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Valve Radio and Audio Repair Handbook by CHAS MILLER

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Chapter 22

Finding faults on IF amplifiers

Chas E. Miller

The valves used in the IF amplifiers of the very early superhets were ‘straight’ screen grids, but these were soon replaced by variable-mu types and then by RF pentodes. In all cases complete failure of the amplifier can be brought about by a screen-grid HT supply resistor going open circuit. In some cases a single resistor was used to supply either two IF amplifiers or a single IF amplifier and the frequency changer, the current passed being high enough to warrant the use of a wire-wound type. Always check these for signs of overheating. Some firms, notably EMI, slipped sleeving over such resistors, in which case look for signs of its having become brittle with heat. Any overloading may ...

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