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Old 3rd Dec 2017, 8:12 pm   #1
G6Tanuki
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Default A Hallicrafters S27

Last weekend Dog [AKA D-HUND] and myself took a trip down to Devon to visit forum-member "Mikebay" and his inamorata in order to collect a Hallicrafters S27.

After sharing a coffee and some interesting conversation [even their cat seemed to take to me!] we returned home (via an hour-long dogwalk on Dartmoor in the sleet..._). The S27 has spent the last week in my 'office' in order to dehumidify. 20% relative-humidity for a while is always good to dry things out.

Looking at it - it's clearly been "got-at" - not entirely unexpected given that it's 70+ years old.

There's a "Pye"-type coaxial connector fitted to the rear panel as an antenna connection and the front panel's been marked to indicate this (though it's rather rubbed-off).

The S-meter on the front panel has been replaced with something distinctly non-original, and the S-meter zero pot has been removed.

The output-transformer's been replaced with a rather-larger non-standard one [could the S-meter and zero-pot have been reworked to make room for this?]

The mains-transformer looks like a non-standard one too - it's jammed up too well against the voltage-stabiliser valve to have been original - but if it is a modification it's really quite a historic one.


I'm pondering: unless I can get a genuine replacement output-transformer, should I fit a more-suitable smaller one [maybe converting from P-P output to a single-ended output stage? Easily done just by pulling out one of the 6V6s] in order to replace the S-meter-adjust pot in the right place?


And, I'm also wondering - the fitting of a Pye-style coax socket and the labelling on the front - given the history of these radios being used for interception of German 'beams' transmissions could this modification have been a genuine WWII-era one?

Whatever, it's going to spend a few more weeks drying-out before I show it mains.
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Old 3rd Dec 2017, 9:11 pm   #2
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Default Re: A Hallicrafters S27

Does the mains transformer have a voltage selector switch on it?

I've owned two Hallicrafters, SX17 and SX28, both fitted with 115/230v dual voltage transformers as original.

Lawrence.
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Old 3rd Dec 2017, 9:15 pm   #3
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Default Re: A Hallicrafters S27

It's an iconic receiver, in my opinion - and the history made it a must have for me. I've yet to see one with an original o/p transformer - mine at least had a replacement push-pull item. Problem is that the original had a tertiary winding for the tone control (odd to have a comms receiver with a "Bass-boost" facility!). A bit deaf, of course, by modern standards - not helped by the fact that a PO had attempted to tweak it up to 2 metres. I'm afraid mine only gets used for broadcast FM. Got to love the acorn valve front end, though.
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Old 3rd Dec 2017, 10:14 pm   #4
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Default Re: A Hallicrafters S27

Hello G6Tanuki

Like others, I have (or had) several Hallicrafter products of 1940s vintage and the transformers always seem to be a talking point, so to take each in turn.

Mains transformer, Hallicrafter used two types the "normal" one for the American market which only supported 115v volts. The other was a "universal" type made for the international market and is identified by being rather slimmer and taller that we would normally expect, and with a voltage switch on the top.

Output transformers. These would seem to be the Achilles heel of these sets and are regularly changed. I have not had to do this on my sets (yet) but American hobbyists have identified one in current manufacture which fits the bill.

My research into S27 use during the "battle of the beams" makes no mention of the sets being modified in any way - but that does not mean they weren’t!


Hope that helps.

TYJ
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Old 4th Dec 2017, 10:06 am   #5
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Default Re: A Hallicrafters S27

Two posts moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=141897
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Old 4th Dec 2017, 4:19 pm   #6
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Default Re: A Hallicrafters S27

Hi All. THis one doesn't have any visible voltage-selector on the mains transformer though I have yet to take the case off and look underneath. The size/shape of the mains Tx just doesn't scream 'original' to me. It's definitely not slim and tall, indeed I'd describe it as distinctly squat and lumpen.

I'll have to investigate output-transformer replacement, as I can't reinstate the "S-meter Zero" pot with the current kludged-in-place transformer-impostor where it is.
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Old 4th Dec 2017, 5:57 pm   #7
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Default Re: A Hallicrafters S27

The output transformer was probably replaced because of the odd secondary impedance, 500 and 5K ohms, instead of the usual 4 to 8 ohms normally used with a speaker. If the original Hallicrafters speaker was purchased with the receiver, it would've been fitted with a matching transformer.
BTW, the original issue Hallicrafters speakers are as valuable as the receiver, or possibly a bit more.
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Old 4th Dec 2017, 5:58 pm   #8
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Default Re: A Hallicrafters S27

Quote:
Originally Posted by G6Tanuki View Post
Last weekend Dog [AKA D-HUND] and myself took a trip down to Devon to visit forum-member "Mikebay" and his inamorata in order to collect a Hallicrafters S27.
...
After sharing a coffee and some interesting conversation [even their cat seemed to take to me!] we returned home (via an hour-long dogwalk on Dartmoor in the sleet..._).

How exciting! I've always been intrigued by these since reading how a supply of them was bought up by the RAF wholesale from a shop in Tottenham Court road and fitted, in short order to British bombers in WWII...

What a wonderful introduction to your set and I hope you have many happy hours and not too much frustration, bringing it back to life and use!
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Old 4th Dec 2017, 6:31 pm   #9
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Default Re: A Hallicrafters S27

I hadn't realised that the original O/P Tx was designed to load into a couple of 'unusual' impedances - there are two pairs of binding-posts on the rear of the chassis that are too gungey at the moment to read the impedances marked on them.

[An impedance-matcher built into the speaker was not unusual: the HRO Senior had the same arrangement, though *that* had the anode current to the output valve flowing out to the speaker and back again. Destructive screen-current in the output-valve if you didn't connect the speaker!]

The story about a RAF 'officer' [actually one of R.V.Jones's team - technically a civilian - in a hired uniform] being ordered to go to Webbs Radio in Soho and get all their S27s is told in the "Secret War" BBC series and associated book: apparently he bought their entire stock - on credit!
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Old 4th Dec 2017, 6:38 pm   #10
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Default Re: A Hallicrafters S27

Hallicrafters SX28's also have output transformers with high impedance outputs fed from a push pull output stage, so hardly designed to feed Hi Z headphones.

What surprises me is that the matching speaker units also had a matching transformer to match normal Lo Z speakers fitted in the cabinets.
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Old 4th Dec 2017, 6:47 pm   #11
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Default Re: A Hallicrafters S27

Looking at the [barely readable] S27 schematic I have here, it appears that the headphone-socket wiring also has association with the grid-circuit of one of the push-pull 6V6s, as well as a contact that would appear [reading through the scanning-artifacts] to open-circuit one of the O/P Tx secondary-windings-to-rear-binding-posts.

Plug in [high impedance] headphones and get them connected to the grid-drive of one 6V6 and at the same time disconnect the speaker?

I really need a more-legible schematic (and I use that particular word precisely because it's what Hallicrafters would have called it...)
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Old 4th Dec 2017, 7:08 pm   #12
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Default Re: A Hallicrafters S27

This one's not too bad

http://http://gmcotton.com/ham_radio/misc%20manuals/Hallicrafters/Hallicrafters_S27%20VHF%20Reciever_Service%20Manua l.pdf
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Old 4th Dec 2017, 7:11 pm   #13
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Default Re: A Hallicrafters S27

Having found a more-legible schematic [for the US Navy RBK27 version] which includes above- and below-chassis photographs, it's now really clear that the mains transformer on my example is an impostor. The impostor is a *lot* bigger than the real one.

Similarly, I can now see what's been done with the output-Tx and the S-meter zero pot: Pot has been moved to the space where the output-Tx should be [which means there's no way to access the adjustment...] and the impostor output-Tx has been fitted [at an angle] onto the space where the pot and its bracket used to sit.

You can actually see the edge of the output-Tx if you look through what was the S-meter-zero pot hole.

I'm hoping it should be possible to move the original pot and its bracket back to its rightful place without too much effort, then see about getting a replacement output-Tx that fits.

Much of the wiring is rotten [it's the cotton-over-rubber type] - insulation turns to dust when poked. I'm not going to be showing this beastie any kind of mains for quite a while!
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Old 4th Dec 2017, 7:12 pm   #14
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Default Re: A Hallicrafters S27

Brilliant! Thanks! Thats the official Hallicrafters version, I'd been working from the US Navy [RBK27] equivalent.
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Old 4th Dec 2017, 7:20 pm   #15
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Default Re: A Hallicrafters S27

Glad to help, like Mr Astral I've been interested in these since reading 'Most Secret War' and living in sight of RAF Wyton's runway.

Andrew
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