RCA AR88D Comms Reciever

[Tedzed1]

Purchased on a "shall I, shan't I, oh bugger it I will" moment at the recent Golborne BVWS meeting.

The model plate is missing but I assume from looking around the internet it is an early "D" example, in that the 2.5 ohm output is on the same tag-board as the antenna connections, with the 600 ohm output on a separate tag strip above this, I believe later variants had the connections the opposite way round, but please correct me if I am wrong.

The Receiver is in working condition, but needs some attention, all bands work, but seems to pick up a lot of mains produced noise and the noise limiter is not working, well to say it is not working is incorrect as, when it is operated, the set goes virtually silent, with only a very low distorted output that you have to put your ear to the speaker to hear, but possibly understandable considering it seems 99% original, it has had an aerial socket fitted to the rear which is connected straight to a tag on the first I.F can, and also a similar connector fitted to the front panel which I have yet to trace.

So it seems a slowly done capacitor replacement session is in order, as they all seem to be original, even one of the "bathtubs" has started to leak.

I would also like to source a replacement dial glass, as mine is cracked in two places in front of the signal meter, and I would like some advice on repairing the front panel, to mask the two holes that have been drilled, the backlash on the gearbox seems fine.

So any advice on repair or restoration of this beast would be most welcome. It may be a slow restoration, but I would like to make as good a job as I possibly can on this receiver.

(I understand the BFO can be modded to enable better reception of SSB signals.)

[Andrewausfa]

Nice! I'm after a 'D' in the New Year.

The BFO mod is easy, two bits of thick-ish wire connected to two valve pins, you can just about see it here (bottom centre valve base) on my LF.

My LF really came to life when I replaced the grey coffin shaped capacitors in the coil boxes. A big improvement came when I changed some 33K resistors, again in the coil boxes.

Not that you should be worrying about the above at the moment! Do what Al suggested and make sure it's not going to kill your output transformer first.

Andrew

[pwdrive]

Hi Paul, yes take your time and do a good job it will be worth it, main mechanical problems usuallly end up with the tuning reduction box, they are prone to backlash if it's had a hard life, appart from the usuall checks in the reduction drive there is not a lot you can do normally, if yours is ok it's still worth doing some routine maintainance on it, clean, regrease/oil, check/adjust endfloat on the appropriate shafts, make sure the anti backlash gears are offset correctly etc.
Them bathtubs do leak, replace/restuff as is your wish. Check the resistors on the bias board etc.
The S meter looks like a non original add on, this is common, the S meter circuit on these receivers isn't the best but it should function as an indicator.
As you are probably aware it employs an Xtal filter so if you feel the need to re align the receiver bear that in mind as you will have to determine the exact Xtal frequency with a sig gen then align the IF's to that frequency, there has always been a bit of a debate on the IF alignment involving Xtal filters, Ideally a sweep and display is required but you can use the standard sig gen and meter method and get very good results as it is only a single Xtal design, remember that the phasing control is not an adjustable front panel operated feature so that setting it as a preset will be quite critical, you will probably be aware that the Xtal filter at its narrowest setting is for single single reception on CW. If when all is done the single signal reception does not stand out then remember that you need safe crackers fingers to get the tuning bang on the nose, it's very criticall, if no joy then first would be to strip the Xtal holder clean and dry the Xtal and re assemble, if that fails it is possible that the Xtal for whatever reason is totally inactive or there is not enough coupling (check associated capacitors etc) or over damped.
Yes there is a mod for improving SSB reception using the BFO, the most common and simplest is to increase the BFO injection voltage by making up a short length of twisted wire capacitor for the coupling and adjusting to suit, that's what I have on mine.
You are lucky as it still has the topside RF screening can, quite often they are missing as is the one underneath that screens the tuning gang capacitor.
OK, going to upset you now, I have a NOS in original packaging perspex window for mine (not fitted yet) NOS ones are hard to find but good seconhand ones do turn up at auction etc from time to time, but there will be competition, same goes for an original manual, having said that one went on Ebay the other night for under £15 an HRO one from the same seller went for around £25, neither really prohibitive, it depends on your take on these matters.
Not sure about the socket on the rear apron, Diversity/RIS maybe? you would need to draw a basic circuit diagram showning its relationship with the rest of the circuit.

Them drilled holes on the front panel:
They can be filled and made to disappear no problem, the problem will be getting a paint/texture match.
I would be inclined to fit those slightly curved (outwards) black blanking grommets until you really decide which way you want to go, screwing up a front panel refurb will look bad whichever way you look at it.
If I remember right, turning the noise limiter full on does reduce and distort the signal, the criticall point is just before the distortion occurs, you need some repeated fast risetime interferance ideally to hear the limiter working, I have something down our way that ticks regular like a clock, the noise limiter will cope with that no problem and should do with spark/ignition type interference. I can't remember off hand what type of noise limiter is used in the AR88 without consulting the book of words, my Hallicrafters SX28 uses a Lamb limiter which works spot on.

Have fun and don't drop it on yer toes...

Lawrence.

[pwdrive]

OK thats four owners so far, any more?

Lawrence.

[pwdrive]

Hi Paul, just had a look at the circuit diagram, the noise limiter looks like the clipper type not noise canceling type, basically it clips anything above a preset level as determined by the control pot, advancing below the peak signal modulation level will introduce distortion , advancing to far will clip most if not all of the signal modulation. Beware of some of the circuit representions regarding some of the resistors I think they use "m" as opposed to "k" in some positions, this was pointed out by Al (skywave) in another post on a different subject.

Lawrence.

[Skywave]

Welcome to the RCA AR88 owners club!

► Re: the aerial socket fitted to the rear, which is connected straight to a tag on the first I.F can. ◄
That is obviously a modification and is almost certain to be for the purposes of connecting a Q-multiplier.

As regards tuning backlash, yes, a very common problem. When I first got the one I own now, the backlash was very bad. The cure is to remove the gearbox completely, strip it down, thoroughly clean it, re-assemble it - being careful to set up the anti-backlash gears correctly - re-lube and re-fit it.
The rub in all of that is the "set up the anti-backlash gears correctly". Not an easy task by any means, but I made myself a jig to support the frame and four clamps to hold the gears in their tensioned positions whilst they were refitted: made the task a lot easier. But it is still a very tricky job - and it can be done - just requires plenty of patience, perseverance, a tidy bench - and plenty of time. And don't be surprised, when you refit it, if you find that you then have to remove it and do it all again - because the backlash is still not acceptable . Generally speaking, though, an inter-gear tension of about two teeth is usually about right, but it does depend a great deal on the springs - they do tend to become 'tired' with age - and consequently weak. Sad to say, that is a symptom that I personally recognise all too readily!

Overall, though, when all the necessary 'standard' overhaul work is done (which often is not a lot), you end up with a receiver that holds its own on the S.W. bands, even today, after all these years.

Al. / Nov. 20, 2012 //

[Skywave]

Beware of some of the circuit representations regarding some of the resistors I think they use "m" as opposed to "k" in some positions, this was pointed out by Al (Skywave) in another post on a different subject.
Lawrence.

Exactly: the cct. uses the abbreviation "MEG" where you and I would use "kilo" ~ as in 100 k-ohms; the cct. states 100 MEG. (Whoa! How much?)
It's an abbreviation, the origination of which I could never understand: to me, just seems plain stupid.

Al.

[Tedzed1]

WOW, thanks for the response everyone, seems I will not be stuck for advice on this.

So to start with, first job will be that cap that feeds the control grid of the A.F. on the output valve, secondly will be the cap that feeds the tone control.

I think my biggest problem will be the noise limiter, as soon as it is switched to MAN. N.L. or A.V.C. N.L. the sound dies, the noise limiter control next to the signal meter does not make any difference, the receiver stays quiet with the control operated end to end.

As to the glass dial panel, I wonder if a good dial scan transferred to glass would look OK, must admit I have never seen it done on one of these.

I will also replace the bathtubs, I am not too worried about originality, so will just replace them rather than restuffing which seems quite a dirty task.

This is the first stage that I will tackle.

(now shall I ask Santa for a Triple Diversity Rack for Christmas?)

[Skywave]

Noise limiter: bearing in mind that we know that some modifications have taken place with this receiver, the noise limiter cct. might have undergone a modification. Over the years there have been countless designs for noise limiters of the 'pulse suppression' type - of which the one in the AR88 is an example.
So, it might be a good idea first to examine the wiring and components around the N/L valve(s) and compare what you find there to what is shown in the cct. diag.

Changing the subject completely - and this is simply a sporadic afterthought - if you find that you need to replace the PSU reservoir / filter capacitors, please note that most of them do not have their -ve plates connected to chassis . . . an exam. of the cct. diag. reveals why. I mention that since that has been known to catch some people out . . . . .

Al.

[pwdrive]

Hi Al, yes a Q multiplier...I forgot that one.

Hi Paul, you might or might not be aware of Henry Rogers american boat anchors website, here is the link anyway for those that might be new to this field:

http://www.radioblvd.com/ar88.htm

The standard meter mod. is on there.

The site is a very good read, he also keeps and maintains a serial No: log for these classic receivers as well as for the HRO, Hammarlund, Hallicrafters, BC348 etc. He welcomes any submissions, some of my serial numbers are on his list somewhere, there is an email contact somewhere on that site.
From memory I think the AR88D S/no: is stamped on the rear chassis apron? He generally does not respond to chit chat types of emails ie: "my radio doesn't work wot's wrong with it" but anything in a historical context he is usually interested in and will reply so if you can find the serial number it would be worth submitting it to him.
My own AR88D is stacked in a pile at the moment so can't do any voltage comparisons etc that might help, others might be able to help though.
As regards the perspex window I suppose it could be replicated, graphics wise it's fairly simple.
The white is the awkward one, maybe some of the other forum members have ideas to deal with that.
As regards the noise limiter then as Al has said many of these receivers were heavily modified so it would be worth checking around that section for any obviouse signs, failing that it will be the standard fault finding proceedure, it sounds as though the biasing on the limiter diode is up the creek maybe?

Lawrence.