User's Manual

ALPHA 9500 Interim OPERATING MANUAL

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port. Also on the mains board is a step-start circuit. This circuit consists of a relay and a resistor,

which are time-sequenced to limit the inrush current into the ampliﬁer when it is ﬁrst turned on.

5.5.2 High Voltage Board

The main high voltage for the ampliﬁer is created on this board using a full-wave bridge rectiﬁer

and a bank of capacitors. This power supply has two 10-ohm resistors, one in the positive (B+)

lead, and the other in the negative return, which goes to the tube cathode. The combination of

these two resistors limits the surge current in the case of a B+ arc. The voltage across the resistor

in the negative return is used to monitor tube plate current in the control board. This voltage

is also used to generate the “hard fault” condition. When the power supply current exceeds

about 2 to 2.5 Amps, a relay operates to open the coil circuit of the mains tap relays on the mains

board. When these relays release, the ampliﬁer goes to the power-off state. This hard fault circuit

operates independently of microprocessor control. All power supply ﬁlter capacitors on this

board have bleeder resistors which will discharge the capacitors in less than 60 seconds. If it is

necessary to work on this board, it is nevertheless recommended that the discharged condition

be conﬁrmed with a voltmeter, due to the remote possibility of bleeder resistor failure.

5.6 Master Control Board

The master control board is the heart of the ampliﬁer. It is based around a PIC microcontroller,

the master of the I2C bus. This microcontroller communicates with each controller on the other

boards in the ampliﬁer. It is used to monitor all the critical voltages and currents in the ampliﬁer,

as well as the input power and output forward and reﬂected power. It uses these converted

values to control the ampliﬁer’s operation and to send data to the front panel, so that the correct

LEDs are lit and the stepper motors move to the correct positions. A standard 9-pin RS232 serial

port is provided for control and monitoring and is found on the back of the Alpha 9500. A USB

port is also provided. Either port may be used, but only one may be active at any one time. The

ampliﬁer automatically senses when a PC is attached to the USB port, and uses that port. If

nothing is connected to the USB, the ampliﬁer automatically switches back to the RS232 serial

port.

5.7 Display Board

This is the largest board in the ampliﬁer and spans the entire inside front panel. It has 3 micro-

controllers on board, one each to control the stepper motors, the LEDs and seven segment display

and push buttons and ﬁnally the sound generator (not yet implemented).

5.8 Cathode (Input Match) Board