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This model was introduced
in the early 40's to replace or augment the HRO in applications
demanding a more onerous duty than basic operational point-to-point
communications such as intercepting coded axis radio messages
during WWII.
The receiver uses octal valves,
mainly the metal cased varieties which are coded 6SG7 and the
like. The convention which is becoming less well known as time
passes is to add a "G" for the large glass bottle,
"GT" for the small parallel sided bottle and no letter
or sometimes an "M" for the metal clad types. Often
it isn't important which type is used but occasionally substitution
is a matter of physical fit and of course screening. It may not
be possible to use a "G" version without an associated
metal screening can because of positive feedback between the
unscreened anodes. IF coil trimmer capacitors or slugs may also
need to be tweaked to compensate for changes in stray capacity.
The two most frequently encountered
versions of AR88 are the "D" which covers 550KHz to
32MHz and the "LF" which included a Long Wave band,
excluded the Medium Waveband and had the rather unusual IF of
735KHz. My example is a sort of hybrid. At some time in it's
career an "LF" case has been fitted to the "D"
chassis and as the Identification Plate, which carries the details
including date of manufacture and serial number is fastened to
the rear of the case I can tell when the box was made but not
the receiver! If there's anyone out there with the "LF"
version in a "D" case I'd be interested in hearing
from them!!
My information is that this
receiver belonged to the same chap that owned the Racal RA17
which is also described nearby. The presence of the Racal chrome
strips is a clue as well! Interestingly, I heard the last owner
purchased a "new" AR88. These were advertised in the
late 60's for about £88 plus £2 carriage, but although
it may look very nice I bet it has to be looked at in respect
of decoupling capacitors and maybe some resistors as both these
type of components go off even when not used. This particular
receiver must have been relegated to a "shack" coffee
table as there are lots of teacup rings on one end.
The mains cable was supposed
to emerge at the rear, but no longer as it had been cut off.
I made some measurements and found it had been connected correctly
but nevertheless before plugging in the new cable, which I fitted
through a new grommet, to raw mains, I tried it on a Variac whilst
monitoring the heater line. This was an insurance policy as the
receiver was designed to work, not only from 240 volts but also
115 volts!
When I'd cranked it up I found
the heater line was 6.3 volts at the 230 volt setting and 6.5
volts at 240 volts, exactly to specification.
I identified the speaker terminals,
connected a speaker, found the aerial input terminal and connected
a length of wire and switched on. Some stations appeared on some
bands but some bands were quiet so there's some work to do. All
the decoupling capacitors have been replaced with Taiwanese ones
but some other capacitors including the 3000pF connected between
the 6K6 output valve anode and ground is original. Unfortunately
after a few minutes the receiver started making crackling noises
and the output transformer primary winding expired. The likely
candidate is that 3000 pF capacitor. I had a nice new Radiospares
"Universal Transformer" which I fitted and connected
up to replace the original which I left in-situ for two reasons;
there's some negative feedback circuitry associated with it and
there's a 600 ohm output winding for headphones. By connecting
the new transformer's loudspeaker winding across that of the
old transformer there's a load across the new transformer when
the external speaker is disconnected (a good thing) and the output
windings on the old transformer are energised, giving headphone
audio and the negative feedback connections. All this without
disturbing the original wiring of course, and there was loads
of room under the chassis for the new transformer. In fact there's
enough room under the chassis to hide away several modern receivers!
I'd already downloaded a schematic diagram from an American Amateur
Radio site and this proved indespensable in fitting the new output
transformer.
A rather strange floppy feel
to the tuning was sorted out by tightening the screws holding
the flywheel in place. What I'd thought was backlash hadn't been
apparent in tuning stations which gave me the clue to the fault.
Pity the S-meter mod hasn't
been carried out because I find that a meter is pretty important
for getting the feel of a set's sensitivity, especially when
the AVC works well as one must judge incoming signal strength
by signal to noise ratio and the "meatiness" of its
tuning. The original S-Meter is said to be a little flat, probably
because it has a 5mA movement.
Like the Racal RA17, servicing
of this model is not easy as, at nearly a hundredweight, a sprained
wrist can result from merely turning it upside down.
A little about the general construction:
the main tuning capacitor in the AR88 is concealed in a metal
box secured by thumbscrews and all the RF coils are under the
chassis accommodated in a substantial pair of metal boxes; one
for the oscillator coils and the other for the RF amplifier coils.
Short wave coils are wound on ceramic formers and all the trimmer
capacitors are ceramic tubular affairs requiring a adjusting
special tool. Although there are terry clips on the sides of
the main RF box for this tool, and presumably one for adjusting
the coils, these have long since disappeared.
Major wound components are all
housed in sealed metal boxes and include the mains transformer,
a pair of HT chokes and the output transformer. There are seven
IF transformers and a BFO coil each mounted in a metal case with
a detachable lid. Unfortunately to make adjustments to the lower
coils in these necessitates turning the receiver on its side..
not an easy task. Why so many IF transformer cans? This is because
the mixer and three IF amplifiers are "quadruple tuned".
This is done to shape the IF response curve giving a flat top
with nice high sides through the expedient of overcoupling. Because
some stages are tuned in this way you get double humps in the
response curve; other stages are peak-tuned thus filling in the
dip in the top. The end result is "high fidelity" reception
in the wider setting of the switched filter. As the receiver
is only a single superhet with a conventional 455KHz IF (see
"Useful Information" elsewhere on this site) a lot
of trouble needed to be taken to minimise image reception. This
is helped by the use of its two RF and three IF amplifiers. Sensitivity
averages around a microvolt for 0.5watt output, falling off slightly
in the highest ranges. There is also a clever noise limiter which
I haven't had chance to try out yet. |
