AR88D
This receiver has recently joined the collection. I've wanted one of
these for donkey's years and saw one recently for sale at a realistic price.
If a receiver is sold "as seen" it is usually worth a gamble as
the price is often more affordable.
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.
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