Central Electronics Model 600L Linear Amplifier
600L_Manual.doc Page 4 of 5 na5rc@na5rc.net
Reflected Power Meter Scale
The 1-Ohm power-sampling resistor is part of a bridge circuit arranged to indicate load
impedance departures from a pure resistance of 52-Ohms. A germanium diode functions as
the null detector in the SWR Bridge and its output is indicated on the meter. The Reflected
Power Scale is in calibration at a carrier input of 500-Watts. At the beginning of the yellow
scale, the SWR is approximately 3:1. The antenna should be pruned if the indication is in the
red portion of the scale. The use of an antenna scope or an impedance bridge with an
electrical half wave repeater transmission line is recommended for pruning operation.
TVI
Television interference may be encountered in some areas. First, determine if interference is
caused by fundamental overload of the TV receiver by installing a high-pass filter at the TV
receiver. If the interference persists, it may be caused by weak harmonics radiated by the
antenna. Since the 600L RF unit is thoroughly shielded, the installation of a 52 or 72 Ohm
low-pass filter close to the output connector should reduce the harmonic radiation. The
attenuation required will vary with the order of harmonic and distance from the TV transmitter.
Plate Overload
In the event of a plate voltage short, the Plate Overload Breaker will trip out. It will continue
to trip when reset if the short persists. It is essentially in series with the Plate Supply Switch.
Load Mismatch Breaker
The SWR Bridge in the output circuit of the 600L is used to control the internal blocking bias
to prevent damage to the 813 in the event the unit is operated without a load.
At the front of the RF unit is a narrow shelf containing a 12AT7, a NE2 neon lamp,
screwdriver slotted control, and a telephone type relay. The control adjusts the sensitivity of
the mismatch trip circuit. If bursts of voice energy cause the breaker circuit to trip while an
antenna is connected, the sensitivity can be decreased by turning the control slowly
counterclockwise. The factory adjustment is made with no antenna for trip out at
approximately 500-Watts input.
If the 12AT7 tube is not functioning, the relay will not operate when the Plate Switch is turned
on. If the relay does not operate, the VR75 will not glow and blocking bias will be applied to
the 813. With blocking bias applied, the exciter will just begin to drive the 813.
SAFETY
CAUTION: DEADLY HIGH VOLTAGE
Due to the large energy storage capacity of the special 45-µF filter capacitor, several minutes
should be allowed before it is discharged with a large, well insulated, screwdriver.
CAUTION! To prevent damage to the RF choke DO NOT DISCHARGE B+ at the 813 plate
cap. Suggest shorting the RF choke mounting bracket or feed through
insulators to the chassis.
The Scroen regulation circuit consists of an 812A series regulator, 6AQ5 control tube, and
two 0A2 reference tubes.
