Troubleshooting in transformerless LF amplifiers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers
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Despite the high reliability of the final stages of modern transformerless LF amplifiers (one of the options is shown in Fig. 1). they sometimes malfunction. They are usually caused by overloading power or terminal transistors, causing them to fail. A malfunction in these cascades is expressed in a violation of the cascade operation mode but with direct current, namely: at point A (see, Fig. 1), a voltage appears that is different from zero (with respect to the common wire).
Ris.1
in cascades. where a capacitor is connected between the output of the amplifier and the load, a voltage appears that is not equal to half the voltage of the power supplies.
To identify a faulty transistor between the common wire and point A, turn on a voltmeter and turn off the collectors of transistors T6, T7, T4 T5 in turn (see Fig. 2). If the voltage at point A is positive, then it is advisable to carry out this operation in the sequence indicated above. If it is negative, then the order of switching off the transistors should be as follows: T7, T6, T5, T4. The collectors are disconnected until the voltage at the indicated point reaches its nominal value. After that, an ohmmeter determines which transistors (with soldered collectors) are out of order.
Ris.2
The health of the remaining transistors can be verified by first short-circuiting the base and emitter of transistor T1, and then transistor T2. In the first case, the voltage on the collector of the transistor must be negative, and in the second - positive.
After replacing faulty transistors, it is necessary to set the quiescent current.
When troubleshooting in the output stages of transformerless LF amplifiers, instead of loudspeakers, their equivalent should be turned on.
Literature
- "Funkschau" (Germany), 1975, No. 10
Publication: N. Bolshakov, rf.atnn.ru
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