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688 RICHARDSON'S BLUEBOOK OF PROJECTION
16.
To Determine the Cause and Correction of HUM ORIGINATING IN AN AMPLIFIER
NOTE: Wherever these charts suggest that tubes or condensers be replaced as a group, it should be remembered that only one of the group is likely to be defective, replacement being made en masse for the sake of quick results. All the discarded parts should be saved and tested one by one at convenience. When the faulty unit is found, the rest can be returned to active stock. However, if the fault is of intermittent nature and cannot be found by subsequent test, it is better to discard all the parts rather than take the chance of a recurrence of the trouble.
WARNING: AMPLIFIERS USING HIGH VOLTAGES ARE DANGEROUS. Whenever the prescribed test allows, line current should be cut off and filter condensers discharged, before work is done on the interior of an amplifier. When the nature of the test requires that the amplifier be in operation, use only well insulated test prods and observe every precaution.
THE TROUBLE TESTS
THE CONDITION
FOUND
NEXT TEST
1.
• Note frequency of hum
Hum of line frequency Hum not of line frequen
cy
No. No.
2 29
2.
Clean all tube contacts, particularly grid and grid clip contacts
TROUBLE CURED Hum remains
No.
3
3.
Inspect speech line at input terminals for unintended ground to shield
TROUBLE FOUND No trouble found
No.
4
4.
Install new set of tubes
TROUBLE CURED Hum remains
No.
5
5.
Tighten holding belts of all transformers and choke coils
TROUBLE CURED Hum remains
No.
6
-6.
Check amplifier external ground connection
Ground line open Ground contact loose or d Ground contact perfect
irty
No. No. No.
7 8 9
7.
8.
ReDair ground line
Clean and tighten ground contact
TROUBLE CURED Hum remains TROUBLE CURED Hum remains
No. No.
9 9
•9.
Using ohmmeter, check speech input line for unintended ground
TROUBLE FOUND Speech input line normal
No.
10
10.
Check power input fuses for high resistance fuse or dirty contact
TROUBLE FOUND Hum remains
No.
II
II.
Check power line switch for dirty contact
TROUBLE FOUND Hum remains
No.
12