MARCONI BD 268

GENERAL DESCRIPTION

HF Broadcasting Transmitter (10 kW) Type BD 268

photo Marconi BD 268

THIS transmitter is of particular interest where economic considerations necessitate the minimum of outlay and running costs, without sacrificing quality of performance and dependability of service.

FEATURES

Air cooling is employed throughout.

Xenon-filled rectifiers are used, capable of operation over a wide temperature range and requiring no conditioning.

Miniature circuit breakers are incorporated which largely avoid the necessity for fuses and contactors.

Full protection of equipment is ensured by magnetic and thermal overload relays.

Personnel are protected from contact with high voltages by a key-type interlock system.

Installation costs are reduced to an absolute minimum. No floor ducts for cooling or cabling are required.

Efficiency is of the order of 50%, and thus running costs are extremely low.

CONSTRUCTION

The transmitter cabinet is built up from four standard units 7 ft (213cm) in height and 3 ft 9 in. (114cm) deep. The whole cabinet as sembly occupies only 37.5 sq. ft (86 sq. dm) of floor space, and is completely self-contained including cooling fans and high-power components.

CIRCUIT

Drive stages. The Drive Unit Type (HD 26) normally employed with this transmitter can be internally mounted.

A harmonic generator unit follows the crystal drive unit, to produce a frequency between 3.2 and 18 Mc/s which is passed to the r.f amplifiers.

R.F stages. There are three stages of r.f amplification. The first is a single tetrode stage; the second employs two tetrodes and is capacitance-coupled from its anode circuit to the grid of the final stage which uses a neutralized triode. The final stage has a balanced output circuit which is coupled into the outgoing feeder. This is a 600-ohm balanced pair.

Modulation stages. A system of high-level class B modulation is employed. Three a.f amplification stages are used to amplify the input signal to a level sufficient to drive the main modulator valves. An input signal of 0 dBm is sufficient to produce full modulation.

The modulator, which uses two triodes, is fed from the a.f amplification chain via a cathode follower stage.

Power supplies. The main h.t supply is obtained from six xenon-filled rectifier valves which require no conditioning. a.c valve heating is employed.

The control and protection circuits are simplified by the use of circuit breakers in place of fuses and contactors.

TECHNICAL SPECIFICATIONS
Frequency range	Broadcasting bands between 3.2 and 18 Mc/s
Power output	10 kW unmodulated carrier to aerial feeder
Frequency stability	To within ±10 parts in 10⁶ over a temperature range of ±15°C
Modulation	High-level, Class B
Scintillation	Not greater than 1 c/s up to 90% modulation
R.F harmonics	Less than 200 mW
A.F input	1 mW into 600 ohm for 100% tone modulation. (An attenuator is fitted to cater for inputs up to +12 dBm in 1 dB steps.)
A.F response	Level within ±1.5 dB from 50 to 7500 c/s
A.F distortion	Less than 3% up to 90% modulation; less than 2.5% up to 50% modulation (50 to 7500 c/s)
Output impedance	600 ohm balanced
Noise level	Below 60 dB relative to 100% modulation, unweighted
Ambient temperature	+45°C max.
Power supply	380-440 V (±5%) 50-60 c/s (±2%) three-phase a.c.
Power consumption	20 kV unmodulated. 22 kW at 30% modulation (1000 c/s) 32 kW at 100% modulation (1000 c/s)
Power factor	0.9 at full output power
Dimensions	Height 7 ft (213 cm) Width 10 ft (305 cm) Depth 3 ft 9 in. (114 cm)
Specifications may change without notice

TUBE COMPLEMENT
RF stages		AF stages and modulator
Number	Type	Number	Type
?	?	?	?

THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES
	ITU	Country	ITU	Country
	BOT	BOTSWANA	COD	CONGO DEMOCRATIC REPUBLIC
	GHA	GHANA	KEN	KENYA
	LSO	LESOTHO	MLA	MALAYSIA
	NIG	NIGERIA	SRL	SIERRA LEONE
	SNG	SINGAPORE	SOM	SOMALIA
	TZA	TANZANIA	UAE	UNITED ARAB EMIRATES
	YEM	YEMEN