Lowe Production HF-250 User manual

HF-250 General Coverage Receiver.
User's Manual.
Contents
Introduction. 2
Getting started. 3
Controls and Connections 12
Operatingthe HF-250. 17
Care of your Receiver. 30
General notes. 32
Optional Units. 33
Circuit description. 34
Receiver specification. 36
Circuit Diagrams. 44
(C) 1995 Lowe Production Ltd.
Bentley Bridge,

HF-250 User Manual HF250 User Manual
Introduction.
The term Communications Receiver was originally used in
its quite literal sense to describe a radio receiver which was
part of a point to point Communications link.
These radio links were normally manned by trained
operators, and Morse Code was the usual method of
transmitting information. Because of the specialist nature of
the system and the fact that the operators were technically
trained the Communications receiver itself was quite often a
complex piece of equipment.
Over the past twenty or so years, a marked change has
taken place in HF spectrum occupancy and there has been a
considerable increase in the use of short wave broadcasting,
air traffic control, news agency transmissions and so on. The
interest in listening generated by this spread of activity has
lead to a demand amongst the general public for receivers
which will enable them to keep in touch with world affairs by
short wave radio.
Clearly, these users of receivers are relatively unskilled in
the handling of complicated equipment and this in turn has
resulted in the introduction of simpler receivers. However,
simplicity of operation has often been accomplished by a
compromise in actual performance, and the results obtained
from some of these simple receivers have been quite
disappointing.
The design and development of the HF-250 was based on
straightforward objectives :-
• To obtain sufficiënt RF performance for the receiver to
operate without problem in crowded bands with many
strong signals
• To combine complete control of a necessarily complex
piece of equipment with easy operation for the user.
• To achieve both the previous objectives within a
reasonable price range.
We believe that these stated goals have been reached, and
that the HF-250 receiver represents a truly new approach to
meeting the demands of the educated short wave listener.
Gettinq Started
The Power Supplv and other Connections.
The HF-250 requires an external DC supply of between 10
and 15 volts. The absolute maximum supply is 16 volts, and
if this is exceeded damage may occur to the receiver. The
supply polarity is negative ground only, and although
reverse polarity protection is built in it is wise to ensure that
any supply is correctly connected. Be sure that the receiver
power switch is OFF before plugging in or unplugging the
power connectot.
In most countries, the HF-250 will be supplied with a
small 12 volt regulated power unit which is designed to
operate from the local mains power. Remember that this
supply will be operating all the time that it is connected

HF-250 User Manual HF250 User Manual
to the mains outlet, and it is a wise safety precaution to
disconnect it when the receiver or clock are not in use.
In the United Kingdom the power supply is fitted with a
three-core mains lead, and the earth connection
(yellow/green) is connected to the negative (Earth
Connection ground) terminal of the receiver.
This provides a reasonable earth connection for the
receiver, but in some cases, where mains-borne interference
is prominent, it will be necessary to provide the receiver with
a good RF earth, either in addition to the mains earth or in
some cases instead of it. If the receiver is used with any
other mains power supply it should be able to comply with
BSI standards relating to Class 2 insulation.
External Loudspeaker.
A small internal loudspeaker is provided in the HF-250 so
that it is self contained, but although it can provide
reasonable all round audio quality, clearly in the limited
space available compromise has to be made. You will find
that if the volume control is set to a high level there may be
some audio feedback caused by vibration induced by the
internal loudspeaker. If it is necessary to operate the
receiver for long periods at high audio levels the use of an
external loudspeaker is recommended.
Because the HF-250 is capable of giving a high quality audio
signal, we suggest you use a good external loudspeaker, a
small bookshelf type Hi-Fi unit is satisfactory. We can
provide a suitable unit as an optional accessory with the
correct connecting lead for the HF-250. Any external
loudspeaker should have an impedance of 4 to 8Q.
Record Output.
Many keen listeners like to tape record any interesting
stations they hear, and a low level audio output has been
provided, also a pair of relay contacts for switching on the
tape recorder in connection with the built in timer.
The record out socket accepts a 3.5mm mono jack plug and
provides a level suitable for feeding into the line input of
most tape recorders or amplifier systems. An attenuating
resistor should be added in the lead if feeding directly into
the microphone input of a cassette recorder. The output
level at this socket is not affected by the Volume or Tone
controls, so that the loudspeaker can be used to monitor
whilst recording. The record output can also be used for
driving most types of receiver ancillary equipment such as
RTTY, Facsimile or Morse Code decoders. The output level
is about 350 mV RMS from a source impedance of 5k.
Types of Signal.
The HF-250 is equipped to receive most types of
transmission likely to be encountered within its tuning range,
and although most users will be familiar with these, here is a
short section on this topic that may be useful to the
beginner.
AM (Amplitude Modulation).
This was the earliest method used of audio modulation of an
RF carrier wave, and is still almost universally used for long,
medium and short wave broadcasting. An AM signal is fairly

HF-250 User Manual HF250 User Manual 7
easy to tune in, and given a reasonable signal strength, the
receiver may not need to be spot on in frequency. However
when conditions are poor, AM can be difficult to resolve
-one particular problem is frequency selective fading and this
is discussed later.
AM Selectivity.
A radio signal occupies a certain portion of the radio
spectrum which is known as its bandwidth.
The bandwidth of an AM signal is twice its highest
modulation frequency, and because of this broadcasters are
restricted to transmitting audio frequencies below 5 kHz so
that they do not occupy too much spectrum. In the long and
medium wave broadcast bands, station frequencies are
separated by 9 kHz (10 kHz in the USA) so there is little or
no overlap of adjacent signal bandwidths. In the short wave
bands however, the stations use a nominal 5 kHz spacing,
and some broadcasters do not abide by any rules at all, so
there is considerable signal overlap.
The HF-250 is provided with four different filter bandwidths
because of this very problem. If you are receiving a strong
signal in a clear part of the radio spectrum then you can use
the 10 kHz filter and obtain the best fidelity. The stronger
and closer adjacent stations are, the narrower the filter you
will need, and the more muffled the sound will be because
high frequencies are removed.
The 7 kHz filter provides a good compromise for most
medium wave conditions, and the 4 kHz filter for short wave.
The 2.2 kHz filter can be used under severe conditions, but it
is really only suitable for speech reproduction.
When AM mode is selected on the HF-250, the 7 kHz filter is
initially switched in. If you want to change to a different filter
you can use the filter select function. You may find that
reception of a station is improved by tuning the receiver
slightly above or below its carrier frequency.
This is quite a useful technique if there is a strong adjacent
signal that you don't want. As long as the carrier signal is
within the receiver's filter then all will be well, but if you tune
too far or select a narrower filter then the signal will become
distorted.
SSB (Single Sideband).
An AM signal can be considered as a carrier wave combined
with two identical sidebands which contain the modulating
audio signal. It is possible to remove one of the sidebands
without losing any vital information, and immediately halve
the bandwidth occupied by the signal. In practice the carrier
wave is also removed (or partially suppressed) to improve
transmission efficiency, and the result is a single sideband
transmission.
SSB transmissions are used extensively for voice
communication, particularly to aircraft and shipping, and also
by radio amateurs.
It is possible to use either of the two initial sidebands of a
signal, so there are two distinct types of SSB transmission;
Upper Sideband (USB) where the sideband frequency is
above the carrier frequency, and Lower Sideband (LSB)

8HF-250 User Manual HF250 User Manual
where it is below. Nearly all commercial transmissions are
USB, as are amateur transmissions at frequencies above 10
MHz. At frequencies below 10 MHz radio amateurs use LSB
by convention.
To receive an SSB transmission, the receiver must re-insert
the missing carrier signal. If this is not done the signal will
sound just like Donald Duck, - try listening to an SSB signal
in AM mode for this effect. For correct reception the receiver
should be tuned exactly to the carrier frequency.
The HF-250 has a very slow tune rate on its SSB mode to
facilitate accurate tuning, but you will need a steady hand.
The pitch of the received voice will change as you tune
through the signal, but only at one tuning position will it
sound like a natural voice.
A 2.2 kHz bandwidth filter will just accommodate the audio
frequencies used for voice transmission, and this is the filter
most commonly used for SSB reception. The HF-250 will
automatically select this filter for LSB or USB modes, but
under good signal conditions selecting the 4 kHz filter may
offer improved clarity.
CW (Continuous Wave, ie Morse).
Morse code is usually transmitted by interrupting a single
carrier wave, and it occupies a very narrow bandwidth. In
terms of ability to get a message through under difficult
propagation conditions morse is an efficiënt method,
although modern error-correcting digital data systems are
also very good. CW signals are received in the same way as
SSB signals, with the carrier inserted in the
receiver
producing a beat note with the incoming signal. In the CW
mode the HF-250 provides an 800 Hz offset between the
display and the intemal carrier, so that a note is heard at
800 Hz when the receiver is tuned exactly to the signal.
The HF-250 initially selects the 2.2 kHz filter in CW mode
and this should be used for finding and tuning signals. As an
alternative, a narrow 200 Hz filter is provided, and its use will
greatly reduce the background noise, allowing the Morse to
be more easily read. Careful tuning is needed to place the
signal at the peak of this filter, which is centred on 800 Hz.
RTTY (Radio Teletype).
The method of sending teleprinter messages by HF radio
link is to use two closely spaced signals, transmitting one or
the other to send binary data. Each teleprinter character is
encoded into a different sequence of tones which are
transmitted in a bewildering combination of different speeds,
tone shifts, and codes. RTTY signals are tuned in SSB
mode on a receiver, but require a special terminal unit to
decode and display the actual text.
FAX (Facsimile).
Pictorial information (often meteorological data) is
transmitted over HF radio links for reception by shipping. As
with RTTY, a special facsimile decoder and display or printer
is required for its reception.
FM (Frequency Modulation).
When the DU-250 detector option is fitted to the HF-250, the
receiver will receive FM signals. In the context of an HF
receiver this means narrow band FM, which occupies a

10 HF-250 User Manual HF250 User Manual 11
bandwidth of around 12 kHz. This is not to be confused with
broadcast FM transmissions which have bandwidths in
excess of 150 kHz, and are normally transmitted at VHF or
UHF where there is sufficiënt spectrum to accommodate
them.
FM signals in the HF spectrum are usually found either in
the 27 MHz Citizens Band or in the 28 MHz amateur band. It
is typical of FM receivers that they produce a large amount
of noise when there is no signal at the aerial. To overcome
this a squeich system is employed to automatically turn off
the audio output unless a signal is detected. The HF-250
squeich system may be turned on or off manually by using
the filter select control. The filter bandwidth is fixed at 12
kHz in FM mode.
AM Propagation and Fading.
During AM signal reception it is possible to experience
severe fading problems, particularly after nightfall.
This is mainly due to the signal reaching the receiver by
several different paths from the transmitter, and it is most
common after dark because this is when the ionosphere
reflects most HF radio signals. Fading occurs when the
signals arrive at the receiver in antiphase (having travelled
different distances) and then cancel each other out. This will
only occur at a few specific frequencies at any one instant,
hence the term frequency selective fading.
If a selective fade reduces the carrier level of an AM signal,
but leaves the sideband levels unaltered, a receiver with a
conventional AM detector will not be able to
correctly
reproduce the signal, and the output will be distorted. There
are two techniques that can be used to improve the situation;
ECSS, and Synchronous (or Phase-Locked) AM detection.
ECSS (Exalted Carrier, Selectable Sideband).
The ECSS technique makes use of the fact that with a good,
selective receiver, capable of resolving SSB, an AM signal
can be passed through the SSB filter which is only wide
enough to allow one sideband through. The filter must
attenuate the carrier signal by at least 20dB for this
technique to work with any success.
The receiver can be used in the SSB mode with the
incoming AM carrier tuned to zero beat, and the
accompanying sideband treated as a true SSB signal. Either
the upper or lower sideband can be selected using either
USB or LSB mode, so interfering stations can often be
eliminated.
The improvement in intelligibility is often dramatic, and it is
well worth trying out ECSS and developing the ability to use
it. The HF-250 when used with its 2.2 kHz filter is ideally
suited to ECSS reception.
AMS (Synchronous AM).
The difficulty in receiving music signals with the ECSS
method is that it is very difficult to match the receivers
injected carrier exactly with the frequency of the incoming
carrier. Any difference results in a frequency shift of the
audio signal, and the consequent loss of harmonie
relationships.

12 HF-250 User Manual HF250 User Manual 13
The synchronous AM detector in the DU-250 option uses a
narrow deviation phase-locked oscillator to replace the
incoming AM carrier. When phase locked, this oscillator is at
exactly the same frequency as the carrier signal, and does
not have to rely on absolute receiver tuning accuracy.
Incoming carrier level changes make no difference to the
signal detection provided that there is some carrier for the
oscillator to lock on to.
Synchronous AM reception can be selected using the
information carried in the upper or lower sidebands, or both
together. Use of this facility can considerably improve
reception under difficult conditions.
Controls and Connections.
(1) Signal strength
Calibrated S1 to S9 and 10dB, 30dB and 50dB above S9.
The S9 signal strength is set at 50 microvolts p.d. at the 50
Q antenna socket.
(2) Mode Button.
Enables one of ten available reception modes to be selected
by use of the MHz Up and Down buttons (16 and 11)
CW For reception of CW (Morse) signals
LSB For single-sideband signals, (Lower sideband).
USB For single-sideband signals, (Upper sideband).
AM For reception of AM (broadcast) signals
AMS For AM reception using the synchronous detector.
DSB, USB LSB. (DU-250 option).
AMSU Synchronous AM reception using upper sideband.
(DU250 option)
AMSL Synchronous AM reception using lower sideband.
(DU250 option)
FM For reception of narrow-band FM signals. (DU-250
option).
Press this button and the mode indicators will flash. Now use
the MHz Up/Down buttons (16 and 11) to change mode.
When you have selected the mode you require, press the
mode button once more to return to tuning function. The
display will also flash as a reminder that you are in mode
select function.
(3) AMS detector lock indicator
Is illuminated when the synchronous detector is correctly
locked to the received signal.
(4) Frequency display
A 6-digit back-lit LCD showing the received frequency to the
nearest 100 hertz. Frequencies below 1000 kilohertz are
shown directly and those above in megahertz, with a decimal
point separating MHz and kHz.
(5) Main tuning knob
For tuning the receiver and selecting memories. The rate of
tuning is adjusted according to the receiver's mode and the
speed of rotation of the tuning knob.
(6) Headphone socket
The headphone output jack for a Standard 1/4" plug. Mono or
stereo headphones may be used (stereo phones operating

14 HF-250 User Manual HF250 User Manual 15
in mono), when the headphones are plugged in the intemal
loudspeaker is disconnected .
(7) Volume control and Power switch
Controls the volume in the loudspeaker and at the
headphone output. When turned fully counter-clockwise to
the click position, the receiver power is turned off, leaving
only the clock displayed.
(8) Tone control
Varies the audio tonal quality in the loudspeaker and at the
headphone output. The control provides either high
frequency cut or low frequency cut, with a flat response in its
central position.
(9) Mode Indicators.
These indicators show the reception mode currently
selected.
(10) Fast Tuning.
When this button is held pressed then the receiver will tune
in I.OKhz steps.
(14) R F Attenuator button
Displays, and allows the user to change, the state of the
attenuator. A fixed 20dB of attenuation may be switched in or
out.
(15) Filter select button
Enables the receiver bandwidth to be displayed and
changed. In LSB, USB, AM and AMS modes, four different
filters are available :- 2.2, 4, 7 and 10kHz. In CW mode, two
:- 2.2kHz and 200Hz. In FM
squelch facility on and off.
(11+16) Megahertz fast tuning buttons
Tunes the receiver in one-megahertz increments up or down
the HF spectrum. The tuning will continue automatically if
either of the buttons is held pressed, and will wrap around
when at the end of its coverage.
(12) Memory mode select button
Switches the frequency display into memory mode and then
enables memory preview. Turning the main tuning knob
allows each memory to be viewed in turn. Pressing the
memory select button again returns the display to frequency
mode. Memory mode LED is illuminated when the frequency
display is showing memory information.
(14) Memory channel mode button
In memory mode, switches channel monitoring on and off.
This allows each memory to be reviewed, i.e. turning the
tuning knob to a memory number that has a programmed
frequency in the set will receive that frequency.
(15) Memory recall button
In memory mode, transfers a frequency from the selected
memory to the receiver tuning.
(16+11) Memory store buttons
In memory mode, transfers the current receiver frequency
and mode into the selected memory, over-writing any
previous content. Both store buttons must be pressed
simultaneously.
mode this control turns the

16 HF-250 User Manual HF250 User Manual 17
(17) RS232 socket
For connection to your computer for remote control, also a
pair of relay contacts for operating a tape recorder in
conjunction with the HF250 timer.
(18) 50Q / Whip antenna socket
For connection of antennae terminated with 50Q or 75Q
co-axial cable. The cable should be fitted with a PL-259 plug
to match this socket. Also used for mounting the telescopic
whip aerial when the WA-250 option is used.
(19) Antenna select switch
Determines which antenna socket is connected to the
receiver input. The Whip position will only operate if the
internal WA-250 amplifier is fitted to the receiver.
(20) 600 n antenna terminal
For connection of wire antennae not terminated with co-axial
cable. Connection Ground terminal connected to the case of
the receiver. A good earth connection will improve reception
and reduce interference, particularly with a long wire aerial.
(22) FM squelch level adjustment
Only present when the DU-250 detector option is fitted, this
sets the signal level at which the squelch circuit turns on the
audio output.
(23) Record output jack socket
Afixed level signal is available here that is unaffected by the
volume or tone control settings. The level is suitable for
feeding into the line input of most tape recorders and for
driving RTTY decoders.
(24) External loudspeaker jack socket
For connection of an external loudspeaker of 4 or 8 Q
impedance. Inserting a plug into this socket will disconnect
the internal loudspeaker.
(25) 12V DC power input socket
For connection of a suitable power source. Ensure correct
supply polarity (as marked on the panel) and that the voltage
does not exceed 15V.
(26) External Mute Control
For connection to a transmitters mute output, i.e. ground to
mute.
Operating the HF-250.
Volume and Tone Controls.
The volume and tone controls affect the level and quality of
the sound from the loudspeaker or fed to the headphones.
The signal from the record out socket on the rear of the
receiver is not altered by these controls._The tone control
can provide quite comprehensive audio filtering facilities.
When in its central position the response is flat, but when
turned to the left high frequencies are reduced and this can
be used to lessen the unpleasant whistles from interfering
stations. When turned to the right low frequencies are
reduced and the clarity of speech is often improved.
The volume control also functions as a power switch, and
turning it fully counter-clockwise will turn the receiver off,