Standard Frequency Series Generators
Maintenance
14
M-2008-7-15
7.7 TROUBLESHOOTING HIGH MA
SYMPTOM
MOST COMMON PROBLEM
HOW AND WHERE TO TEST
Rotor Runs READY lamp
lights BACKUP CONTACTOR
comes in and EXPOSURE
LAMP lights during exposure.
Some X-Rays may be emitted
from the X-ray Tube, but
Milliamperes are HIGH.
HIGH
MA.
X-Ray Tube Filament
Characteristics are not the
same. (Incompatible X-Ray
Tube).
Initially, check output pulses at the X-Ray Tube.
If pulses are correct, you must adjust filament voltage lower.
The present X-Ray Tube does not have the same filament
characteristics as the one connected previously. In some
instances it may be necessary to lower the voltage strap on the
MA stabilizer so that the large filament voltage can be lowered
to achieve the proper MA. Selections are
240 - 250
or
260
VAC
. Should you lower the MA Stabilizer Strap, recalibrate the
large filament properly. After you calibrate the large filament
properly, you may find the small filament operating at too
low MA. To correct, increase the small filament resistor strap.
Calibrate all mA stations, measure KVP and Timed Impulses
out of the X-Ray Tube.
SCR Half Waving
To prove, run a Simulated
Load Test.
The simulated load test at
the right of this part will prove
the Primary High Tension
Switching Circuit is or is not
operating properly
SIMULATED LOAD TEST
1. Remove
P1
&
P2
power cord from the Primary of the
High Voltage Transformer
.
2. Place a Volt Meter between
H1
and
H2
terminals on the
X-Ray Control Motherboard. Adjust Major and Minor kVp
selectors until 120 VAC is obtained.
3. Turn unit OFF. Connect 120 VAC lamp from
P1
to
P2
at
the X-Ray Control. Then place the Volt Meter from
P1
to
P2
. Select
2 seconds
on the timer and make an exposure.
Lamp should light and voltage should be 120 VAC. If not
go to the next test.
HIGH MA
(continued)
4. If you read 60 to 70 VAC, the SCR’s are half waving.
There are three causes possible, beginning with most
likely:
I.
Timer to SCR Gate Harness has
An OPEN lead. Push pins in tight.
II. The SCR itself is defective
III. One of the Timer Pulse
Transformers is OPEN.
(See next page).
AFTER A SHORTED OR HALF-WAVING SCR, DEFECTIVE
SCR HARNESS OR TIMER HAS BEEN REPLACED,
RUN A SIMULATED LOAD TEST TO ENSURE PROPER
OPERATION OF THE SCR OUTPUT.
NOTICE
REPEATED FIRING OF SCR’s HALF-WAVING WILL
EVENTUALLY CAUSE THE SCR TO SHORT.
IN THIS
CASE YOU WOULD REPLACE THE SCR ONLY TO
FIND ANOTHER PROBLEM, SUCH AS KICKING THE
CIRCUIT BREAKER OR LOADING THE HIGH VOLTAGE
TRANSFORMER
. NORMALLY YOU MAY SUSPECT THE
HIGH VOLTAGE TRANSFORMER WHEN THE PROBLEM
IS THE HALF-WAVING SCR.
THIS IS THE REASON FOR
ADMINISTERING THE SIMULATED LOAD TEST AFTER
REPLACING A SHORTED OR HALF-WAVING SCR. THE
SIMULATED LOAD TEST IS THE ONLY WAY TO BE SURE
ANY X-RAY CONTROL IS FUNCTIONING PROPERLY.
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