SBS PA250 User manual

PA250
POWER AMPLIFIER
TECHNICAL MANUAL

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 2
CONTENTS
1Introduction
2Safety, Electrical hazard
3Safety, Toxic hazard
4Safety, RF hazard
5Unpacking
6Controls and Connectors
7Installation
8Specifications
9How to contact sbs
10 Technical Section Contents
11 Declaration of conformity
No part of this manual may be reproduced or transmitted in any form or by any means, electronic or mechanical,
including photocopying, recording or information storage and retrieval systems, for any purpose other than the
purchaser's personal use, without the express written permission of sbs.
Information in this document is subject to change without notice and does not represent a commitment on the part
of sbs. sbs shall not be liable for any direct, indirect, consequential or incidental damages as a result of the use or
misuse of this equipment, handbook or any related materials.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 3
1Introduction
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The PA 250 uses a very efficient power supply design, combining a toroidal
transformer and switch mode regulator which are ideally suited to transmitter
applications where the rejection of supply borne voltage spikes and surges is of
prime importance. Woven mat PTFE circuit boards bonded to the massive heat
sink/ground plane are used to achieve remarkable thermal stability at the surface
mounted component level, leading to excellent reliability.
Installation is very straight forward thanks to the PA 250's fully featured
metering and control. The meter shows PA forward and reflected power,
voltage, current and temperature allowing setup and long term monitoring to be
a simple repeatable process. Output power is set from the front panel and
forward and reflected power alarm thresholds are easily set and previewed on
the meter. A remote control/monitoring socket allows status information to be
connected to a telemetry system and for external muting of the RF output.
The PA 250 design uses high reliability components specified to deliver full
output power, continuously over many years with no routine maintenance
requirement. It is built to exceed the requirements of ETS 300 384: January
1995, Radio broadcasting systems standard for VHF FM sound broadcasting
transmitters, specifying equipment for radio transmission.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 4
2Safety, Electrical hazard
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This unit contains high voltages which could be fatal. YOU MUST ALWAYS
ISOLATE THE UNIT FROM THE ELECTRICAL SUPPLY BY COMPLETELY
DISCONNECTING IT BEFORE ATTEMPTING TO OPEN THE CASE.
THIS EQUIPMENT MUST BE EARTHED.
Do not expose this equipment to rain or any other source of water.
In common with all electrically operated equipment, only suitably trained
competent personnel should attempt to adjust, modify or repair this equipment
or operate it with the cover removed. In case of query please contact your local
agent or sbs.
Any unauthorised adjustment, modification or repair of this equipment may
invalidate any warranty and/or safety approvals that apply.
Please read all of this manual and familiarise yourself with the controls before
attempting to use this equipment.
To ensure safety, it is the responsibility of the user to install and operate this
equipment in a manner that is within the manufacturers specifications.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 5
3Safety, Toxic hazard
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This equipment includes devices which contain Beryllium Oxide which is a highly
toxic substance. Inhalation or ingestion of even tiny particles could be injurious
to health or even FATAL!
Extreme care must be exercised when replacing and discarding components
which may contain Beryllium Oxide. If any such device is physically damaged,
you should seek expert advice, e.g. by contacting the device's manufacturers.
All such devices must be disposed of in accordance with local regulations. In
the UK your local council will have a toxic waste disposal department who will
be able to advise you. Elsewhere you should contact the responsible authorities.
NEVER DISPOSE OF A DEVICE CONTAINING BERYLLIUM OXIDE WITH
GENERAL WASTE.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 6
4Safety, RF hazard
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RF BURN HAZARD:
Removing the top cover of the unit will expose components operating at high RF
voltages. The power MOSFET transistor and components in the RF output and
filtering networks pose a very high risk. YOU MUST ALWAYS ISOLATE THE
UNIT FROM THE ELECTRICAL SUPPLY BY COMPLETELY DISCONNECTING IT
BEFORE ATTEMPTING TO OPEN THE CASE.
Antenna systems connected to any radio transmitter/amplifier will also cause RF
burns. Expert advice should be sought about the safe installation of such
systems.
RF EXPOSURE HAZARD:
Antenna systems should be installed such that exposure by any person to RF
fields cannot exceed safe limits. The permitted limits vary from country to
country. Expert advice should be sought about the safe installation of any
antenna system.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 7
5Unpacking
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This package should contain:-
1x PA250 Power Amplifier
1x IEC Power lead
1x PA250 manual
If any items are missing or damaged please inform your supplier immediately.
Initial Checks
Ensure that the PA250 has been set to the correct line voltage for your country.
The standard version is set to 230V.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 8
6Controls and Connectors
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Front Panel:
POWER LED Green indicates power is applied and that the power
supply is operating correctly, red indicates failure of
the switch mode power supply.
P.FWD LED Green indicates that output power is above preset
alarm level. Red indicates that the output power is
below that level. Orange indicates that the external
mute (via rear panel control connector) is active.
P.REF LED Green indicates that the reflected power is below the
preset alarm level. Orange indicates that reflected
power is above the preset alarm level. Red indicates
that the VSWR cutback has come into operation which
occurs at above 25W reflected power.
TEMP LED Green indicates that the RF MOSFET case temperature
is below 100 Celsius and red indicates that it is above
100 Celsius causing the temperature cutback to
operate.
METER Through the range selector switch, the meter may
provide an indication of temperature, forward power
(300W FSD), reflected power (30W FSD), power
supply voltage and power supply current.
POWER O/P ADJUST Adjusts the output power between minimum and
maximum. A small flat bladed trimmer tool is required
to make this adjustment.
PUSH TO DISPLAY Pushing this button displays the forward or reflected
power alarm preset levels on the meter. Use the
selector to switch the meter to read forward or
reflected power. Hold in the button and the meter
indicates the level at which the associated alarm will
operate.
P.FWD ADJUST Use this control in conjunction with the above button
to preset the forward power alarm level. A small flat
bladed trimmer tool is required to make this
adjustment. The amplifier output power is not
affected by this adjustment.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 9
P.REF ADJUST Use this control in conjunction with the above button
to preset the reflected power alarm level. A small flat
bladed trimmer tool is required to make this
adjustment. The amplifier output power is not
affected by this adjustment.
Rear Panel:
AC POWER Filtered IEC female connector with T6.3A fuse in pull out
drawer.
Earth point M4 threaded insert for earth bonding if required.
OUTPUT 50 OHM N-type female connector.
INPUT 50 OHM TNC female connector.
CONTROL/MONITOR 25 way D-type female connector.
Pin 1 Power supply OK
Pin 2 Forward power OK
Pin 3 Temperature OK
Pin 4 VSWR cutback active
Pin 5 Reflected power OK
Pin 14 Interlock
Pin 15 Interlock
Pin 19 External mute (ground to activate)
Pin 20 External power indication (unbuffered)
Pin 21 External power control
Pin 24,25 Ground
All outputs are open collector and low in their normal
state. The output transistors are BC184L's, which can
sink up to 100mA max. with an absolute maximum
switched voltage of 30V.
The interlock pins must be connected together for the PA250 power supply to
operate. They should not be connected to any other point.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 10
7Installation
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Installation should be straight forward. It is strongly recommended that the unit
is mounted on support rails within the rack. The case will get warm in normal
operation. A space of 1U (44mm) above and below the unit should provide
adequate ventilation to the case. To cool the PA, two fans take in air at the
front panel and blow the heated exhaust air out the back directly into the rack.
If this air flow is restricted in any way the PA250 will have to operate at a
higher internal temperature which may have a detrimental effect on its service
life. In extreme cases the thermal cutback may operate to keep the internal unit
temperature at a sustainable level by reducing the RF output power.
All cables, particularly RF should be of a high quality low loss type.
Terminations should be secure and follow the instructions supplied with the
individual connector. Control cables should be screened.
Due consideration should be given to the safety of the installation, in particular
to the AC power wiring. Attention is drawn to the section of this manual
relating to electrical safety. Where necessary, the antenna down lead should be
grounded to a safety earth via an adequately rated grounding plate.
A drive of minimum 5W is required at the RF input. Maximum permissible input
is 8W. No internal adjustment of the unit is required during installation. A green
Power LED indicates that the unit has an electricity supply input and that the
switch mode regulator, responsible for supplying power to the RF amplifier, is
operating correctly. Output power is adjusted by the use of a small flat bladed
screwdriver or trimming tool. Turn the Power O/P Adjust control with the meter
set to read P. FWD on the selector. Set the required power level. If the P.REF
LED turns red the reflected power cutback has come into operation so the power
adjust may have limited effect. Turn off the unit and correct the fault with the
antenna or down leads.
Once the required power is being displayed on the meter, push in the small Push
To Display switch and the meter will indicate the forward power alarm level. If
the power should drop to this level during operation, a forward power fault will
be flagged by the front panel LED turning red and at the control/monitor socket.
While holding in the push switch, turn the P. FWD Alarm Level Adjust control
until the meter shows the level at which this alarm is required to operate.
Similarly the reflected power alarm level can be set by turning the P. REF Alarm
Level Adjust with the push switch held in and the meter set to display P. REF on
the selector. If the reflected power should rise to this level a fault will be
flagged by the P. REF LED turning orange and through a signal output from the
control/monitor socket. This alarm is designed to give an early indication of an
antenna fault. The reflected power cut back will operate above 25W reflected.
The P. REF LED will turn red in this condition and a separate failure signal will be
output. Adjustment of the P.REF Alarm Level will not effect the cutback circuit.
An indication of the efficiency of the amplifier can be gained from the PA.V and
PA.A ranges on the meter.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 11
8Specifications
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RF Input: 5-7W
RF Output: 20 - 250 W (front panel pre-set)
Input / Output Impedance: 50 S
Spurious outputs: < -90 dBc (87.5 MHZ to 137 MHZ @ fc>± 0.5 MHZ)
Harmonic & spurious output: < -72 dBc (30 MHZ to 1 GHz)
AM Noise: < 0.5% @ ± 40 kHz deviation
Minimum return loss: 12 dB at full power (foldback above this level)
Power Supply: 230 V ± 10%, 50/60 Hz
Environmental conditions: Temperature : -20°C to 45°C
Humidity: #90% RH, non condensing
Dimensions: 88 mm high (2U) x 443 mm wide x 420 mm deep,
including handles and connectors, 12.5 kg weight

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 12
9How to contact sbs
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For all enquiries write to:-
sbs
PO Box 100
Hastings
East Sussex
TN35 4NR
Or telephone [phone] within the UK, [phone] from outside
the UK.
Or fax [phone] within the UK, [phone] from outside the UK.
Or email [e-mail] for sales enquiries or supp[email protected] for
technical support.
Alternatively visit our web site: http://www.sbsfm.com/

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 13
10 Technical Section Contents
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10.1 Introduction
10.2 Circuit diagrams
10.3 Circuit description
10.4 Alignment
10.5 Fan replacement

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 14
10.1 Introduction
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The PA250 is internally divided into two sections. All RF circuitry is housed in a
screened enclosure and is contained on two PTFE based PCBs, PAIN and
PAOUT. The PAMASTER PCB is mounted outside of the RF section. It provides
the power supply and all control and monitoring for the RF amplifier section.
Power is fed into the RF enclosure through two ‘feedthru’ capacitors. A filtered
D type connector takes all other signals between the two sections. The D type
connector is mounted on the PATERM PCB within the RF box. This routes the
power and temperature sensor signals back to the PAMASTER PCB and
distributes power to the fans.

PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 15
10.2 Circuit diagrams
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