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Old 3rd Jan 2009, 11:10 pm   #1
Mark Spurlock
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Smile RCA AR88D Comms Receiver restoration - idiot's guide sought

I'm looking to start a restoration of an AR88D that I've had in my garage for the past 3 years. Problem is, if I' honest, I'm a bit of an 'armchair restorer' of valve sets; whilst I've collected them for over 25 years, and dabbled with the restoration of old classics like Bush DAC 90 and 90As, such work has rarely progressed beyond basis replacement of wiring and capacitors.

I've started with a circuit diagram for the AR88, but the first despair was that resistance between live and neutral checked in at 8 Ohms....making me reluctant to run even a low voltage through the set to try and recondition the caps (no, I don't have a variac), also the by-pass caps are all leaking a treacle-like orange goo (more despair!). And then there's the lace-covered wiring that I am dreading replacing, lest I forget what tracked where....

Does anyone know of a 'beginner's guide to restoring this classic? Any suggestions warmly welcomed!!

Thanks

Mark
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Old 3rd Jan 2009, 11:30 pm   #2
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Default Re: AR88D restoration - idiot's guide sought

Mark, I have got all my info from this excellent forum which is packed with a huge amount of very knowledgable folks, plus articles found on the internet. The ones I have are:
'Getting the Most from your AR88'
'Getting the Utmost from your Ar88'
'Ar88 fault finding'
'Buying and renovating the AR88'

The goo coming from the bypass caps is the oil as they are paper-in-oil caps. It probably means they are electrically 'leaky' as well. I am in the process of changing those on my set. I have actually replaced them with convential leaded components as I am more interested in a working set than authenticity. Your milage may vary.
The most important caps to replace are C118/C117. I didn't and blew my output transformer primary. Replacements for these are expensive so I have used an old transformer from a defunct valve tape recorder and paralleled it with the old.
It can be a bit of a daunting task at first looking at the '88 circuit but if you work logically and take time you can bring it back to life successfully. As for the 8ohm across neutral and earth, I imagine this is the DC resistance of the mains transformer primary and should be ok. Perhaps someone else may be able to comment on that one.

Charlie
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Old 3rd Jan 2009, 11:46 pm   #3
Sean Williams
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Default Re: AR88D restoration - idiot's guide sought

The AR88 is not that difficult to work on, and if you look at each part of the circuit carefully you will see lots of similarities to normal domestic radios.

I am suprised that you have mentioned problems with the wiring - the varnished cotton covered cable used in the AR88 seems almost indestructible, I wouldnt attempt to change the loom unless it is really bad!

Caps, yep, they do leak - but ones that appear dry are usually OK - the main smoothing block can be prone to leakage - dont worry I have spares for these in large numbers!

A word or two regarding alignment - do not touch it, untill you are 200% sure the set is correct electrically - unless the phantom bodger has been in there with a screwdriver all should be well - there are 27 seperate adjustments just for the IF stages..... (oh, they all interact too, so a better way of going mad I've yet to find...)


The resistance you read sounds reasonable - it isnt a direct short, so should be ok - pop the output valve out, and the rectifier, and apply some mains - see what happens.

Meter the resistance between HT+ and chassis - if there are no shorts then pop the reccy back in, and measure the HT volts on warmup - next stop will be the grid pin on the OP valve - if it is positive then that cap needs changing - do it now, or you will be shopping for a nice new output transformer.....

Cheers
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Old 4th Jan 2009, 9:41 pm   #4
Mark Spurlock
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Default Re: AR88D restoration - idiot's guide sought

Hi Sean / Charlie

Thanks to you both for such quick, and encouraging replies - most helpful.

I will heed what you have said and run through the various recommendations.

Sean, you mention that have spare caps (for sale?); please would you advise availability, etc. as I would like to replace these early in the piece.

Re the IF alignment, I am not planning to touch this if at all possible; don't have the confidence or the kit just yet. Also, a mate of mine (now 83) used to be a radio operator on the Zambesi during the war, and spent ages in search of either a B28 or AR88LF; he got the latter, but it was so out of alignment (courtesy of a roving, merry screwdriver, no doubt) that he parted company with it with great sadness; so I noted a lesson there!

Thanks again

Mark
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Old 5th Jan 2009, 3:50 pm   #5
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Default Re: AR88D restoration - idiot's guide sought

Quote:
Originally Posted by cendoubleu View Post
Mark, I have got all my info from this excellent forum which is packed with a huge amount of very knowledgable folks, plus articles found on the internet. The ones I have are:
'Getting the Most from your AR88'
'Getting the Utmost from your Ar88'
'Ar88 fault finding'
'Buying and renovating the AR88'
The manual is worth having, even if the most useful parts are the circuit diagram and the parts list.

The obvious document missing from this list is the REME (Royal Electrical and Mechanical Engineers) Notes.

It's about 50 pages of in-depth maintenance material including aligning the IF sections. It looks as if someone decided that the manuals for the AR88D and AR88LF were basically useless and did the job properly.

I've got a copy which I really don't want to scan, partly because it looks as if it's already a copy of a copy. See if you can find it on the WWW, or maybe someone here will already have it scanned and turned into a pdf.

One problem with adjusting the IF alignment is that it's very easy to break the adjusters. Search on this forum for threads.

Pete.
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Old 5th Jan 2009, 6:08 pm   #6
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Default Re: AR88D restoration - idiot's guide sought

Quote:
The obvious document missing from this list is the REME (Royal Electrical and Mechanical Engineers) Notes.

It's about 50 pages of in-depth maintenance material including aligning the IF sections. It looks as if someone decided that the manuals for the AR88D and AR88LF were basically useless and did the job properly.
I assume you're saying that this is better than the RCA Manual? I found the 44 page RCA Manual for the similar CR88 (AR88 with adjustable crystal phasing control) to be very useful. It runs to about 44 pages and describes the IF alignment procedure in detail with many drawings showing the IF response curves.


Quote:
One problem with adjusting the IF alignment is that it's very easy to break the adjusters. Search on this forum for threads.
The compound used to lock the adjusting screws dries very hard and if you try to adjust the brass adjustment screws without removing it the heads will split. I think the compound is shellac which can be dissolved in a 50/50 ethanol/acetone mix, but I haven't tried it. If you have a sweep generator it might be best to try an alignment CHECK before resorting to realignment. The curves should open up symetrically on each side of the IF.
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Old 5th Jan 2009, 6:37 pm   #7
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Default Re: AR88D restoration - idiot's guide sought

I see that numerous documents relating to the AR88, including the REME document mentioned above, are available from the VMARS site here:-

http://www.vmarsmanuals.co.uk/archive/files_index.htm

They look very useful.
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Old 5th Jan 2009, 9:13 pm   #8
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Default Re: AR88D restoration - idiot's guide sought

I was referring to the 25 page AR88D manual titled 'Instructions' which the OP will definitely need to have, as well as the articles mentioned by Cendoubleu.

The REME notes I have are

E772 - Technical Handbook - Technical Description. This is on the VMARS site mislabelled as E722

and

E774 - Technical Handbook - Field and Base Repairs, also on the VMARS site. This is detailed hands-on instructions including setting up the IF stages with a Cossor 343 wobbulator.

The VMARS site has a few others I don't have and their versions are a year or so later than mine.

The AR88D instructions manual is also on the VMARS site. I've got a printed RCA version, but the version on the VMARS site has a supplement on the S-meter.

I've also got "Supplemental Instructions for Aligning AR88 IF Stages in the Field". This appears to be an internal RCA technical note from 1942 and describes setting up the IF stages without a wobbulator using an RF voltmeter. That contains detailed curves and instructions. Using an RF voltmeter is an inferior method.

Pete.
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Old 17th Jan 2009, 12:18 am   #9
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Default Re: AR88D restoration - idiot's guide sought

Mark

just wondered if you had made any in-roads on the AR88 restoration and if so how are you progressing?

Charlie
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Old 18th Jan 2009, 6:18 pm   #10
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Default Re: AR88D restoration - idiot's guide sought

Hi Charlie

I have struggled with time of late, so progress is slow. However, I fired the unit up (gently) using a 12V to 240V vibrator unit, which I ran from my car - I previously tripped the main RCD at home when I attempted to run the set from thte mains - as the missus and her mother were 5 minutes from the end of a weepy film, I got nothing but grief.........

When connected the set 'pulses', in that the lamps glow, then fade, then glow, etc. I presume that this might reflect the state of the smoothing capacitors?

I am planning to try and acquire a set of capacitors and start the general replacement, as well as trying some of the tests suggested from this thread.

We have a radio rally here in Frome on June 28; if I need spares, my guess would be that this venue would probably be ideal.

Cheers

Mark
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Old 18th Jan 2009, 6:30 pm   #11
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Default Re: AR88D restoration - idiot's guide sought

Mark, I have found a few spares that might help.

PM me for details.

Run it off the mains - you will never see what is going on with the set if you try other supply methods.

Cheers
Sean
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Old 18th Jan 2009, 8:01 pm   #12
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Default Re: AR88D restoration - idiot's guide sought

Mark,

If you're renovating old equipment, try putting a 100Watt mains bulb in series with the equipment. Run it like that for a while - it will help the old electrolytics to re-form without the shock of sudden full voltage.

Old stuff liikes to wake up slowly. I should know

Paul
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Old 18th Jan 2009, 9:20 pm   #13
Mark Spurlock
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Default Re: AR88D restoration - idiot's guide sought

Hi Paul

Thanks for that useful tip - course, one day soon, 100W filament bulbs will be a thing of the past.......isn't change a wonderful creation of Brussels..
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Old 18th Jan 2009, 9:29 pm   #14
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Default Re: AR88D restoration - idiot's guide sought

Let's have no EU knocking, or CFL versus Filament bulb rants here please. It's been done before several times. Stock up on 100W filament bulbs for lamp limiters while you can.
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Old 10th Feb 2009, 10:40 pm   #15
Mark Spurlock
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Default Re: RCA AR88D Comms Receiver restoration - idiot's guide sought

Just as an update. After getting bored with all the snow, I managed to blag a bit of quality time, alone in the garage with the AR88D (sad, I know..). I replaced a couple of paper-wax caps, prepared the 100W series lamp and gently fired the set up, having located a speaker that checked in at 2.8 Ohms, and coupled this to the 2.5 Ohm output at the rear of the set.

When set to 'REC', the resistance of the set must be quite low (presumably due to the low impedance primary of the transformer?), as the 100W bulb glows quite brightly; there is no discernable AF output via the speaker, and no clicks or pops when the wavechange switch is moved. I then tried switching to 'TRANS' mode (idle curiosity) to note that the resistance of the set appears to increase, with a noticeable fall off in the 100W bulb's luminosity; when I switch back to receive there is a healthy 'click' from the speaker - something must be getting through.

Someone has clearly added another jack socket at the front of the set, but I'm not certain whether they intended this for low impedance output, so I'm loathe to plug the speaker into that, just in case the o/p transformer suffers (it might be dead already, I guess, but maye those few odd clicks infer life in the old dog yet.....

Also, some bright spark appears to have linked all three lugs of the antenna-ground terminal board to the chassis - I've taken a guess and separated the third (ant) terminal from this cosy arrangement and shoved a length of wire in across the garage in vain hope.

As we speak, the set is sat out there, with the 100W in series; I intend to give it a couple of hours and put it away for the night, but, before I do, please may I throw a few questions out to the hitherto helpful audience: what should I try next? Full mains voltage and a few preparatory prayers? Should I try and measure valve voltages (as previously suggested; help appreciated here, as I only have limited knowledge of what I'm doing!!). Also. does anyone know where I can get a replacement voltage selector switch; I trashed the cover of mine by standing the set on it (I know...).

All best, and thanks for any feedback.

Mark
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Old 10th Feb 2009, 10:43 pm   #16
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Default Re: RCA AR88D Comms Receiver restoration - idiot's guide sought

I'd start by measuring the HT voltage.
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Old 10th Feb 2009, 11:56 pm   #17
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Default Re: RCA AR88D Comms Receiver restoration - idiot's guide sought

A few respondends have mentioned the risk of blowing the audio O/P transformer if the coupling capacitor to the audio output stage is leaky.

An excellent article in a past Radio Bygones article pointed out a funadamental design weakness in the output stage and recommended the inclusion of a cathode bias resistor and bypass capacitor. This addition affords some protection to the O/P transformer and it controls the anode current more effectively than relying on the valve characteristics!

If you need me to I will try and find the relevant RB issue - it was some years ago.

Good luck and get it GOING.

David
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Old 11th Feb 2009, 12:47 am   #18
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Default Re: RCA AR88D Comms Receiver restoration - idiot's guide sought

If the set is showing no signs of distress remove the 100W bulb. This should increase the HT voltage.

Your most likely problem is leaky bath tub capacitors and other leaky capacitors, including those in the tuning compartment which are hard to get at.

With all valves except the rectifier removed the set should draw zero HT current. That means there should be no voltage across R30 the screen dropper resistor. If you can measure a voltage there you probably have a leaky screen decoupler capacitor/s.

There should also be no voltage across the series combination of R43, 44 and 45. If there is you'll need to check every capacitor on the HT line.
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Old 11th Feb 2009, 12:27 pm   #19
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Default Re: RCA AR88D Comms Receiver restoration - idiot's guide sought

If I recall correctly, the TRANS position removes the HT from the valves, hence you'd expect the HT to rise. The TRANS position doesn't seem like a sensible thing to select in a set with possibly dodgy caps.

If you've checked everything you can check and replaced at least the caps likely to damage the O/P transformer, I'd say apply an audio signal to the audio stage and check that works, then 455KHz to the IF stages and work forwards. Start with low signal levels and raise them.

Have a good close look at the mods. They may be incomplete or nonsense.

Pete.
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Old 11th Feb 2009, 12:34 pm   #20
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Default Re: RCA AR88D Comms Receiver restoration - idiot's guide sought

If you're going for the inject a signal approach I'd go for injecting AF signals at the output valve control grid and working backwards. The trouble with working forwards is there is all sorts of IF switching in the way.

Taking voltage readings of the 250V HT, stabilised 150V HT supply and screen grid voltages is a good approach too, as it will soon show up any leaky caps.
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