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Old 2nd Jun 2012, 7:06 pm   #1
Radio Wrangler
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Default Two AR88s

When a tiny sprog in the 1960's I saved diligently for years. I wanted an AR88 like my schoolfriend's father had. Shortwave mag and PW had glowing articles on them.

The one I bought £33/10/- in then money was odd. Its tuning scale was all yellow with black writing, it didn't have alternating bands in reverse video so to speak. I wasn't sure about this. The panel lettering didn't put the frequency bands in megacycles by the band selector switch. Had I got some lesser model? It did have a real RCA S-meter calibrated in dB above one microvolt, which was good.

It turns out I'd bought a very early AR88, predating the AR88D and AR88LF models. I only found this out recently. The front panel was engraved, not silk-screened.

It's not been under power since the 1980s and all components are originals.

While in storage at my parent's, something got dropped on it and the shaft of the tone control pot sheared off at the groove for its circlip. It's seen life and the brass screw tops on the IF trimmers are chewed up from a previous owner. Various other trimmers have suffered from the attention of pliers, not proper tools.

The bandswitch detents are imperceptible.

I know, I know, this is a deserving case for a proper rebuild. I learned a lot from this set. I've owned it far longer than RCA, the British government and everyone else all put together. It is part of me.

OK, I've got me a winter project.

Today, at the Livingston ARS mini convention, I bought a second AR88. An AR88D and a jolly handsome beast. In accordance with Murphy's law, the opening button (a switch) on the Range Rover chose today to die, so the radio had to get loaded onto the folded rear seat via a rear door (took a few people to do it safely)

My problem is that this new (to me) AR88D is a fine looker, and I'm feeling guilty about using it for parts. The old AR88, my long-time companion has a very early serial number, about the fourth one known to currently exist. I think the old set is the one to restore. Am I right?

I intend doing a complete paper capacitor replacement, whatever way I go. If the wiring is flaky I may make a pin-board and make a new harness in kynar-insulated wire. I'd keep the big block smoother if either is good, and I'd go for re-stuffing the bathtubs. I intend to put low noise high stability metal film resistors throughout, for reliability and to have the old thing at it's best.

I intend doing a much more legible circuit diagram, over several pages, to make life more fun than trying to read RCA's demented spider version.

I'm a very happy camper indeed. 45 quid got me a set in very good condition, complete with both of the rare screens (tuning capacitor cover, and overall RF section top cover) and also the trimming tools. And an original manual.


Other booty from the rally was equally good: 0-35v 5A HP power supply (25 quid) and an HP 246 noise source for another 25 quid. I designed the replacements for the 346 noise source family, so I sort of wanted one for old time's sake. I have a proto of the new smart noise source.

A couple of stalls had 5 old radios A couple were old baseboard jobs, one tall cathedral cabinet job needing a lot of work and a couple of woodies. Would have gone for pennies if anyone interested had been there.

Me I'm smiling fit to bust and I've got this https://www.vintage-radio.net/images...2/censored.gif heavy AR88 still to get out of the car.

Eeee a grand day out.

David
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Old 2nd Jun 2012, 7:50 pm   #2
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Default Re: Two AR88s

I would try to find another set and use that to make 2 good ones from your old one and new purchase.
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Old 2nd Jun 2012, 8:48 pm   #3
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Default Re: Two AR88s

Good luck with these fine old beasts- I wouldn't like to make the choice, either!

I'm stumped as to why '88s turn up without these screens- what thought process do people go through not to refit them? I might understand it if they were made of platinum... Same with the trimmers/Allen key, not clipping them back in place after working on the set is simply sloppy.

I wouldn't trust the old smoothing block- on the one currently in my posession, I fitted a 150mA A/S fuse between rectifier cathode and reservoir, even though I'd fitted new 4.7uF 630V polyesters in the old can- plenty of room. Shame to have that nice tranny exit left. I found that, having punched out the two little retaining rivets, dunking the block in boiling water for a few minutes meant that the inner slid out of the can quite cleanly and easily. The original solder seam had cracked, letting much of the oil out all over the bottom of the case- at least this meant it wouldn't go pop when subjected to boiling.

Look forward to a few more AR88 posts, Colin.
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Old 2nd Jun 2012, 11:20 pm   #4
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Default Re: Two AR88s

Hi Colin,

I solved the car problem by pulling it out of the case to save weight, using a stiff pair of leather gauntlets and lifting the chassis out myself. I've had a bit more serious look at it in the house.

Well, the reason for sale is clear. Someone's given the old set a fair amount of cleaning work and has had a go at the gearbox. I thought the tuning knob shaft had been bent, but the its bearings are loose, the inner end of the shaft can wander three-dimensionally.

The tuning and logging discs are contacting, the flywheel is a bit loose.

The phantom screwdriver twiddler of old Glasgow town has been on the loose. No problem for me. The gears look OK. It has the later and less robust end-stop/tuning scale gear but they appear intact. All the bits I'm interested in look good. No oil or gunge out of the bathtub seals. Certain well-known resistors have been hot.

So, I think I rebuild my original one, it's more historic. I'll strip it to a bare chassis, clean everything properly and start reassembly. The wiring harness doesn't look too bad a task to remake. Both units have brittle insulation, so Kynar seems the way to go.

If I'm going so far, I ought to take out all the known weaknesses, so 4.7u 630v polyester jobs for the main block reservoir sounds good. If the can doesn't come apart well, I can always cut some aluminium plate and weld-up a new can with a slide-in capacitor support frame, so that the mounting screws lock both together.

It's a lot of work, but worth it.

David
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Old 3rd Jun 2012, 3:11 pm   #5
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Default Re: Two AR88s

I bought an AR88. worked well for years Then the output transformer failed.
Took a while to find a replacement as Colomor had just sold their last ones but through this forum I did find one.
In the meantime someone offered me a D and an LF for spares. Both turned out ot be too good to break, in spite of the D falling off its trolley into the garden pond it still works.
One has a non original output transformer and the other has non original smoothing caps.
So should I look for another for spares or look for a space in my local care home?
Don m5aky
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Old 3rd Jun 2012, 4:50 pm   #6
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Default Re: Two AR88s

You could all take a leaf out of the book of Onewatt (Chris), another member on this forum - Collection currently holds 19 complete AR88s.... I think there are a few scrappers as well

I might still have a few spares here, I donated most of the remainder to Bletchley Park.

Personally I wouldn't scrap any feasable receiver.

Sean
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Old 4th Jun 2012, 9:22 am   #7
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Default Re: Two AR88s

Nineteen?! That's a ton. In two stacked rows, he'd have a pretty good storage heater.
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Old 4th Jun 2012, 9:31 pm   #8
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Default Re: Two AR88s

David, I quite agree regarding doing things well here, these are solid sets (literally and metaphorically) built to a good commercial standard and leading-edge in their day. Even when using modern components, making a good job of things respects the aspiration of the original designer(s). I hate seeing professional kit with half-baked, sloppy and even dangerous codges, At least the hobby has matured and moved on from the days of the "Make your AR88 Natty with Nuvistors/Make your R390 Marvellous with MOSFETS"-type vandalistic gibberish that circulated when this "just make it good, we'll fret about the price later" kit was more plentiful and less respected.

My present AR88 was one of the best cosmetic examples I'd seen, with the smoothest gearbox and apparently electrically original- I'm hanging onto it! I didn't worry too much about the puddle under the rear apron, I expected to do at least some work on any set of this age. It seems to be a bit of a mongrel, with engraved front panel but "wasp" tuning scale, the panel is finished in olive drab with a very fine, even crackle finish, rather than the more common, coarser, black crackle. I'll admit I know next to nothing about what seems to be quite a complex subject, variant-tracking.

The cracked solder seam on the filter block meant that the terminal panel prised away easily, I recovered the hollow screws and their insulating bushes. I folded up a mounting bracket for the new capacitors, with a bottom panel drilled to replicate the "widescreen dice" pattern of the original 5 connections and the 4 holes of the original outer case. I tend towards pragmatism, rather than exacting originality but this above-chassis component is such a characteristic feature of the original that I'm glad I bothered.

The wiring is in pretty good nick, I left it when re-capping, etc., throughout but I ought to rewire, really. One day...

Colin.
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Old 5th Jun 2012, 1:27 am   #9
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Default Re: Two AR88s

Hi Colin,

I understand the people saying they wouldn't use a viable set for parts, but I sought one for that purpose and it was quite a promising one which turned up. Th gearbox in it need rebuilding. It seems to have been apart recently, the gearbox that is, and there's been a lot of effort in cleaning the chassis without removing anything. If i took up the earlier suggestion and went looking for a third set as donor for fixing sets 1 and 2, I'd also soon wind up with 19 or more.

No, the one I've had for all those years is rather special to me, and I want to get it back to full as-new performance. There is a serial number database and mine was the fourth earliest, last time I looked. The AR88s (not D not LF just plain AR88) were earlier than the bulk of them, and were equipped with S meters, not intended for those triple diversity racks. What were they used for? Development for the systems that came later?

I'm going to redo the early set properly, and then I'll be able to pass on quite a few spares for others... though I may hang on to the audio output transformer

I can make a replacement filter capacitor case. A couple of years ago I finally packed in my gas welding gear and bought a professional grade TIG system, so things in mild steel, stainless, aluminium or copper aren't difficult. In the past I've designed and wound my own transformers. Only the meter and the valves would be a problem.

Cheers
David
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Old 6th Jun 2012, 11:40 pm   #10
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OK, I've just had a look inside my old AR88, the early serial numbered one, and had a bit of a surprise.

It has what looks like PVC wiring.

It looks original. The cable forms and lacing are professional and look to have gone in when it was assembled, even the RF screened areas are the same.

The chassis is gererally a lot nicer than the later receiver I bought last weekend. No rust. No rust on the transformers, chokes, and the reservoir block.

There is an old sticker on the right-hand trunnion plate. I remember the R1556 bit but the rest is interesting.

TYPE R1556
REF No 10D/2360
SERIAL ATB3297
MOD No 213/1
DATE 5-9-55

That serial is different to the four digit one stamped on the rear drop of the chassis. R1556 is the RAF designation. Is that an RAF serial number, different to the RCA one?

There's no oil under the main reservoir block.
Some of the bathtubs have red gunge around the seals of their tags.. it looks like red hermetite gasket stuff, very thick indeed.

That capacitor has been replaced and it must have been back in the 60s, before i got it. The paxolin board carrying the HT resistors for the negative bias has one cement cased RS wirewound on it. The dropper to the VR150/30 is a green Welwyn type wirewound. The bathtubs are marked Cornell-Dubilier and Aerovox..

I need to strip the thing for a proper cleaning, and the IFTs need attention. Most importantly the detent on the band switch has been useless for a long time.

The cabinets are different. Location in the early set at the rear is by a two pronged folded bracket spot welded to the centre of the rear crosspiece, plugging into two rectangular holes in the lower middle of the rear drop of the chassis. The later cabinet has a pair of taper ended pins plugging into round holes at either side of the rear drop of the chassis.

Both chassis have the holes for either cabinet.

I suppose I'd better get some photos of this plastic wiring, and compare with the brittle cellulose in the later box.

Polythene existed in WWII, and the AR88's secret weapon was its polystyrene coil formers for the highest frequency band.

Or what was done to it in September '55?

Any thoughts, ladies and gentlemen?

David
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Old 7th Jun 2012, 7:01 am   #11
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Default Re: Two AR88s

I have today just paid for an AR88D in reasonably good condition. With a lot of spares. Looking forward to picking it up tomorrow morning. Can't wait.

Robert.
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Old 7th Jun 2012, 10:03 am   #12
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Default Re: Two AR88s

Hope it's a good 'un, Robert. Occasionally, I have a hankering for an 'LF to complement the 'D, then I'm reminded that one large, heavy piece of steel around the place is enough...

David, as with anything of slightly mythical status, I've heard all sorts of lore about the AR88- as usual, some of it is credible, worthy and useful, much of it needs taking with varying amounts of salt and some of it is self-evidently or easily provable to be nonsense. I was once told that many (some?) RAF sets were rewired with PVC-insulated wire after the war. Decades later and in a completely different setting, I remember considerable competition amongst us trainees as to who had produced the best-aligned components, neatest connections, most consistent harness terminations etc on workshop samples. Perhaps sets that passed through MOD workshops really would have had "better than factory" wiring?

The stabiliser feed resistor (R30, 2.7k) has a reputation for going open-circuit in the long term- it's markedly over-run in the maybe 20 seconds between the directly heated rectifier supplying HT and the remaining valves starting to draw current. Thereafter, it runs on the limit of its rating (4W in my manual), I know that wire-wounds are supposed to be able to withstand short-term overload but maybe repeated thermal shock doesn't help with longevity. It's not in the optimum place for cooling, either. Maybe, any "usual suspects" were changed at this supposed rewire- the MOD seemed to like those "green vits". The wire-wounds in the bias chain aren't run hard but look to be of only average commercial quality, maybe they just conk out occasionally. Excusable after all those years.

Colin.
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Old 7th Jun 2012, 11:48 pm   #13
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Default Re: Two AR88s

Thanks, Colin.

I like those green vitreous resistors too. They seem to take a direct nuclear strike and come out smiling.

So my old AR88 is a very early one, but it's not terribly original. PVC wasn't around in WWII, I'm fairly certain, so one of Her Majesty's minions rewired it in probably 1955. A decade later it was in the hands of a school kid in Huddersfield, who had it bead blasted and stove enamelled in hammered silver finish.

My intent up to last night had been to strip the thing right down, even to removing valveholders and cleaning everything. I was then going to reassemble it from scratch, making a pin-board for making a new wiring harness. I was intending using new reliable capacitors throughout, upping the power rating of the over-run wirewound resistors and going for low noise metal film for all the others.

I've got to get invasive to replace the detent section of the bandswitch... the other musts are the bathtubs and the micamolds and THAT capacitor. The main reservoir block comes under the category of 'heading them off at the pass' I'm also going to re-draw the schematics and make them more readable.

I think some schematic files and a PDF of a pin-board for making the harness could be useful to others.

I'm talking myself into the full harness job.

The wirewounds in the bias area are somewhat singed.

Decisions, decisions

David
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Old 8th Jun 2012, 9:40 am   #14
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Hi David,

If the panel around the bias chain resistors shows more than mild darkening/dust convection, it could be an indicator that excess HT draw has been going on for quite some time- high up on the list of likely causes is cumulative leakage from all those wretched micamoulds. Hate the things, lurking squat electronic pestilence, about as welcome as Colorado beetles. If they aren't the No.1 scourge of US equipment of the era, they're pretty high on the list. I have an eradicate on sight, no quarter given policy- there's no reward for compassion here. I know from experience that the one I leave skulking in a deep recess of a coil-box will smile slyly and stab me in the back one day.

A little while back, I worked on a BC348, as an experiment I left it running on a current-limited power supply (heaters disconnected) over a few days whilst I got on with other things- I got familiar with the tell-tale down-drone as another micamould revealed its sweaty, fizzling guilt.

Your comment about a substituted RS resistor in the bias chain reminded me that the AR88D is specified with the 6K6GT as standard- not sure if this applies to all variants, I've heard different opinions. It seems to have a reputation for being weak-kneed, I've seen its durable, respected cousin the 6V6GT substituted in several. The 6V6 works "OK" as a straight plug-in with the -18V (or thereabouts) grid bias of the circuit as it stands but there's a tad more available power output and somewhat less distortion with around -13V bias. It might be that the original 160 ohm was substituted with 120 or even 100 ohms- this also reduces the maximum negative bias available at the RF gain control but possibly not significantly. I doubt that a bit more quiecscent current causes significant problems, in mine I used 120 and 39 ohms in series to give the choice, it's run like this for years. After all, many sets will have run with high capacitor leakage for a long, long time without transformer distress.

Colin.
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Old 8th Jun 2012, 8:55 pm   #15
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Default Re: Two AR88s

Quote:
Originally Posted by Radio Wrangler View Post
OK, I've just had a look inside my old AR88, the early serial numbered one, and had a bit of a surprise.

It has what looks like PVC wiring.

It looks original. The cable forms and lacing are professional and look to have gone in when it was assembled, even the RF screened areas are the same.

The chassis is gererally a lot nicer than the later receiver I bought last weekend. No rust. No rust on the transformers, chokes, and the reservoir block.

There is an old sticker on the right-hand trunnion plate. I remember the R1556 bit but the rest is interesting.

TYPE R1556
REF No 10D/2360
SERIAL ATB3297
MOD No 213/1
DATE 5-9-55

That serial is different to the four digit one stamped on the rear drop of the chassis. R1556 is the RAF designation. Is that an RAF serial number, different to the RCA one?

There's no oil under the main reservoir block.
Some of the bathtubs have red gunge around the seals of their tags.. it looks like red hermetite gasket stuff, very thick indeed.

That capacitor has been replaced and it must have been back in the 60s, before i got it. The paxolin board carrying the HT resistors for the negative bias has one cement cased RS wirewound on it. The dropper to the VR150/30 is a green Welwyn type wirewound. The bathtubs are marked Cornell-Dubilier and Aerovox..

I need to strip the thing for a proper cleaning, and the IFTs need attention. Most importantly the detent on the band switch has been useless for a long time.

The cabinets are different. Location in the early set at the rear is by a two pronged folded bracket spot welded to the centre of the rear crosspiece, plugging into two rectangular holes in the lower middle of the rear drop of the chassis. The later cabinet has a pair of taper ended pins plugging into round holes at either side of the rear drop of the chassis.

Both chassis have the holes for either cabinet.

I suppose I'd better get some photos of this plastic wiring, and compare with the brittle cellulose in the later box.

Polythene existed in WWII, and the AR88's secret weapon was its polystyrene coil formers for the highest frequency band.

Or what was done to it in September '55?

Any thoughts, ladies and gentlemen?

David

David,

its probable that the set was converted from a standard AR88 to a R1556 in 1955. I can't find my copy of the conversion instructions, but as I recall it involved fitting the standard (RCA) S-meter, changing the aerial input socket to one of those large RAF types, and adding another 2-way connector on the back panel (whose purpose I can't remember either right now).

So I suspect 1955 was the "creation date". Its possible that they decided to rewire in PVC at the same time, though that is only speculation. It could equally have been done by some later owner.

One other point for all AR88 owners to note is that the audio output transformer going open circuit is a very common fault. Replacement transformers were available from Colomor at one time, though I believe John Reynolds bought most of their spares from them, and they have since been dispersed once John passed on.

The problem that causes this appears to be the mode switch. As you turn the set off you have to go through the "TRANS" position, where most of the set is disconnected from the HT+ rail. The purpose of this is to mute the set when you go to "transmit" (i.e. "TRANS"). But the problem is that without the normal load on the HT rail, its voltage can go pretty high. On mine (and all the others I have tried) you get a nasty crack in the LS as you go to the TRANS position, indicating there is a high voltage transient as well, as the HT current is suddenly cut.

I haven't checked all this out in detail, but my suspicion is that excessive voltage on the HT rail is stressing out audio output transformer - to the extent that it eventually goes belly up (open circuit). One possible fix (again I have yet to try this) would be to move C96 from the rectifier side of L50 to the junction of L49 and L50, i.e. in parallel with C97. This would change the HT filtering from C-L-C type, to an L-C filter. This has the desirable characteristic of dropping the HT voltage, and also making it much more stable with variations in load current. Whether this mod will actually work (and cure the audio o/p transformer problem) needs careful investigation - its dependent on exact circuit values, actual load currents and so on.


Richard
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Old 8th Jun 2012, 11:03 pm   #16
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Default Re: Two AR88s

Hi Richard,

Another course of action could be to simply link the HT contacts on the power/mode switch (unless, of course, one genuinely is using it in its intended mode- I would contend unlikely). As it stands, it constitutes a rather brutal "withstand" test on the components left in ciircuit. A quick check on my set reveals this contact as easily accessed- with the set upside down, connections at the one and two o'clock positions of the switch, closest to the headphone jack (!!),

Colin.
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