Radio today (Jan-Dec 1938)

All of these points may be quickly checked by measuring the voltage between the center tap of the transformer primary and the reed terminal of the vibrator socket. This voltage should read 5.5 volts or more. If the check is satisfactory, the vibrator should be tested for proper operation either in a vibrator tester or by the substitution of a new Mallory replacement vibrator. Sticking or shorted vibrators are usually caused by "projections" being built up on the contact points. These "projections" (contact transfer) are the result of an unbalanced condition in the circuit. A careful check of the "buffer" condenser should be made. If this condenser is open or the capacity not as specified, it should be replaced with a Mallory oil filled condenser, Type VB or OT having the specified capacity. Never change the specified capacity of this condenser unless specifically instructed to do so. Low "B" voltages Check the points given below as the cause for low "B" voltage. 1. Battery voltage low. 2. Corroded fuse clips. 3. High switch resistance. 4. Weak rectifier tube. 5. Defective buffer condenser. (Caution: See preceding instruction on buffer condenser replacement). 6. Defective filter condenser. 7. Worn vibrator. (Check in tester or substitute new Mallory replacement vibrator). 8. Check for troubles in radio which will cause low voltage such as shorted cathode resistor, by-pass condenser, shorted transformer, defective tubes, etc. Intermittent operation 1. Generally caused by troubles in the receiver, such as defective antenna insulation or connections, defective wiring, defective tubes, etc. Other sections of the ency clopedia specifically explain this method of servicing these troubles. 2. Intermittent vibrator operation usually caused by worn vibrator nearing the end of its life. 3. Loose connections in the power pack. 4. Defective rectifier tube. Unusual mechanical noise Unusual mechanical noise from the vibrator may be caused by: 1. Vibrator touching out parts and vibrating against them or causing other parts to vibrate. Correct this trouble with a cardboard pad around the vibrator. 2. An old vibrator nearing the end of its life. 3. Loose case screws, or loose parts in the radio set. Electrical hum from speaker Hum from the speaker is usually caused by: 1. Defective filter condensers (low capacity). 2. Microphonic tubes. 3. Microphonic condensers. (Usually variable condenser). 4. Loose chassis screws. 5. Poor grounds in radio. Don'ts 1. Never change the specified capacity of the buffer condenser (unless circuit matching is carefully checked with oscillograph). 2. Never attempt to repair a vibrator. Filling contacts or bending springs destroys the factory adjustment which has been carefully made with expensive instruments. 3. Never replace a vibrator until you are sure it is defective. 4. Never hesitate to write Mallory for specific information and help. A practical vibrator test, which will give the service engineer as good an indication of the vibrator condition as the tube tester does of tubes, will probably be of extreme interest to many in THE/ffec END LEAD (center lead) Centralab Resistor Circuit of the vibrator tester suggested by MalloryYaxley and described on these pages. * TUG-PROOF ... a mighty important reason for changing to CENTRALAB'S new Center Lead Resistors. For these new Fixed Resistors have their ends so firmly attached that you will never experience any trouble with loose ends no matter into what "tight hole" you may be forced to solder. Yes . . . the same strong ceramics—hard as stone . . . and as impervious to moisture, vibration and shock as the previous types. End leads brought out through bakelite insulation. Baptized with fire at 2500 degrees . . . they are still the best Resistor bet of all. For original equipment or replacements . . . select Centralab's new Center Lead Resistors. l\otv available . . . five to a card in one-half and one teatt ratings . . . 100 ohms to 5 megohms. Centralab Milwaukee, Wis. Division of Globe Union, Inc. BRITISH CENTRALAB. Ltd., Canterbury Road. Kilburn London, N.W.6, England FRENCH CENTRALAB CO. 118 Avenue Ledru-Rollin Paris XI, France July, 1938 49