Radio today (Jan-Mar 1939)

Fig. 2. If the hum decreases when the input is shorted, then the hum is in a previous stage. If the hum level remains the same, it is being introduced in the stage under consideration. A quick method of checking proper polarity of the voice coil and field coils is to short the primary of the output transformer as shown in Fig. 3-A. Where pentode tubes are used, merely connect the screen and plate together at the socket. When the primary is shorted, the hum output of the set should increase or remain the same, when the coils are properly poled. If the hum decreases, it means that either the field or the voice coil connections should be reversed. Often a substantial reduction in hum output can be obtained by properly poling the field and voice coils. This is of particular importance when replacing the speaker, cone, output transformer, or field coil. (Naturally, if no objectionable hum is present, it is rather useless to make such a test.) Hum due to insufficient filtering of the grid bias supply is checked by connecting additional capacity across the bias supply source as illustrated in Fig. 3-B. A decrease in hum means that more filtering should be employed. Either an increase in the filtering resistance or condenser, or both, may be necessary. Care should be taken that the total grid circuit resistance does not exceed that recommended by the tube manufacturers for the tube employed. Checking hum-bucking circuits Should it be found that the hum is in the previous stages, the same tests are made for these stages as have been outlined for the power output stage, except for the poling of the speaker. When hum canceling is employed, it may be necessary to adjust the amount of induced hum stages to effect a more perfect balance. If it has been determined that hum bucking is employed, and it is of insufficient value in the previous stages, it can be increased by decreasing the value of the plate coupling resistor in a resistance-coupled stage. Also the filtering to the stage might be made less effective so as to increase the hum bucking voltage. On the other hand, too much hum bucking voltage introduced in the voltage amplifier stages, is evidenced by a decrease in the hum when the input to the output stage was shorted as in Fig. 2. This is treated similar to the case where no hum-bucking is used, but when the hum is introduced in a previous stage. The plate coupling resistor can be increased or the filtering of the power supply can be increased. In many sets hum is kept at a low level by restricting the bass response of the receiver. Servicemen can employ this same method that the manufacturers sometimes use. Of course, the tone of the set will be affected. Induced hums Induced hum by coupling of circuits or components is traced to the stage in the manner described previously. Hum picked up by a transformer is determined by opening the primary at the B plus connection. When hum voltage from the power supply is isolated from the transformer as stated above, the hum will decrease if it comes from the power supply. If the hum does not decrease, it means that it is being magnetically induced into the transformer. This induction may be reduced by relocating the transformer or turning it at an angle. Sometimes magnetic shielding is a satisfactory solution. Hum that is induced into the wiring may be cured by moving the hot leads to a position where the induction is reduced. Or the hum induction can be decreased by shielding the leads. Generally speaking induced hum is not obtained in a receiver unless major changes have been in the receiver. One receiver that wTas brought into Radio Today's laboratory had a bad hum that was caused by induced voltages into a stage coupling transformer. It seems that a universal replacement had been used and the core was in a different position than that originally used. Since the transformer was near the speaker field coil, a large amount of hum was picked up by the transformer. Changing the angle of the transformer reduced the hum. but it was impossible to mount it in such a position. Simplest and cheapest solution was to use an exact duplicate unit, which proved satisfactory. If hum chasing is done on a stageby-stage basis, listening to the output as various parts of the stage are shorted out or changed, it will be found quite simple and quick, On the other hand, absolute measurement of hum voltages may be found to be confusing. As in the other tests described in dynamic testing, a check under actual operating conditions is most practical. FIRST AID FOR SICK RADIOS Lee Houseman of the Dallas Radio Hospital has sent a snappy red and white "ambulance" whirling through the Texas city's streets for the. past five years and has found it a smart business builder. Replete with medical cross, instruments, but racks instead of the usual stretcher, the first aid wagon arouses lots of comment. A smartly-dressed "doctor" leaps out, diagnoses the ills of the ailing receiver. If nothing serious is wrong, the medico gives first aid on the spot. Otherwise the "patient" is hauled off to the hospital for a major operation. The ambulance can't be confused with one for humans because it is simply a sedan painted and doctored up appropriately. SERVICE COST RECORDS IMPORTANT The need for keeping accurate records of expenses and income is far more important than most servicemen realize. If every serviceman figured out how much it costs him to do business, it is likely that much of the cutthroat price competition would cease. No man wants to do business at a loss, but many servicemen in their anxiety to get the work quote prices far below a fair profit, only because they do not know they are losing money. Everyday we hear experiences of how servicemen quote a price about half of their actual cost — and these' same servicemen after being shown how they are losing money, are willing to charge a fair price — what's more important, they get it. In order to help the serviceman keep acquainted with his operating costs, Radio Today is conducting a survey of service costs and profits. Each questionnaire that is submitted will be individually figured and the participant given pertinent data concerning his business. A questionnaire form was printed on page 35 of the January issue. If you wish to participate, please do so at once. Serviceman Robert Leslie of Ontonagon, Mich., wrote us as follows when he submitted the operating figures for his business: "I run a one-man business and would have been in the red badly, if I had not followed your advice in Radio Today. I now sit down and keep track of every hour and charge ?2 an hour on every radio I service. And had I taken Radio Today five years ago, I would be able to have a nice little bank account now." Fig. 3-A. If the speaker field and voice coil are properly poled, the hum will increase when the primary is shorted. Fig. 3-B. If the hum decreases when the bias supply filter capacity is increased, then the hum occurs in that circuit. February, 1939 27