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AR88 receiver

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AR88 receiver

Post by Hartley118 » Wed Jan 23, 2013 11:56 pm

I've been enjoying my AR88 for a couple of years now and appreciate its legendary sensitivity, reliability and audio quality.

However, it does have an intriguing problem and I'm currently summoning the willpower to risk slipped disc, hernia etc to take it down from its shelf and get inside there to deal with a niggle.

Performance is generally fine except on BBC Radio 5 live on 909kHz (Brookmans Park) which has always suffered an annoying heterodyne whistle. The scientist in me finds it rather satisfying that the 2nd harmonic of the 455kHz IF is 910kHz and, when bang on tune to the station, the heterodyne is about 1kHz. So I've kinda lived with the phenomenon, which is independent of input RF signal level.

However, I can't believe that the AR88 was really meant to be like this and am now thinking it's time to deal with the problem. I have a minimalist (lazy) approach to restoration and prefer to replace only the components which I find sufficiently defective to cause a performance problem. For example I turf out wax capacitors in critical grid coupling applications, but tend to leave them in decoupling positions where a few meg leakage is neither here nor there. The word 'recap' jars with me - especially in something as big as the AR88, but I do have a hunch that I may have a decoupling problem here.

Before getting deep into valve testing etc, I thought I'd ask:

Has anyone encountered this kind of IF heterodyne problem with the AR88, or any other set, and how did you fix it?

Martin

 
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Re: AR88 receiver

Post by turretslug » Thu Jan 24, 2013 12:51 am

Agreed- it does whistle on R5 909kHz, and so do many sets with 455kHz IF. Many people speak of the '88 as if it were the Holy Grail- it's not, it's a very good radio indeed but there are compromises. Whilst it has what were state-of-art RF/IF pentodes and excellent low loss materials (ceramic valve-holders and switch wafers, polystyrene coil-formers etc.), it is heavily productionized with simple mild steel chassis and minimal complication. As well as a tuning drive that has more in common with alarm clocks than laboratory equipment, there is minimal screening/isolation in the IF/detector section. This I find surprising- with all those IF amps (3x 6SG7) and double-tuned IFT''s, you'd think that there would be a bit more sectionalising, even the decoupling is "sufficient", rather than generous. Detectors are famed for IF harmonics (goes with the territory) and the AR88 has less screening around the 6H6 base than many much more modest contempararies. It would only have been a few more cent's worth of mild steel. Furthermore, whereas most receivers with multiple IF stages have them arranged in linear fashion, the '88 has them wrapping around each other, as if they had to be squeezed in after everything else had been placed. Odd.

I keep meaning, amidst everything else that goes on in life, to fold uip some plate-work and install around the detector/IF and see if it has any effect on the "909 whistle"- when i replaced the original "bath-tub" oil-filled capacitors, it left convenient chassis holes that would accomodate screening. I don't like butchering original metalwork!

Food for thought...

Colin.

 
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Location: Cambridge

Re: AR88 receiver

Post by Hartley118 » Thu Jan 24, 2013 11:32 am

Thanks for your reassurance Colin that the whistle on my set may be just typical of the design. I guess that IF signal is leaking back into the RF circuits.

Somewhere I have a screening box which I think was used on the naval version: it's supposed to fit round the RF/IF sections. I'll dig it out and try it, though it wouldn't be surprising if it made the whistle worse!

I was interested in your comments on the AR88 design. As far as I know, RCA intended it as a semi-consumer set to bring radio reception to the farms of 'Middle America' as well as its Ham applications. Then came war in Europe and massive UK government demand.....the rest is history! That may explain some of the economies, though its unnecessary weight doesn't really fit with that theory.

Cheers,

Martin

 
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Re: AR88 receiver

Post by turretslug » Thu Jan 24, 2013 11:59 am

IMHO, you're spot on as regards it being a semi-consumer product- it's a "posh radio" that got lucky (if WWII could ever be described as lucky!). I assume that US techies and accountants put their heads together in looking for an off-the-shelf mass-producible product that offered "bang for buck". I imagine that its weight simply reflected the construction techniques of the time and the desire to use stansard, cheap rolling-mill stock rather than resorting to lighter materials like the aluminium ear-marked for hundreds of thousands of aircraft or die-casting.

I'm interested in the remark about screening for a naval version- I'd heard that navies, in particular, were paranoid about super-hets and demanded very well suppressed and screened kit. I can see why- one local oscillator amongst thousands of others in a busy city would be inconspicuous but a lone signal in the midst of thousands of square miles of salty water is crying out for trouble.

There are many on the forum who have much more knowledge and experience than me in RF matters but I'm guessing that the whistle under discussion would be very difficult to get rid of completely. No harm in a bit of experimentation though, even failure represents learning curve!. For R5 in the workshop, I resort to various devices with other than 455kHz IF- thermionic in winter!

Colin.

 
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Re: AR88 receiver

Post by Terrykc » Thu Jan 24, 2013 12:10 pm

As the AR88 is of American design and manufacture, 455kHz would have been the obvious choice of IF.

455kHz is the standard American AM IF frequency and, without looking it up, I doubt if there are any AM transmissions on 910kHz anywhere in America!

The problem you have was first noticed en masse in this country when the far-eastern designed-for-America 'vest pocket' radios flooded the UK in the early 60s. However, the youngsters, with whom these sets mainly found favour, were rarely interested in listening to the BBC Home Service, as it was (now R4, of course) on 908kHz.

The answer is simple - by which I mean that it is easy to say or write! - move the IF frequency!

I can't comment on the practicalities, having never touched an AR88, but you might like to consider the following points:

How easy is it to adjust the IF transformers? It should be simple, provided that the adjusters are not of a type to seize up or disintegrate at the slightest touch.

The modern AM IF in Europe is 470kHz but that might be outside the range of available adjustment so a more modest change is called for. A 5kHz change to 460kHz will produce an 11kHz beat with 909kHz which will be way outside the IF response (before even considering audibility!)

Obviously, all the local oscillator frequencies will change by 5kHz as result so some minor RF realignment will be required for the 'perfect finish'.

I doubt you are the first to ponder this problem so, perhaps, a Google search might bring you into contact with others who have attempted this and any trials and tribulations experienced as a result? Unless we've got a few AR88 experts here, of course ...!

 
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Re: AR88 receiver

Post by turretslug » Thu Jan 24, 2013 12:34 pm

Terry, that's an option I did chew over when squeaking the IF strip with PM5326 wobbulator and easy enough to do but the AR88 has a crystal filter at a nominal 455kHz- only a simple single device but useful for HF CW and, on balance, what the set is about. A bit of a waste for R5!

Of course, one could get a new crystal cut for 460 (etc.) kHz but we're now getting into what I characterise as mission creep...,

Colin.

 
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Re: AR88 receiver

Post by Terrykc » Thu Jan 24, 2013 12:51 pm

turretslug wrote:...the AR88 has a crystal filter at a nominal 455kHz- only a simple single device but useful for HF CW ...

Oh dear! Well, I did say that I'd never worked on an AR88, Colin!

 
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Re: AR88 receiver

Post by Hartley118 » Thu Jan 24, 2013 4:29 pm

Thanks for the interesting information. I'm delighted to know that my R5 heterodyne is an interesting technical feature and not a fault. Might try a bit of shielding round that 6H6 though - as you say Colin, we'd expect that to be the stage where the harmonics are mainly generated.

As for changing the IF, don't think I'm sufficiently geeky for all that technical fun, games and risk. And I have plenty of other radios for R5......only 30 miles from Brookmans.....Rather a waste of an AR88's sensitivity!

Martin

 
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Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: AR88 receiver

Post by Terrykc » Thu Jan 24, 2013 10:49 pm

Hartley118 wrote:... As for changing the IF, don't think I'm sufficiently geeky for all that technical fun, games and risk ...

Quite a simple operation, Martin - or at least it was until Colin threw a (crystal) spanner in the works ...!

 
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Re: AR88 receiver

Post by turretslug » Fri Jan 25, 2013 10:29 am

Is that more or less useful than a chocolate spanner? :=D

Yep, shame about that rock, otherwise a slight shift of IF to quench the whine would make for a simple and worthwhile outcome. Willing minds meet immoveable (quartz) object. Always very satisfying when a theoretical consideration becomes a practical "in the flesh" solution and, I find, really helps fix what's going on in the grey matter.

Not quite so daunting or "geeky", really- with a more typical, basic, fully tuneable IF strip, you'd reach for the trusty, basic workshop generator (from the likes of Advance etc.) that had a few decades under its belt and a few bumps and thumps en route, you'd set it to 455kHz and twiddle along the line- it might actually be producing 453kHz or 458kHz or something thereabouts. By time you'd finished, the radio would be working fine and you'd have shifted the IF without even knowing it (certainly, as Terry says, once you'd done the job properly with front end as well and got the LO to get the tuning scale spot on.). Unless, of course, you'd been unlucky and brought a beat into audibility on a favourite station...


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