
checking low-voltage power supplies, check the -100
C402, C403, and C404 first.
volt supply first, for it affects all supplies. Check
the other supplies in the following order: -12.6 volt,
5-30. REPAIR.
+lo0 volt, +250 volt. The +12.6 volt supply may be
checked in any order. The following describes a
5-31. ACCESS TO POWER TRANSISTORS AND
method of checking the +250 volt supply; a similar
procedure may be used for the other supplies. FAN MOTOR.
5-32. Figure 5-3 shows necessary disassembly for
a. Check F402. If blown, check resistance from replacement of power transistors or fan motor. Pro-
+250 volt line to ground (see table 5-3). ceed as follows:
b. Check transformer output. Note
Take care not to dislodge the vertical am-
c. Check supply output. Note whether it is high,
plifier tubes from their sockets.
low, or contains excessive ripple. Typical ripple is
less than 8 mv peak-to-peak, when the instrument is
a. Remove cabinet.
not triggered or free-running.
b. Remove eight screws holding fan shroud to rear
(1) Output Too High. Disconnect base lead ofQ402
chassis.
and note any change in supply output. If out-
put decreases, check Q403 and Q404. If no
c. Rotate shroud slightly to allow clearance for
change, check Q401 and Q402. transistor, and remove shroud assembly from the
instrument.
(2) Ou ut Too Low. Disconnect collector lead of
bany change in supply output. If
5-33. Replacement procedure is the reverse of the
output increases, check Q403 and Q404. If no
change, replace Q404 collector lead and check above. Install shroud with blank heat sink slot at
9 o?clock as viewed from rear of instrument.
Q401 and Q402.
Note 5-34. REPLACEMENT OF SEMICONDUCTORS.
The voltage changes involved may be small. 5-35. Excessive heat can destroy semiconductor de-
If so, alternately connect and disconnect the vices. When soldering or unsoldering transistors or
lead in question while observing voltmeter. diodes, place a heat sink such as long-nose pliers on
lead of component between its body and point to which
(3) Excessive Ripple. Low or high line voltage heat is applied. In addition, isolate oscilloscope from
and excessive or insufficient loading of sup- ground or ground body of soldering iron to prevent
plies can cause excessive ripple. leakage current from damaging component.
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~ 5-4 01255-1
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ACEC
