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Old 11th Nov 2012, 2:12 pm   #1
Mans Veldman
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Default R1556A High imp. phones

I finished restoring my R1556a (AR-88) receiver. It has got a full mechanical and electrical restoration including a recap of the leaky bathtubs.

After a re-alignment it plays again as it should. Besides the low imp. phone connection the R1556A also has a high-impedance phones connection at the front and although the white plastic plate looks a bit amateuristic it is original. I did see this kind of white plastic indicationplate on several original war-time photo's.

The high-Z connection is established by connecting it, through a capacitor, to the primary side of the output transformer. This capacitor (paper/wax) also proves the originality of the high-Z connection because it has been stamped aug. 1944. This is the only recognizable stamp. I cannot read it's value anymore. It looks something like 0.x7 uF but could easily be something else. Measured on my C-meter it says 113nF but that is of not much value because it already shows several mA leakcurrent at 150V.

I want to replace it but could'nt find anything about it in RCA manuals or British EMER's. My guess is that it's a 470nF/500V C, but I am not sure. I guess anything between 100nF and 1uF will do, but I like to know the original value. Does someone now the exact value, or has documentation about the R1556A specifically?

I attached some photo's. The bathtubs where leaking oil and the rubber seals deteriorated. I completely gutted the bathtubes including the seals en solderlugs (taking precautions in dealing with the PCB containing oil/residue). After that I restored them using Philips capacity less feedthroughs. Although not original anymore it looks very acceptable to me.

'73 Mans - PA2HGJ
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Old 11th Nov 2012, 4:04 pm   #2
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Default Re: R1556A High imp. phones

For the hi-Z headphone connection, 0.47uF sounds about right- response will be about 3dB down around 300Hz at the LF end for 2000R phones.

For safety, use a couple of 1uF X-rated in series or a 0.47uF Y-rated cap if you can find one.
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Old 11th Nov 2012, 4:56 pm   #3
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Default Re: R1556A High imp. phones

I have an R1556A and the capacitor feeding the Hi-Z headphone jack is 0.1uF.

Nice front panel by the way. Mine has a milky appearance.
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Old 11th Nov 2012, 5:43 pm   #4
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Default Re: R1556A High imp. phones

Very interesting!

Nice work, Mans.

My R1556 has a label inside, on the outer face of the right-hand front-panel triangular gusset plate stating that it's an R1556 and that it was modified in 1955 mod number 213/1
It differs from yours in that it has a PVC wiring harness, presumably dating from 1955.

I'd always thought that someone later had drilled the extra hole in the front panel where you have the high-Z phones socket. So now I know better. The toggle mains switch I knew was 1955 vintage because it's included in the beautiful wiring harness. The mains switch section on the mode switch must have been a known weakpoint then.

What I've also found, which I hadn't noticed before seeing your photos, are three small tapped holes in the panel, one of which lines up with the left end of your plastic legend plate. It looks like there was anothe mounting screw at the right end and one along the bottom. Mine didn't have a plastic plate when I got it, and someone had added another switch in the high-Z jack hole.

This is a very early AR88 that got converted into R1556. The front panel is engraved and it carries a real RCA S-meter. The tuning scale is all black figures on yellow background.

I'm planning on rebuilding it this winter... and now I don't have to fill in a switch/jack hole I thought shouldn't have been there!

Thanks,
David
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Old 11th Nov 2012, 6:20 pm   #5
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Default Re: R1556A High imp. phones

Intriguing: though I can't provide an answer to your query, the 'modifications' to your AR88 front panel match up with a couple of unexplained bunged-up-holes in the front-panel of my AR88.

[they're filled with rather accurately machined plugs on mine - kinda like someone wanted to reverse the modification?]

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Old 11th Nov 2012, 6:50 pm   #6
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Default Re: R1556A High imp. phones

Yes interesting. If you had not researched the white panel Mans, I would have assumed it must be a much later addition and not original. If some sets were modified or produced to accomodate this addition, why white? Was it just available? Was it to ID this particular version of the set, or simply to assist in finding that end in semi darkness? My 88 has the central jack socket but no blanks or markings on either side. It is labelled Slave 31 [top right] and I've been told by Sean W that it was probaly part of a diversity unit. Personally I would find the white panel a bit irritating [especially on such a great restoration] even if it is absolutely original but as I've done nothing much with mine yet, perhaps I shouldn't really comment.
Dave W

Could this high impedance Mod have had anything to do with the output stage design, so famous for blowing up the Output TX I wonder?

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Old 11th Nov 2012, 7:11 pm   #7
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Default Re: R1556A High imp. phones

For reference, here are a couple of pics of my somewhat-scabby AR88D [it spent five years in an unheated shed] which has clearly had the modifications reversed:

I wonder what the small hole bottom-right is for?
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Old 11th Nov 2012, 8:44 pm   #8
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Default Re: R1556A High imp. phones

Mans, that is a nice front panel.

No holes for the additional phones socket or mains switch on mine, though the octal power option socket on the rear panel had been replaced (proficiently) by a 5-pin type I recognised as being used by the UK military. Perhaps individual branches of UK services added particular mods to suit their own purposes. I replaced this with a blanking plate punched to suit a press-in type IEC filter. (I have a lot of sympathy with keeping things original but I used to do quite a bit of safety testing/appraisal so I'm a bit chairy about primary-side stuff. One hard-and-fast rule I have is no cutting/drilling of original metalwork, it doesn't take much to cut/fold adapter metalwork and use existing fastener points)). I'd heard that the original mode switch mounted mains switch was electrically fragile but hopefully my fitting of a 220ohm NTC inrush limiter gives it an easier life.

I have to confess I didn't go as far as opening up and restuffing the bathtubs, I simply fitted new caps on a point-to-point basis- luckily the IO valveholders and plentiful chassis tabs make this straightforward. Your bathtub shot certainly shows up the difference in volume between old and new types! I like the feedthrough idea.
I regard the white tubular Aerovox capacitor in two of your pictures as something of a liability- unusually for a tone-correction capacitor, it's connected between O/P valve anode and chassis, making it a potential O/P transformer killer in the event of short-circuit. It might have been a quality component in its day but O/P transformers are scarce now, I replaced mine with a 2700pF 1600V polypropylene as cheap insurance,

Colin.
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Old 11th Nov 2012, 9:05 pm   #9
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Default Re: R1556A High imp. phones

Thanks everybody for all the replys so far, what a magnificent forum this is.

For the moment I fitted a 0,47uF/400V polyester capacitor. The cabling runs from the bakelite phonejack through cable lugs mounted on the RF-Deck. The wire used has cotton/rubber isolation, as frequently used in that era.

Quote:
Originally Posted by Station X View Post
Nice front panel by the way. Mine has a milky appearance.
My receiver also was in quite a state. Although fully original it was very dirty/greasy. This was my luck, because where there's grease it won't rust. I've taken the RF-deck from the receiver and thoroughly cleanded the chassis and RF-deck. I cleaned the front panel using a degreaser en brush. After that I rinsed with water, dried it and then rubbed the front panel with a tiny bit of parrafine oil.

Quote:
Originally Posted by dave walsh View Post
Yes interesting. If you had not researched the white panel Mans, I would have assumed it must be a much later addition and not original.
Apart from the white panel there are other modifications wich seem to date from WWII period also. On the back there is a socket and plug mounted. According the indication it should be used to connect "Control Unit 88". If the Control Unit is not used a shorting plug should be placed. The socket and plug carry the typical Air Ministry markings (letters A & M and the crown).

The socket is wired as follows: R55 (connected at the RF Gain pot.) is taken from GND and connected to the socket on the back. The other pin of the socket is connected to GND so with the shorting plug in place R55 is again connected to GND, otherwise it is connected to "Control Unit 88". Presumable "Control Unit 88" is used to mute the receiver when used with a transmitter or maybe Control Unit 88 is used in combination with Diversity Receive. Does anybody know this "Control Unit 88"?

Also visible on the last picture is the hole on the right side where once an alternative Aerial Socket was mounted. My first impression was someone mounted a PL-259 socket, but that doesn't fit. The hole is bigger, and the screwsholes are wider apart.
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Old 11th Nov 2012, 9:14 pm   #10
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Default Re: R1556A High imp. phones

Quote:
Originally Posted by turretslug View Post
I regard the white tubular Aerovox capacitor in two of your pictures as something of a liability- unusually for a tone-correction capacitor.
Yes, you are right about that cap. I didn't mention it but this Aerovox is also oilfilled (and mine was sweating oil). You can quite easily push the metal can out of the cardboard enclosure. After that I cleaned the enclosure with degreaser and restuffed it with a 2,7nF/500V polyester cap. Luckily modern capacitors are a lot smaller
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Old 11th Nov 2012, 9:15 pm   #11
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Default Re: R1556A High imp. phones

Tanuki, if you look due left of the plugged mains toggle hole, you'll see a tiny plugged screw hole, and another along where the lower edge of the plastic would have gone.

These are the mounting screws for the plastic modification plate, and somebody really did a job on reversing the mods. I wonder why? Is it the original panel of the set?

I think I'll stick with the toggle mains on mine. I'll also bridge the HT contacts on 'trans' so that the set can't be run with HT unloaded.

I don't fancy having the high-z phone socket. I remember now, I got a shock off the damned thing, the blocking C was leaky. Later, a switch went in the hole for a crystal calibrator I fitted in about '68 and removed in the early 1980s.

David
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Old 11th Nov 2012, 9:30 pm   #12
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Default Re: R1556A High imp. phones

You're right - I hadn't noticed that second small hole! As far as I am aware this is the original front-panel on this particular AR88.

It's always intriguing to dig back through the modification-history of antique gear and try to work out what was done and why. This particular AR88 also had a strange antenna-socket fitted to the rear when i got it - a diecast aluminium coaxial job with threaded retainer, which looked rather like the sort of thing that you'd find on WWII radar gear. But not the "Pye"-type we know and love from 19-sets etc.

I removed this odd socket because i couldn't source a plug for it. If anyone's interested i could take a photo of the hole where it was fitted..

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Old 11th Nov 2012, 9:50 pm   #13
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Default Re: R1556A High imp. phones

MY R1556A also has the white panel, so I assume it was part of the conversion from an AR88. It also has the connection for "Contol Unit 88" which I've never been able to find any information on. I assumed it was some kind of muting unit.
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Old 11th Nov 2012, 9:50 pm   #14
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Default Re: R1556A High imp. phones

I've still got it in place.

And I too never found a plug

David
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Old 11th Nov 2012, 9:52 pm   #15
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Default Re: R1556A High imp. phones

Mine has the correct shorting plug if that's what you're referring to.
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Old 11th Nov 2012, 9:54 pm   #16
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Default Re: R1556A High imp. phones

Wasn't 'control unit 88' the metering and diversity control panel in the 3-pack rack?

Either that or something from 'Get Smart' I always thought.

They could never have done a 4-pack AR88 diversity system, the risk of creating a gravitational singularity is just too high.


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Old 11th Nov 2012, 9:58 pm   #17
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Default Re: R1556A High imp. phones

I think we've crossed purposes, Graham, Tanuki was talking about the huge coaxial antenna socket that got added as part of the R1556 setup, and I also have the 2-pole AGC connector and its shorting plug, which is what I think you've just mentioned.

One mystery about AR88s... to say so many of them went in racks and didn't have S meters etc, there seems to be no shortage of table-top cabinets for them. Odd. Quite different to the RA17 situation.

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Old 11th Nov 2012, 10:04 pm   #18
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Default Re: R1556A High imp. phones

It took me some time to remember, but now I've recollected.

In episode 2 of the 1977 BBC documentary "The Secret War" there is a shot of a Wireless Operator using a R1556 with the same white plate on the front panel.
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Old 11th Nov 2012, 10:37 pm   #19
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Default Re: R1556A High imp. phones

Quote:
Originally Posted by Mans Veldman View Post
I want to replace it but could'nt find anything about it in RCA manuals or British EMER's.
Mans

the R1556 designation came from the RAF. So you won't find anything about the modifications in either the RCA manuals or EMERs. EMERs were specifically British Army documents.

There were quite a few variants (and sorry I don't know anything about them): R1556, R1556A, R1556B were covered by A.P.2567A, and R1556C was covered by A.P.2980K. I've not seen copies of any of these docs, but no doubt someone will have a copy.


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Old 11th Nov 2012, 11:11 pm   #20
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Default Re: R1556A High imp. phones

Ah yes Mans I've got the first two Secret War episodes myself but didn't notice the white plate. After being told how rare the series was I picked up 2 and then 1 during a World War 2 Weekend in Ramsbottom. Strange!

An 88 control unit for Diversity, then perhaps it relates to my slave set?

Re the gravity problem [David] I recently read a copy of Secret Listeners
[volounteer civilian monitors WW2] describing an AR88 being delivered to a home address by two men who struggled with it.

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