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Old 16th Aug 2019, 10:26 pm   #1
Triggermann
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Default AR88 - New keeper

I have just acquired this AR88 from an SK estate and could do with some help as I try to resurrect it.

It's history is supposed to be...
An SWL bought it sometime in the 1950's from a surplus sale, used it till the 60's then it sat unused in a brick heated house (UK).

So 50 odd years later I step in. I have downloaded user books and circuits.
My first steps with the radio are to examine and make good any damage / aged items. AC won't be applied until it's electrically safe, and then via my Variac.

In the meantime can anyone comment / direct me to the below information..

1. Exact type of AR88, age etc?
2. Serial number, where is it? Mine has 001912 stamped on the top right hand corner of the rear panel and 002222 hand written on a paper modification sheet on the RF Shield.
3. Modifications.. clearly the signal meter is not standard, there's red light and a twist in fuse holder either side of the PHONES socket and a what looks like a female 7/8 coaxial socket on the rear panel. Is this an RAF R1556 or similar?

Thanks Andy
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Old 16th Aug 2019, 10:49 pm   #2
Silicon
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Default Re: AR88 - New keeper

The appearance of the Range display panel may suggest that it is an early model.
Later models had alternating white and black bands. The writing on range 1 should tell you if it is a D or an L version.

If you look inside you may find a label on the RF screening cover which identifies the model. It may just have AR88 on it. Have a look at that label at the rear behind the grill.

My AR88D may be have been modified by the RAF. It has an ON/OFF switch where your fuse holder is. Mine has a second headphone socket where your red light is mounted.
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Old 16th Aug 2019, 11:07 pm   #3
Triggermann
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Default Re: AR88 - New keeper

Thanks. Does this picture help re the range 1 writing?

Better photos once I get it in the shack.
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Old 16th Aug 2019, 11:30 pm   #4
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Default Re: AR88 - New keeper

It's a nice early AR88 - neither D nor L, with the shiny paint and engraved lettering.

73,

Colin.
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Old 17th Aug 2019, 12:19 am   #5
Silicon
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Default Re: AR88 - New keeper

Range 1 appears to be 535 - 1600 kcycles.

If you are offered an instruction manual for an AR88D and an AR88L, go for the AR88D which has the same frequency range.
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Old 17th Aug 2019, 4:10 am   #6
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Default Re: AR88 - New keeper

I have one similar, but not exactly the same.

All-yellow tuning scales in the left hand window.
Engraved panel (copper plated under the paint)
Big antenna connector.
UK modification label on the outside of the panel/chassis reinforcement gusset (You have to slide the set out of the case to see if yours has one. Mine gives the date of modification and says R1556)
Bakelite 2-pole AGC connector mounted sideways facing on rear chassis drop (Not there in your photo. Used for diversity coupling.)
PVC wiring harness (Seems to have been fitted at time of modification in the early fifties)
Genuine RCA illuminated S-meter.
Same tuning ranges as the AR88D
Just has numbers around the range knob (Later sets had tuning ranges in Mc/s)
Extra holes in front panel (for second phones jack, sometimes toggle mains switch)

It looks like you have an original AR88, no suffix. It's had some forces mods, but not all the known ones.

Slide it out of the case and see if there's a label.

Nice speaker!

David
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Old 17th Aug 2019, 4:41 pm   #7
dave walsh
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Default Re: AR88 - New keeper

Just a few general comments trigger. It does look like you may have something rare but I have no expert knowledge like David and others. I picked up an early AR88 a few years ago from a Forum Member [it still has'nt had much attention]. I was intrigued that there was a label marked Slave 31 on the front panel and DIVERSITY printed left of centre on the rear of the chassis. [I would think it's much later than yours]. I later found that Bletchley Park had constructed a replica Diversity set-up, thanks to Sean Williams. I might even get to see it soon as I will be in Oxford.

My first priority was to ID the set. I found the Radio Boulevard Western Historic Radio Museum site. This is an absolute mine of info with a section devoted to "AR-88 Series-Serial Numbers and Serial Number Log" in PART 2! So far so good but I couldn't locate a serial number on my set. Lots of people were very helpful and various places were suggested but no success. I got very frustrated with this Despite looking at the back of the chassis again and again I hadn't spotted it until finally there it was [faintly] stamped into the top right hand corner of the rear panel [just like yours]. I really can't explain what happened there The Museum Site is great but they say themselves that the ID process is partly guesswork and difficult. It's certainly very complicated for "a bear of little brain." My Serial No is 009755 and this is directly listed on page 7 of the web site in the AR-88D [Camden] section, so that's where it was apparently built.

I get a headache just looking at all this info but the closest I can find to your 001912 is under CR88A where 001700 is listed and then 001923 so perhaps 001912 is somewhere in between? or maybe I am on the wrong track all together! There will be more expert advice/interpretation I'm sure. I hope this might help. Apologies if you know it already.

Dave W
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Old 17th Aug 2019, 5:39 pm   #8
m0cemdave
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Default Re: AR88 - New keeper

Quote:
I later found that Bletchley Park had constructed a replica Diversity set-up, thanks to Sean Williams. I might even get to see it soon as I will be in Oxford.
Bear in mind that the working replica diversity setup (based on the Knockholt Wireless Intercept station) is part of the Computer Museum, not the BP Trust site. See here for opening times etc:
https://www.tnmoc.org/
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Old 17th Aug 2019, 5:48 pm   #9
Triggermann
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Default Re: AR88 - New keeper

Thanks all for your comments / advice. I have done work on two KW2000's, TS530/820/830's so hopefully I've learnt to look, ask and be careful.

Today I emptied the case and gave it a general clean. I always like to work on clean things.
It looks complete with no obvious missing bits / cut wires. The meter scale looks DIY, yet the mountings look factory produced.

I think this radio will not be restored visually, just electrically. I like its patina.

Next up... what will I replace before trying AC through a variac. I could be busy this winter.

Andy
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Old 17th Aug 2019, 7:45 pm   #10
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Default Re: AR88 - New keeper

Yours seems to have the original cotton over rubber wiring which will be hard as pot and crumbly by now.

General advice is to do ALL those bathtub capacitors. Look for high value resistors (valve screen feeds - the one in the RF assembly is a right sod to get to)

Check the series droppers in the -ve path of the power supply. These are used to make some negative bias voltage for the RF gain control and AGC. THe resistors are rather stingy for the power they dissipate and can go high in value and break up.

Look around for the brown rectancular "Micamold" low value capacitors. These are paper capacitors faking mica parts! Hunt and pounce!

The most important parts to swap:
1) the coupling capacitor (two in parallel to make up the value in the original set) from the 6SJ7 anode to grid 1 of the 6V6. THAT capacitor. Failure kills output transformers and they are unobtainium.
2) the tone correction capacitor from the 6V6 anode to chassis. Use a modern part and rewire it across the primary of the output transformer. Where it currently is, it's perfectly placed to ruin the output transformer.

It might be getting to the point where I'd contemplate following the RAF's lead and making a new harness. I'd probably use aviation grade Kynar insulated wire that doesn't melt at soldering temperatures to make life easier.

David
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Old 17th Aug 2019, 7:46 pm   #11
dave walsh
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Default Re: AR88 - New keeper

Same but a little different Triggermann. From the photos, it looks like you also have the DIVERSITY marking on the back. The cover [with info] to the right of the two valves, is missing on mine and the two covers to the left are square [ish] rather than round. The front panel seems to be very tidy!

Thanks for the correction and details m0cemdave. I could have sworn it was Bletchley but then I couldn't find the serial number for my set

Dave W
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Old 17th Aug 2019, 9:49 pm   #12
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Default Re: AR88 - New keeper

At first sight it appears to have 'stuff' fitted around the lower-left-side controls - historically the RAF had bunch of AR88s modified with power-switches/headphone-sockets in this area - R1556.

But yours seems to have a pilot-light in this area?

Yours also has the big-and-obscure coax-socket fitted above the antenna/ground tagstrip on the back - which is another classic R1556 thing.

And the oval, unoccupied cutout to the left of the antenna-connectors over towards the power-input/voltage-selector: that was in times-past occupied by an interconnector to link the AGC lines to a diversity-control system.

Out of interest - what colours does yours have for its mains power-lead?

Mine is red/yellow/black!
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Old 17th Aug 2019, 10:40 pm   #13
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Default Re: AR88 - New keeper

The metal cased "bathtub" capacitors will almost certainly need replacing. they are oil filled with what may be PCB oil so try not to get any on your hands and dispose of any cleaning rags sensibly. I have seen restorers open them up and restuff them but it is a messy business. I restored an AR88 for a local Amateur Radio Club last year and just cleaned the sticky residue off the bathtubs, disconnected them and wired in replacement yellow polys as point to point components, some across the valve socket pins which was tricky to do as the wiring is laced and hard to trace unless you use a multimeter with a buzzer. I was lucky as the HV HT caps were not leaking either physically or electrically. You should be able to download manuals from VMARS. Cheers, Jerry
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Old 17th Aug 2019, 11:35 pm   #14
dave walsh
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Default Re: AR88 - New keeper

I've got the large obscure coax socket replaced with a standard modern coax version. There's a small and large pin single audio socket [associated with seventies stereo systems] to the left of R21 [adjustable] and parallel to the oval socket which has the original 600 Ohms Output strip fitted on my set! There's the mains selector and the other socket? diagonally below. Is that related to the Diversity set up? I note that the OP's receiver has Camden written on the back plate which suggests it's an AR-88D like mine but the serial number may not tie in!

Dave W

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Old 18th Aug 2019, 12:28 am   #15
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Default Re: AR88 - New keeper

Your receiver has been exposed to moisture at some time.

The first check should be the mains transformer and the wiring associated with it.

If you remove the rectifier you won't have to worry about the HT side for now.

A soak test of the mains transformer with a lamp limiter and a 1 Amp mains fuse would be a good starting point.

I have an AR88 that works with the original wiring and bathtub capacitors.
However Davids capacitor alterations No. 1 and 2 are essential.
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Old 19th Aug 2019, 3:28 pm   #16
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Default Re: AR88 - New keeper

Yes, there's a little bit of rust and general air of neglect, but nothing too serious and I suspect that many radios of this age (and weight!) will have spent some of their time in a corner of a shed or garage. It certainly looks as though it will smarten up nicely and it looks to be in largely original condition with no grim carving-about.

It looks as if the 1st RF stage valve/toob (6SG7) has been replaced with a 717A type- this is essentially a 6AK5 on an octal base with a a distinctive "mushroom" envelope. These were available in large numbers as surplus post-war and part of the appeal was that they could be plugged straight in with no socket rewiring. There was a craze for "hot-rodding" older sets in the 'sixties as transistors started to dominate and owners of valved sets developed paranoia that they might not be hearing that last faintest 10m potential contact, and lots of lore developed that fitting XXX wonder-bottle would bring in icicles cracking on Pluto. The 717A has a small anode area and was intended to be operated at low power, so beware that the AR88's circuit design doesn't over-stretch it. Also, it's a "straight", rather than variable-mu valve, so ideally its AGC ought to be potted-down on the AGC diode's load resistor. It may simply have had the AGC disconnected and been wired for high gain.

Good luck, these are still marvellous sets when aligned and operating well.
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Old 19th Aug 2019, 5:54 pm   #17
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Default Re: AR88 - New keeper

Just realised something I should have mentioned earlier- as standard, the AR88 was fitted with a 5Y3GT rectifier, your set looks as though it has something like a 5T4 or possibly a 5Z4 fitted. It's not at all unusual to find one of the various other 5V heater International Octal base rectifier family fitted to AR88s- they have differing heater currents and forward voltage drops between types but will generally basically "work" in the AR88 with its relatively modest HT current requirements. Note however that the steel envelope types could possibly become painfully live in the event of an internal fault- the envelope is connected to pin 1 of the base but this is left floating and not connected to chassis in the AR88 as it was designed around the glass envelope 5Y3GT.
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Old 19th Aug 2019, 8:01 pm   #18
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Default Re: AR88 - New keeper

I seem to remember that the AF output transformer primaries tend to fail on these radios as a result of a surge in anode current when switching from stby to on. May be worth a con check. I don't think the AF output valve has a cathode resistor and a mod was recommended to add one to bias the valve off a bit and limit the current. Cheers, Jerry
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Old 19th Aug 2019, 8:08 pm   #19
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Default Re: AR88 - New keeper

Yes, be careful of using the "STBY" position of the power switch - in "STBY" the rectifier is basically unloaded so the filter-capacitors charge to the peak-voltage delivered by the mains transformer. Then when you switch to "REC" you dump this high voltage into the rest of the circuit.

The RAF 'R1556" modification bypassed the 'STBY" position on the switch. I guess the problem was known about back in WWII-days?

Some people wire a 100-Ohm resistor from the cathode of the output valve to limit the current.

As a safeguard, wire a couple of NE-2 neons across the primary of the output transformer = - these will strike if subjected to more than 80-or-so volts, so 'clamping' any switch-on/off voltage surges in the transformer and hopefully saving it.
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Old 19th Aug 2019, 8:49 pm   #20
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Default Re: AR88 - New keeper

What killed my AR88's output transformer was the matching speaker becoming disconnected due to a loose plug on the back of the speaker. I managed to obtain a new O/P TX, but I did away with the plug and soldered the wires directly to the speaker.
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