I I
AR 1,11
This receiver is still fairly widely
available on the secondhand market
from near -mint condition to sheer
grot - depending upon the previous
owner. For a modest sum (about
£50) an AR88 can be acquired in
excellent condition.
You should aim for one which has
not been modified. The construction
of the receiver is of a very high
standard and unless any modifications
have been executed to a similar
standard and properly documented
they are likely to detract from its
performance and reliability. The
AR88 is rather like the VW Beetle - it
tends to go on for ever!
The AR88 has a long history,
starting life in the late 30s as a
general coverage receiver but
intended also for amateur use (hence
the AR). This version is not often
seen on the market; it covers the
range 550kHz to 32MHz and is fitted
with an S meter. However do not
confuse it with one which has had an
S meter added!
The 1939-45 war and the need for
reliable communication receivers
resulted in large-scale production of
26
First produced in
the 1930s, the
RCA AR88 still
commands a big
respect from
amateurs. Full
description by Bob
Henly, C.Eng.,
MIERE, G31HR.
the AR88 in various forms including
a CR series (not to be confused with
the Marconi CR 100, etc). Two
models are generally available now.
These are the AR88LF and the AR88D
The main difference between these
two models is that the AR88LF has a
lower intermediate frequency and a
low frequency tuning range 75 to
150kHz. The AR88D is better suited
to amateur use and this article will
therefore concentrate on that model.
First, let us see what it is. The
AR88 is a single -conversion super -het
with a total complement of 14 valves
(those ancient glass things referred
to in a recent article). The receiver is
shown in block -schematic form in
fig. 1.
It comprises two tuned RF
stages and 3 IF stages. The two
tuned RF stages are essential in view
of the relatively low intermediate
frequency of 455kHz in order to
reduce its response to second -channel
interference. The coverage is
continuous from 530kHz to 32MHz in
six over -lapping ranges.
The two RF amplifier stages use
6SG7 valves and are designed to
provide only sufficient gain ahead of
mixer to give the required signal-to-
noise ratio. The principle purpose of
the RF stages is to provide selectivity
ahead of the mixer in order to
reduce second -channel responses.
This is its main weakness; image
rejection of 500kHz is about 120dB
but at 30MHz it falls to about 40dB.
Fig. 2 shows the manufacturer's
performance figures for sensitivity
and image rejection. Although
current practice would probably use
much less gain ahead of the mixer in
the interest of cross -modulation
performance the AR88 in fact acquits
itself very well when compared with
many modern equipments on this
score.
The mixer is a 6SA7 and the local
oscillator is a 6J5. The supply to the
local oscillator and the BFO is
stabilised at 150 volts by a VR150/30.
The Intermediate Frequency (IF)
stages comprise a single crystal
bridge filter followed by a three -stage
tuned amplifier using 6SG7s. The
inter -stage coupling circuits between
first and second, and between
second and third stages, each use
four tuned circuits. These have
variable coupling links which together
with similar links in the crystal filter
circuit enable IF bandwidths between
400Hz and 6kHz to be switch -selected.
In addition, two further positions of
the selectivity switch give 'fidelity'
bandwidths. The IF bandwidths are
summarised in fig. 3.
The second detector is of the diode
envelope type using half of a 6H6.
The second half of this valve provides
carrier -derived AVC which is applied
to the RF stages and the first and
second IF stages. AVC is selected by
a front -panel switch and a novel
feature is that the RF gain control
which also operates on the RF stages
and the first two IF stages alters the
AVC delay.
A 6J5 is used as a Beat -Frequency
Oscillator (BFO) for the reception of
CW. The anode supply is stablished
at 150 volts. The frequency is
adjustable from the front panel over
a range of approximately +/- 3kHz
about the IF. Output from the BFO is
injected at the grid of the third IF
amplifier ensuring a good injection
level. A second 6H6 is used as a
peak noise limiter with the clipping
level adjustable from the front panel.
Audio output is provided by a two -
stage amplifier comprising a 6SJ7
followed by a 6K6 output stage
which is designed to deliver 2.5
watts into either a 2.5 ohm or 600
ohm load. The front panel jack
provides approximately 10mW into
20,000 ohms from an additional
winding on the output transformer.
The integral power supply - which
contributes a large part of the weight
- operates from either 110 or 230
volt AC and uses a 5Y3 rectifier. A
socket on the rear apron is provided
for operation from external power
supplies.
Both rack and table mounted
versions can be found. It is built like
the proverbial battle ship; overall
dimensions are 19.25in x 11.0in x
19.25in deep and it weighs about
80 lb. Both mechanically and
electronically it is very stable and
although its physical size may be
rather daunting it represents a very
good buy.
The panel layout and location of
the controls is shown in fig. 4. The
central feature is the tuning control
and the two tuning dials. The main
1st & 2nd
RF Amplifier
Antenna
Frequency
changer
Bandpass
Crystal
Filter
Local
oscillator
Fig. 1
Sensitivity
Band
No. Megacycles
1
2
3
4
5
6
I -F rejection at 600 kc is 100,000.
.6
1.0
1.5
1.7
3.0
4.3 .6
4.6
8.0
11.5
12.1
16.4
16.4
22.5
22.5 2.5
28.0
in Microvolts
for 0.5 watt
.5
.9
1.0
.6
.6
.8
.8
.7
1.2
.7
1.3
.8
1.2
Fig. 2
Performance data
dial on the left comprises seven
concentric scales: one for each range
and an inner logging scale. Calibration
is in MHz except on range 1 where it
is in kHz. The logging scale comprises
a number of numbered segments
each representing one revolution of
the vernier scale. The vernier dial
occupies the centre of the panel and
has a single scale graduated from 0
to 100. The two dials are coupled
together, to the tuning control and to
the main tuning capacitor by a train
of split gears which result in a very
smooth 'feel' with zero discernible
'backlash'. This is probably one of
the most attractive features of this
receiver and is one item which
requires careful inspection before
purchase - of which more anon.
How well does it perform under
today's conditions? The answer (in
the author's opinion) is: quite well.
As a CW receiver it is superb and
the crystal filter is quite adequate for
most situations. The addition of an
audio filter such as Datong FL1 or
FL2 more than compensates for any
inadequacy. Its performance in the
1st & 2nd
IF Amplifier
AVC
3rd
IF
2nd Det
AVC
BFO
260 v
150 v
6.3 v
ac
Receiver diagram
Antenna Input in
Microvolts for
6 DB Signal-
Noise Ratio
.9
1.4
2.2
1.0
.95
.9
1.3
1.2
1.1
1.3
1.2
1.3
1.4
1.5
1.3
presence of strong adjacent -channel
signals is very good - 7MHz is a
good test of any receiver and the
AR88 can certainly hold its own. Its
main weakness is on SSB. This
weakness is due to three main
things. Firstly the tuning rate on all
amateur bands is a little too high to
make the initial acquisition of a SSB
signal easy. Secondly the IF selectivity
curve is not sufficiently steep -sided
to resolve SSB to its best advantage.
Thirdly the BFO is not preset for
upper and lower side -band. The first
is overcome to a great extent by
practice and some owners have
fitted an additional, out -board
epicyclic drive to the main tuning
control to slow it down. The IF
response can be modified a little by
careful alignment (of which more
later) or the filter can be replaced by
a more suitable type, but this is
major surgery. As far as the BFO is
concerned a variable control is
invaluable for CW and therefore it is
necessary to determine the two
settings for upper and lower sideband
respectively with a little patience and
record them for future use by two
discrete marks on the front panel.
AF Gain
Power
supply
Antenna Input in
Microvolts for
20 DB Signal-
Noise Ratio
4.6
8.0
12.0
5.0
4.8
4.5 14.5x103
8.0
6.8
6.0
6.6
7.0
7.0
8.0
8.0
7.0
A
RF Gain
Tone control
Mains
[...
input
Image
Rate
>106
106
2.4x105
6x10°
2x103
4x103
1.5x103
103
400
400
200
27
TIIH
AR 88
In general, sensitivity is adequate
but the signal-to-noise ratio on the
HF bands, ie, above 15MHz could be
better. This is generally improved by
the use of a low -noise pre -amplifier
ahead of the receiver. A tuned pre-
amplifier or even an antenna tuning
unit will improve image rejection
above 15MHz.
Although the electrical and
mechanical design is inherently
stable there is often a problem with
frequency stability and re-setability
above 15MHz. This derives from
several causes - mainly a direct
result of the age of the specimens
available. Firstly some 6J5s tend to
be microphonic and careful selection
is necessary. Thumping the front
panel is a good test and tapping the
valve envelope. On a secondhand
receiver of this vintage there are
often bandswitch problems. Generally
this is not due to wear or damage
but simply dirt and grease which has
accumulated over the years. It can be
solved by the careful application of
switch cleaner. This involves the
removal of the coil -pack covers and a
large number of nuts. Do make sure
that you replace every one and when
you gaze on the beautifully
constructed coil do resist the
temptation to touch it any more than
is necessary!
Finally there is instability caused
by ageing of components which can
cause both frequency instability and
gain instability. The worst offenders
are capacitors used for decoupling.
The capacitors used in the AR88 for
this purpose are either moulded
mica for small values or oil -filled
paper for the larger values and both
have a very good reliability record
approach this problem with care! If it
proves necessary to replace any
moulded mica capacitors in the RF,
LO or IF sections then use only
polystyrene of a suitable value and
working voltage. The author
discovered in his receiver that a
previous owner had replaced all RF
and IF coupling capacitors with Hi -K
Disc ceramics; this included
decoupling capacitors in the Local
Oscillator and the BFO. Their
replacement with polystyrene
capacitors made a dramatic
improvement to Local Oscillator drift.
So far no open -circuit resistors
have been found but if gain
signal-to-noise ratio is thought to be
28
or
Above: With the
top casing removed,
this is what you
see. Right:
Diagrammatic
version of the front
panel, showing all
controls, what they
do. The panel
should be
unscratched and
_.,
clean if the price is
around the upper
limit. Make sure the
tuning drive gears
are not worn or
distorted. Below: IF
II
bandwidths for all
switch positions.
below par then it pays to check
screen -grid decoupling resistors/
capacitors - assuming that the valve
concerned has first been tested.
Design of the RF and IF amplifiers is
very conservative and results in
stable performance. If instability is
experienced when aligning the
receiver then the relevant circuit
components should be carefully
so
checked.
Finally, in purchasing an AR88D,
examine carefully the physical state
of the equipment. In the upper end
of the price range the appearance
should be clean and unscratched
with no sign of modification. Check
that the tuning drive gears are not
worn or the assembly distorted.
There should be evidence of grease
on the gears and the drive should be
smooth and positive with no 'slop'
or backlash. Ideally it should come
complete with a handbook, two
trimming tools and an Allen key
clipped inside the cabinet. In many
cases it may have the original
packing crate and a spare set of
valves.
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SELECTIVITY
BANDWIDTH KHz
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14
7.0
3.0
1.5
0.4
NOISE LIMITER
(-\
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SRO
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ARO A vC
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60dB
22
16
11
10.4
8.4
We shall now look at the alignment
of the AR88. In my experience
recalibration is necessary from time
to time but the IF alignment
deteriorates very slowly. Do not
undertake realignment unless you
have the time and patience to do a
complete and careful job; tweaking
up is not recommended!
Ideally an oscilloscope and a
sweep generator is required to
achieve the very best results. However,
with a little care it is possible to
c
c
C.,
SIGNAL
TYPE
AM
CW
SSB
FM
Hook, to engage
in trimmer shaft
Locking
ring
MODE
SWITCH
REC. MOD
REC. CW
REC. MOD
------
OR AVC-NL
MANUAL
MANUAL-NL
-AS FOR CW SELECTIVITY POSITION 3-
MANUAL-NL
USE SELECTIVITY POSITION 3 OR 4
1
achieve very good results using a
high impedance voltmeter and a
stable signal source -
I use my BC221
frequency meter. A signal generator
is preferable however for the RF
alignment, provided that its calibration
is reasonably accurate and it is
stable. Beg or borrow one if necessary!
Beware though some of the cheaper
signal generators on the market have
inaccurate calibration and are
unstable even at 455kHz! It is
essential that, with the signal source
used for the IF amplifier alignment,
one can estimate +/- 7kHz from the
central Intermediate Frequency.
Alignment must commence with
the IF amplifier. Although the
handbook gives the intermediate
frequency as 455kHz this is the
nominal value. The value which we
shall use is the actual value of the
receiver's crystal filter which will
probably differ from the nominal
value by a small amount.
Connect a high impendance
voltmeter of the analogue type (eg:
AVO8 or similar 20000 ohms/volt) on
its 5 volt range to terminal 5 on the
rear terminal strip which carries the
loudspeaker connections; the
AVC
SWITCH
AVC
OR
MANUAL
OR
Trimming
tool
Trimmer
shaft
RF
GAIN
CLOCKWISE
AS
REQUIRED
AS
REQUIRED
I
REQUIRED
CLOCKWISE
1
I
Above: This is a
general guide to
the control settings
on AM, CW, SSB,
and FM signals.
There is, naturally,
more information in
the original manual
- if you can obtain
one. They are
almost as valuable
and rare as the
equipment itself!
Left: Details of the
trimmer and the
tool with which it
can be adjusted. A
well -kept AR88
should come
supplied with two
trimmer tools and
an Allen key inside
the cabinet.
positive lead of the meter being
connected to chassis. Connect the
signal generator via a 0.01µF capacitor
to the frequency changer signal grid
(6SA7 pin). Select Selectivity switch
position 5, mode switch 'REC MOD'
and AVC switch to AVC (fully
clockwise). Sweep the signal
generator either side of 455kHz
looking for maximum reading on the
voltmeter. The signal generator (unmodulated) level should be reduced
if necessary to keep the volt meter
reading around quarter -scale and to
avoid overloading the receiver. The
frequency at which the maximum
voltmeter reading is obtained should
be noted; it should be within a kHz
of the nominal IF.
In the extreme case, where no
output at all can be detected, it will
be necessary to roughly align all the
IF transformers to the nominal
455kHz by injecting a signal at each
IF grid starting with the third and
working back towards the mixer grid
with the selectivity switch in position
2. At each stage the IF transformers
are adjusted for maximum reading
on the voltmeter. However this is an
extremely unlikely circumstance and
AF
GAIN
AS
C 'W
one should look for faulty valves or
components before making any
adjustments.
To proceed with the alignment
assuming that all is well and we
have the signal generator set to the
frequency of the crystal filter as
above. Set the selectivity switch to
position 2 and the crystal phasing
(C75) to its mid -way position. Now
adjust all IF transformers for maximum
reading on the voltmeter working
from the detector back towards the
frequency changer; reducing the
signal generator level where
necessary to avoid overload. Now
select selectivity switch position 3;
set to the signal generator frequency
7kHz above the crystal frequency and
adjust the crystal phasing (C75) for
minimum meter reading. All that
now remains to be done is the
adjustment of the crystal load circuit.
The purpose of this adjustment is
adjust the symmetry of the IF
response about the centre frequency
nd it is very difficult to carry out with
the equipment we are using here.
Ideally we need a sweep generator
and an oscilloscope. The following
procedure can be used only with
patience, otherwise leave the
adjustments as they are. First tune
the signal generator to approx
1.2MHz; set the receiver band -switch
to band 1 and tune for maximum
deflection on the voltmeter, reducing
the generator output as necessary to
avoid overload. Select position 3 on
the selectivity switch and note the
two vernier logging scale readings
for which the voltmeter reading falls
to half the maximum value. Adjust
L34 to obtain as near equal distances
on the logging scale for these two
readings either side of the setting
which gives maximum reading. This
procedure is repeated for selectivity
switch positions 4 and 5 adjusting
L81 and C80 respectively. Note that
the dial readings and the adjustments
will become progressively smaller
and more critical.
All that now remains is to set up
the BFO. Using the above set-up,
tune the receiver for maximum
reading on the voltmeter. Set the
BFO pitch control so that the pointer
is at the 12 o'clock position (check
that the variable capacitor coupled to
this knob is at half -mesh - if not then
slacken the knob and adjust it). Set
the mode switch to 'REC CW' and
adjust L22 until the whistle that is
heard falls to zero -beat. The BFO
should now give approximately 3kHz
variation in pitch either side of zero beat. Return the mode switch to 'REC
MOD'.
Alignment and calibration of the
RF section is quite straightforward
but requires even more patience! It
is possible to achieve very close
agreement with the dial calibration
over the whole of each range even
though only a two -point tracking
29
system is used (this is due to the
design of the ganged tuning
capacitor). The accuracy to which the
dial can be set is only about +/-
5kHz but this is probably superior to
most signal generators with an
analogue readout. If a frequency
meter of the BC221 class is available
- or even a 100/1000kHz
crystal
calibrator then this can be used to
put the finishing touches to the
calibration but the initial calibration
and alignment should be done with a
signal generator. The reason for this
is the poor image rejection at
frequencies above 10MHz -
particularly when the receiver is not
correctly aligned - which could lead
to the receiver being aligned to the
wrong signal.
It will be found necessary to
repeat each sequence several times
before optimum calibration or
performance is obtained. The
procedure is as follows: -
In all cases use the signal generator
with modulated output since this
makes it easier to identify. The
output level should be set to produce
a one or two -volt reading on the
voltmeter and should be reduced
where necessary to avoid overloading.
Set the generator frequency to the
high frequency for the band concerned
and tune the receiver to find it.
Adjust the appropriate trimmer to
bring the signal near to the correct
dial reading. Now set the generator
to the low frequency and adjust the
appropriate trimmer to correct the
calibration. Repeat this procedure
until the two signals appear at their
correct dial readings. The trimmers
concerned should be adjusted using
the tools provided with the receiver.
At the low frequency end this
involves the core of an inductor and
an insulated screwdriver blade is
suitable. At the high frequency end,
trimmer capacitors are involved
which are of an unusual tubular
construction. The AR88 tool provided
for this has a box -spanner at one
end to release the friction lock and at
the other end there is a hook. After
releasing the locking nut so that the
central part of the capacitor will just
move this central part is adjusted by
sliding it in or out by engaging the
hook on the tool in the shaft end -
see fig. 8. Remember to lock the
trimmer when adjustment is
complete.
The RF amplifier is adjusted in a
similar way. The antenna trimmer on
the front panel should be initially set
to the 12 o'clock position. With each
adjustment tune for maximum
voltmeter reading repeating the
adjustments until no further
improvement is obtained. As each
stage comes into alignment the
generator output should be reduced
to avoid overload; the final output
Above: The AR88
with its rear panel
removed. Right: A
simple antenna
tuning unit for the
frequency range
1.5MHz to 30MHz.
500pt variable
CI
capacitor.
35 turns 18
1.1
SWG enamelled
copper wire on 2
inch former (eg a
length of plastic
drain pipe) tapped
at every 3 turns.
12 way
SW1
rotary switch eg
R.S. heavy duty.
To
receiver
level should be around a microvolt
or less.
A final check should be made by
setting the signal generator to a
frequency in the middle of each band
and checking that it appears in the
right place on the dial. Adjust the
antenna trimmer for maximum
output and check the level of the
image signal by tuning the generator
to a frequency which is 910kHz (ie, 2
x IF) above the frequency to which
the receiver is tuned. If the generator
has a calibrated attenuator you can
estimate the image rejection ratio by
adjusting the generator output until
you obtain the same reading on the
voltmeter as you had at the correct
response frequency. The ratio of the
two readings is the image rejection
ratio.
The receiver is now aligned and
ready for general use. One final
touch whilst we have the test gear to
hand is to note the logging scale
readings for each of the amateur
bands - including the three new
ones - using a crystal calibrator if
you have one.
Finally let us look at the operation
of this receiver. It is not intended to
give a blow-by-blow description but
rather to concentrate on how to use
Ll
To
Antenna
it to advantage particularly with the
two modes for which it was not
specifically designed. It was originally
designed for reception of telegraphy
(CW) and amplitude modulated
telephony. With a little understanding
it can be used effectively for SSB
and narrow -band FM also.
The mode switch in the bottom
left-hand corner of the front panel
has four positions; 'OFF' removes
the mains supply completely. In
'TRANS' the HT supply is removed
putting the receiver in a standby
mode. In this position terminals 3
and 4 on the loudspeaker terminal
strip are shorted together and can be
used to energise an external relay for
transmitter operation. In 'REC MOD'
the receiver will receive modulated
transmission whilst in 'REC CW' the
BFO is energised for CW reception.
The switch in the corresponding
right-hand position controls the AVC
and Noise -Limiter and is self-
explanatory. The noise limiter
threshold is set by the control above
the BFO pitch control. There is a
combination of these controls which
should be used as a general guide
for reception of the various modes
but which may well be modified by
conditions prevalent at the time. Let
us now look at some of these in
30
more detail: -
First the reception of CW, ie,
telegraphy. Here we have in effect a
single frequency and all we have to
do if there is no interference is to
tune for maximum signal and set the
BFO pitch control to give the required
beat -note in the headphones or
loudspeaker. In general the receiver
is used with the AVC off so that the
BFO injection does not reduce the
receiver sensitivity. The AF gain is
set fairly high and the receiver gain
is controlled by the RF gain control.
Of course this ideal situation rarely
exists; there is usually an abundance
of adjacent -channel interference and
this is where we learn to 'drive' the
receiver. Clearly we have a case for
using the crystal filter and the
technique is best practiced on a
strong signal such as one of the
broad -cast stations in the 7MHz
amateur band. Switch the selectivity
switch to position 3 and the BFO
pitch to one side of the zero position
- say to 1 o'clock.
As you tune through the signal
note how the strength of the beat -note
is much stronger on one side of
zero -beat than on the other. Shift the
BFO pitch control to say 11 o'clock
and note how the position is reversed.
Put the BFO in the zero position and
the beat -note strength is the same
on both sides of the zero -beat
position. Repeat the exercise with
the selectivity switch in positions 4
This is the view you get if you removed
the bottom housing from the AR88. The
inner casing on the right houses the
oscillator coil pack, by the way.
and 5 and note how very much more
pronounced the effect is.
We make use of this effect in
receiving CW in two ways .
Firstly, if
possible we tune the wanted signal
so that it sits at the top of the IF
response curve - with the BFO on
one side or the other of zero - and
unwanted signals down the side of
the response and therefore attenuated.
If the unwanted signals are very close
to the wanted signal then we can do
one or two things; both using
maximum selectivity. We can adjust
the BFO to the other side of zero -beat
so as to make the interfering signal
differ more greatly in beatnote, or
adjust the BFO pitch to reduce it to
zero -beat. Alternative we can adjust
the receiver tuning to place the
offending signal further down the
side of the IF response at some
sacrifice in the strength of the
wanted signal. All this may sound
difficult but it becomes quite easy
with practice.
Now let us look at SSB. In Selectivity
position 4 it is possible to select each
sideband of an amplitude modulated
signal - and again there are an
abundance of broadcast intruders in
the 7MHz amateur band on whom to
practice. If we tune carefully across
an AM signal with the BFO and AVC
off, two points, one on either side,
should be found where the signal
becomes unintelligible. At these
points we are receiving a single
sideband only and the carrier is
positioned down one side of the IF
response curve. Now switch on the
BFO; a beat note will be heard. As
the beat -note is adjusted for zero -beat
the signal should become increasingly
more intelligible. Note the position
the BFO pitch control and repeat
with the other sideband. The two
positions of the pitch control will be
roughly correct for receiving amateur
SSB and can be 'refined' by first
tuning AM SSB signal for maximum
signal, then adjusting the pitch
control to make the signal
intelligible.
The reception of narrow -band FM,
which is becoming quite popular at
the HF end of the 28MHz amateur
band, is also quite simple - and
there are always plenty of CB
stations on 27MHz to practice on!
The technique, known as slope
detection, uses the IF response curve
to convert the frequency modulated
signal into an amplitude modulated
signal and in doing so there is a loss
in signal strength, but this is rarely
appreciable. The FM signal is tuned
using selectivity position 3 or 4 so
that the centre of the signal lies
down one side of the IF response
curve. Frequency modulation
produces variations about this
central frequency which in turn move
the signal up and down the 'slope' of
the IF response resulting in an AM
signal to the detector. The above
techniques are shown graphically in
fig. 10.
Well, that was a much -abbreviated
run through on how to drive the
AR88; the key -word here is practice.
The receiver will acquit itself well,
even with a piece of wire on the
floor for an aerial - but of course it
deserves better. The aerial input
circuit is designed to match into an
impedance of 200 ohms but I have
found it will work very well with a 70
ohm coax feeder. A random wire
antenna coupled via a tuning unit
(for example fig. 11) will give very
good results and the tuning unit will
help also to reduce image interference
on the 21 and 28MHz bands.
Being a general coverage receiver
it comes ready equipped for the
three new amateur bands of course.
Buy an AR88 and enjoy a real
receiver.
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