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Overcurrent Problems
Overcurrent problems usually cannot be found by
resistance checking, due to the very low load
resistance. However, the load can be selectively
uncoupled by removing laminar bus connections or,
when the trouble is isolated to a board, by removing
the bus to jumper plugs one at a time to find the row
at fault.
(Use logic pages YB411 and YB421 to
assist in finding the proper bus terminals.)
Note: Use care in removing voltage regulator
loads in storage areas. If -3 volts are removed
and the storage driver voltages are still present,
the drivers could be ruined.
If
a
-3
voltage regu-
lator overloads on a storage board, remove the
driver voltages before removing the
-3
volt loads.
Voltage Regulator Problems
Following is a suggested sequence to use when a
voltage regulator is causing a problem:
1.
Replace the overcurrent and regulator cards.
2.
Make sure the reed relay on the overcurrent
card is properly seated.
3.
Check for proper bias on the voltage regulator.
4.
C:hierk for propier ronnections of local or
remote sensing (or both,
if
used).
5. Check remote potentiometer connections
if
used.
6. Check for 150 vac input.
7.
Make sure that all terminal screws, screws
on capacitor bus, and screws on feed-through capac-
itors are tight.
8.
Replace the voltage regulator.
Overvoltage Unit Adjustment
To adjust an overvoltage unit when no external
adjusting device is available:
Battery or
External
DC Supply
Push to Test
_L
A
+
or - Input to
Overvoltage Unit
Under Test
DC Zero Reference
Measure voltage at poinl5 A and B
FIGURE 118. OVERVOLTAGE ADJUSTMENT
1. Make sure de voltage is down.
2.
Remove the overvoltage feed wire from the
regulator being tested (to prevent feeding de back
to the load).
3.
Check that the overvoltage potentiometers on
the overvoltage unit are adjusted to their proper
firing level (center arm to de zero).
(See logic page
YB401.)
4.
Connect an external de supply (or batteries)
and a 5000-ohm potentiometer as shown in Figure 118
to provide a variable input to the overvoltage unit
under test.
5.
Meter the output pin of the overvoltage
unit
with a voltmeter.
(This is the wire that go.es to the
firing gate on the overvoltage unit SCR.)
6.
Gradually raise the test input voltage with
the 5000-ohm potentiometer until a dip in voltage is
noticed at the output. At this point the input can be
read to tell at what level the overvoltage unit fired
the SCR.
7.
If the SCR does not fire, check the components
on the overvoltage unit or readjust the dividers on any
supply being tested that has an output of over
6
volts.
(See logic page YB291.)
Overvoltage occurs when the supplies are at the
following levels. The voltage sensitivity of the over--
voltage card is also shown.
Supply
_Llpper-limit
Overvoltage Card
602
")
56XY
67
When divider output is
48
57
8. 5 volts, the card senses
30
36
the overvoltage condition.
18
22
-12
-22
6
8.5
8.5
3
4.5
4.5
Phone Jack
S lee,e
Tip
D----+--+----0
Power System Service Checks
(3/71)
125
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