The ARRL Handbook for Radio Communications; Volume 6: Test Equipment, Troubleshooting, RFI & Index by Inc. ARRL

Author:Inc., ARRL [Inc., ARRL]
Language: eng
Format: azw3
Publisher: ARRL, the national association for Amateur Radio®
Published: 2018-10-07T16:00:00+00:00

Many “amplifier is dead” problems turn out to be simply a lack of ac power. Before even opening the cabinet of an unresponsive amplifier, be sure that ac is really present at the wall socket and that the fuse or circuit breaker is really closed. If ac power is present at the wall socket, trace through any internal fuses, interlocks and relays all the way through to the transformer’s primary terminals. If the amplifier operates from 240-V circuits, be sure you check both hot wires. (See the Safety chapter for more information about ac wiring practices.)

Hard failures in a high-voltage power supply are rarely subtle, so it’s usually clear if there is a problem. When you repair a power supply, take the opportunity to check all related components. If all defective components are not replaced, the failures may be repeated when the circuit is re-energized.

Rectifiers may fail open or shorted — test them using a DMM diode checker. An open rectifier will result in a drop in the high-voltage output of 50% or more but will probably not overheat or destroy itself. A shorted rectifier failure is usually more dramatic and may cause additional rectifiers or filter capacitors to fail. If one rectifier in a string has failed, it may be a good idea to replace the entire string as the remaining rectifiers have been subjected to a higher-than-normal voltage.

High-voltage filter capacitors usually fail shorted, although they will occasionally lose capacitance and show a rise in ESR (equivalent series resistance). Check the rectifiers and any metering components — they may have been damaged by the current surge caused from a short circuit.

Power transformer failures are usually due to arcing in the windings, insulation failures, or overheating. high-voltage transformers can be disassembled and rewound by a custom transformer manufacturer.

Along with the plate supply, tetrode screen supplies occasionally fail, too. The usual cause is the regulation circuit that drops the voltage from the plate level. Operating without a screen supply can be damaging to a tube, so be sure to check the tube carefully after repairs.

If the power supply checks out okay and the tube’s filaments are not lit, check the tube socket and the pins on the tube itself — overheating can cause solder to flow and intermittent failure. If the tube’s filament is lit, check the resting or bias current. If it is excessive or very low, check all bias voltages and dc current paths to the tube, such as the plate choke, screen supply (for tetrodes) and grid or cathode circuits.

If you do not find power supply and dc problems, check the RF components or RF deck. Check the input SWR to the amplifier. If it has changed then you likely have a problem in the input circuitry (overheated coil, shorted capacitor or bad switch contact) or one or more tubes have failed. Perform a visual check of the input circuitry and the band switch, followed by an ohmmeter check of all input components. Use an SWR analyzer or

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