3.5mm jack plug/socket size?

[CURRY POWDER]

Hello, I am curious about the size of a 3.5mm jack socket, 3.5mm I hear you cry!

I have just acquired a Fi-Cord 1A miniature tape recorder c.1960, the headphone and line out jacks I assumed were 3.5mm mono sockets.

But various 3.5mm plugs don’t fit. However, an 1/8” dia. rod fits perfectly? So, my question is were 3.5mm jack plug/sockets always 3.5mm?

As 1/4” series plug/sockets are just that (1/4”) might 3.5mm actually started life as 1/8”? Also the contacts wouldn’t line up with the tip and body of the plug even if it did fit.

The attached photo is of the socket in question, it seems to be of high quality, both contacts and threaded body and nut all silver plated, the insulation of a durable plastic. Does anyone know the manufacturer?

[BRASSBITS]

You say 3.5mm doesn't fit how too big or too small?

Have you tried a 2.5 jack plug?

[Station X]

1/8" is 3.175mm so a 3.5mm jack plug will be oversized.

A 2.5mm plug would fit, but may be a bit slack.

The spec for plugs and sockets would have specified tolerances such that the largest diameter plug would fit the smallest diameter socket.

[TonyDuell]

I've just checked my Ficord 1A and you're right. It's not a 3.5mm socket. It's smaller. Mine is an original Ficord (with the 4 pin microphone socket, similar to the sockets used on some HT/LT batteries), not the BBC modified one with a DIN socket.

I remember seeing about 40 years ago a cable ending in a jack-like plug that was a loose fit in 3.5mm sockets. I wonder if that was a 1/8" plug. No idea what became of it though.

[CURRY POWDER]

Thanks both, I thought I'd had a eureka moment (about 3 o'clock this morning) and tried a 2.5mm plug, but it's far too small, doesn't make contact at all. I am wondering if it was a different standard (i.e. 1/8") to 3.5mm?, when were 3.5mm jacks first used? I must see if there is any history of jack plugs on the web. Tony.

[Reelman]

There must have been another standard at one time. The mike connector on the Italian Geloso tape recorder is also a non standard “3.5” mm plug IIRC. Sadly no longer in the collection so can’t confirm.

Peter

[CURRY POWDER]

Hello TonyDuell, yes mine is also an original, non BBC variant. It records and plays back fine, It didn't come with a microphone, but luckily I had a 4 pin plug (also as used on older Tannoy speaker chassis) Wondering if the jack sockets were a format never adopted as standard.

[TonyDuell]

The Geloso tape recorder I have (Model G255, the live chassis one) has 5mm jack sockets. This was a standard size at one time (Bulgin made such plugs and sockets for example) but they are very hard to find now.

[emeritus]

You could try checking the diameter using the shaft of a twist drill as a plug gauge, a trick I learned at Plessey.

[Station X]

I think you'll find the OP has already done that.

[CURRY POWDER]

Well, there is another size! 4.4mm PO bantam plugs. Still can't find any reference to 1/8" (3.175mm) need someone with vintage manufacturers catalogues, Bulgin etc.

[CURRY POWDER]

A useful resource; https://ips.org.uk/encyclopedia/connectors/

[TonyDuell]

I think I read that at one time Grundig (and maybe other German manufacturers) used a 6mm jack plug and socket. The 6mm plug would fit (and correctly connect to) a 1/4" socket but not vice versa.

I can't find any reference to a 1/8" jack plug in any of my old catalogues but I may not have looked carefully enough. I will keep looking.

[TonyDuell]

Found it!. There was a 1/8" jack plug/socket at one time.

I have a1968 'STC Electronic Services' catalogue here. Along with the 'standard' (1/4") and Rendar-brand 'miniature' (0.135", which I make pretty close to 3.5mm) it also shows...

An AEI-brand subminiature jack plug and socket with a diameter of 0.125". The plug is a JP200, the socket a JS200, and the latter looks identical to the ones in the Fi-Cord.

So I am pretty sure it is 1/8". But finding the plug now is going to be non-trivial.

[Vintage Engr]

R.S used to have the 1/8" ones in their catalogues in the '60s. The tip is also of a different profile. I will have a look, I did used to have some, but it was over 50 years ago!

David.

[TonyDuell]

As an aside, be warned that some American catalogues call the 3.5mm jack plug/socket a '1/8" phone plug' and the 2.5mm one a '3/32 phone plug'. These are not what you need for the Fi-cord. I would think that 1960's RS catalogues listing a 1/8" jack plug would indeed be the right one (and probably an AEI JP200 under a different name).

I can try to scan the page from the STC catalogue that shows this connector. It gives a dimensioned diagram but annoyingly for us doesn't say where the insulator comes along the 'pin'. The tip seems to be parallel-sided with a rounded end, no little groove as on most other jack plugs

From memory I am pretty sure the plug I had years ago that was a 'loose fit' in a 3.5mm socket was, indeed an AEI JP200. But the chances of me finding it are very small. But if it turns up at least I now know what it is.

[CURRY POWDER]

Well Chaps, thanks, good research there.

I was hoping I was right about there being an 1/8" size, I mean, why would the UK chose 3.5mm as a size for miniature jack plugs over half a 1/4". I guess 3.5mm won out just as VHS tapes and the like did.

Regarding the plug tip profile, that sounds about right, on the socket the tip and body contacts are spaced about the same widths (both a fraction under 1/8")

Tony D, If you could scan your STC catalogue that would be most interesting.

Thanks again, Tony

[CURRY POWDER]

I have reasonable amateur engineering skills and a modest workshop, lathe and mill etc. so I will have a go a making an 1/8" jack plug.

I may make my fortune manufacturing obsolete size jack plugs, NOT!

[emeritus]

Brass tube in 12" lengths in imperial external diameters (including 1/8") is in the K&S metals range of brassware. Model railway and modelling shops , and some hardware shops, often stock their range.

Just had a look in the garage and measured with a micrometer some offcuts of thick wall copper and thin wall brass tube I got from "Proops" decades ago. The copper measures 0.125", the brass 0.126". If they would be of any use, pm me your address and I can post you a few FOC.

[CURRY POWDER]

emeritus, thanks for your kind offer, but I do have some K&S brass tubing, I use it for fuel tank construction in model boats. I was thinking of just that material, in conjunction with a bit of Tufnol for the construction, regards Tony.