AR88D & LF

Several of these receivers have 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.

See more examples below
 

 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.

Below is an AR88LF. This differs from the AR88D in having a low frequency band and does not have coverage beyond 30MHz

This example is fitted with an S-meter

 

 Below are more examples to see at the virtual Radio Museum

 
 
 

See results of fault finding

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