MARCONI BD 260

GENERAL DESCRIPTION

7,5 kW HF Broadcasting Transmitter Type BD 260

photo Marconi BD 260

THE TYPE BD 260 transmitter is an economical equipment of compact design and pleasing appearance. it is designed to give a high fidelity performance and incorporates the most up-to-date developments, special attention being paid to ensuring maximum possible ease of operation.

Two basic editions of the transmitter are available giving a frequency coverage of 2.3-15 Mc/s and 4.0-27.5 Mc/s respectively.

FEATURES

Rapid frequency changing in less than four minutes to any one of up to six spot frequencies.

Air cooling is used throughout resulting in consequent simplification of equipment and reduction in routine maintenance.

The machinery involved is very quiet in operation. All major valves have thoriated tungsten filaments and AC heating is employed.

Improved valve design permits the use of moderate HT voltage

Unit construction greatly facilitates transport and installation, and access doors are provided at the rear of each unit. A system of electro mechanical interlocks ensures safety of personnel and all circuits are protected against overloads.

Complete freedom from fire risk by the elimination of oil-filled components.

CIRCUITS

Radio frequency stages. The transmitter is normally supplied with three drive units, each of which contains a crystal oscillator, a harmonic generator and a power amplifier. Provision is made, however, for fitting additional units up to a maximum of six in order to give a wider selection of frequencies. These may be set up anywhere in the frequency band. Furthermore, an external oscillator may be used to feed the harmonic generator, so permitting the use of an alternative high precision drive.

Signal from the drive unit is fed to two tetrode amplifier stages, which are followed by the two main power amplifier stages. These are of unconventional design and are especially constructed and screened to ensure negligible feedback and so keep distortion to an absolute minimum. The output circuits take the form of double-coupled circuits designed to produce a high degree of attenuation of unwanted frequencies.

The two final amplification stages are similarly designed and consist of a tetrode amplifier followed by a grounded-grid triode stage, both of which are anode modulated.

Audio frequency stages. The audio input is fed through a pre-amplifier, designed to give an exceptionally low distortion and hum level, to a two stage push-pull voltage amplifier which is followed by a driver stage operating under class B conditions. This latter incorporates a modulation control circuit designed to limit the modulation depth to 100%. The modulator consists of two triode valves connected in push-pull and operating in Class B. The modulation transformer and reactor are air cooled. Monitoring circuits are provided to enable the modulator performance to be checked and pick up points facilitate circuit testing.

Power supplies. The main HT supply is derived from mercury-arc rectifier valves, and the low power rectifiers are of conventional design. All valves aud components are rated generously.

TECHNICAL SPECIFICATIONS
Power rating	(to aerial feeder): 7.5 kW from 2.3 to 21.45 Mc/s with gradual reduction to 6.5 kW at 27.5 Mc/s. Capable of 100% modulation
Modulation	High level Class B
Frequency ranges	4-27.5 Mc/s (75-109 m) or alternatively, 2.3-15 Mc/s (130-20 m)
Frequency stability	±1 part in 10⁵. An alternative version of the transmitter is available with an external oscillator, to give a stability of ±20 c/s conforming to the Mexico City recommendations
RF harmonics	Not less than 46 db below the fundamental
AF response	Level within ±1 db between 50 and 5,000 c/s and ±2 db between 30 and 10,000 c/s measured with reference to 60% modulation at 400 c/s
AF harmonics	Not greater than 3.5% from 30 to 7,500 c/s Not greater than 2.5% from 100 to 5,000 c/s at modulation depths up to 90%
Noise level	At least 60 db unweighted and 75 db weighted below 100% modulation
Input level	Not less than 1 mW into 600 ohm for 100% modulation
Output impedance	600 ohm balanced or 75 ohm unbalanced. Max. standing wave ratio 1.4 to 1
Power supply	380-400 V, three-phase four-wire 50 c/s AC mains. Voltage regulation within +5% and -15% of nominal value, frequency tolerance ±2%, phase variation within 1% of the mean
Power consumption	33 kW at 100% modulation (efficiency 34%) with a power factor of 0.9
Dimensions	Height 7 ft (2.1 m)	Width 11 ft (3.3 m)	Depth 5 ft (1.5 m)	Weight 2.3 tons (2320 kg)
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
	CYP	CYPRUS	MLA	MALAYSIA
	NIG	NIGERIA	UGA	UGANDA
	YEM	YEMEN