11
PJ250 GENERAL DESCRIPTION
EXTERNAL DESCRIPTION
The PJ250 is housed in a 3U 19" rack, on the front panel the alarm indicators are placed in a central
position, the mains switch and the meter for the forward and reflected power. On the rear panel the RF
INPUT and RF OUTPUT connectors are located, together with the interlock connectors, one of BNC type
and one VDE female, the main voltage input.
ELECTRICAL DESCRIPTION
The PJ250 is a power amplifier working on the 87.5-108 MHz band with an output power in excess of
250W and a drive level of about 25W. This amplifier uses a RF module composed of four amplification
stages (MRF 317), with power supplies fixed with the RF module on cooling ribs to obtain a high
dissipation. A built in low pass filter suppresses the harmonic contents below the FCC and CCIR
requirements. A protection system protects the amplifier against thermal problems, excessive input drive
power and excessive SWR inside the amplifier or along the feeder. This system provides an automatic
reset to initial conditions when the problem ceases.
METERS AND INDICATORS
The forward and reflected power of the amplifier are read by the analog multimeter (11.Fig.1) and
selected by the selector (12.Fig.l) on the front panel. Various led indicators indicate the alarm conditions
caused by thermal problems (3.Fig.l), excessive input drive power (4.Fig.l) and excessive SWR (5.Fig.l).
A green indicator confirms the amplifier is operational (10.Fig.1).
PROTECTION CIRCUITS
The protection circuits put the amplifier in stand-by in the case of a fault condition. After 90 seconds the
protection reactivates the amplifier if the fault has disappeared. If not, this process is repeated 4 times, at
the end of which the amplifier stays disabled for 15 min; after 15 min, if the trouble persists, the
protection performs another four cycles and then disables the amplifier indefinitely. If during these cycles
the anomaly disappears and the amplifier works regularly for more than 15 min, the counting system is
reset and the original conditions established.
NOTE: the intervals described are nominal.
ELECTRICAL DESCRIPTION
INTRODUCTION
This section describes the overall working theory of the PJ250. For ease of description the amplifier is
subdivided into subassemblies that will be discussed in detail below. The block diagram is illustrated in
Fig.5.
POWER SUPPLY
The power supply comprises of a P.C. board fitted to the cooling fins and is accessible from the lower
part of the amplifier, unscrewing the fixing bolts of the cooling fins as shown in Fig.4. A mains
transformer has a selectable input for voltages between 110 and 240 V and two outputs:
"A": 31 V, "B": 18 V.
The output "A" drives the RF modules. Output "B" supplies the ALARMS CARD; inside this circuit a
rectifying and stabilization circuit provides the +15 V needed by the electronics.
ALARMS CARD
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