8
To reset a "LOAD FAULT" indication:
1. Determine and correct the cause of the problem.
2. Place the amplifier "STANDBY-OPERATE" switch to "STANDBY" and then
back to "OPERATE".
The load fault detector circuit is located on the output filter board and measures the
reflected power presented by the output filter network and the SWR directional coupler.
The sensitivity of the circuit is adjusted by R304 and R601. C324 is a factory adjustment
that controls the impedance of the detector circuit. This capacitor is adjusted for
minimum detector voltage on 7 MHz with a 50 ohm load. Do not adjust any components
in this circuit without consulting the factory.
SWR Considerations
A high SWR (Standing Wave Ratio) causes higher voltages and/or currents to appear at
the output connector of the amplifier. This problem occurs with all amplifiers regardless
of whether tubes or semiconductors are used in the output stage.
Vacuum tube amplifiers have the reputation of handling high SWR's quite well. This
reputation probably exists for two reasons. First, most vacuum tube amplifiers use
adjustable output networks. Second, nearly all vacuum tubes handle momentary
overloads extremely well.
In tube amplifiers that have tunable Pi or Pi-L networks, the only components affected by
a high load SWR are those between the loading adjustment capacitor and the output
connector. When the output network is properly adjusted the SWR cannot affect tube
life.
The TMOS FET devices used in the ALS-600 have nearly the same momentary overload
tolerance as vacuum tubes. The low pass output network in the ALS-600 is much
broader in bandwidth and less sensitive to load changes than the networks used in tube
type amplifiers.
This makes the ALS-600 much less sensitive to frequency and SWR changes than the
best "no- tune" tube amplifiers but not as good as adjustable pi-networks. The ALS-600's
multiple section output network and push-pull output configuration offers much better
harmonic suppression than most tube amplifiers regardless of load SWR.
The only danger presented by a high SWR in this amplifier is that the current and power
dissipation in the output devices may exceed safe limits. Lengthy operation with high
SWR at high power levels can result in heat damage and failure of the FET devices. If
the reflected power exceeds 50 watts the power level should be reduced until the reflected
power indicated on the internal meter is 50 watts or less on peaks. At approximately 75
watts of reflected power the internal safety circuitry will disable the amplifier.
