Testing Vintage Headphones?

[Martin G7MRV]

Hi All,

Im looking for a bit of advice on both how to test, and the wiring/structure, of vintage headphones,

Ive just acquired a pair, marked 'B T-H' and 'BBC'. Physically they look fine, but the cord has just a 'prong' on each, so I've not yet connected them to anything. The cords go into the earpieces rather than to external terminals.

I'd like to know what the arrangement is internally on these, and what is a safe way to test them. Incidentally, just holding the connectors in my hand and stroking them together got me a little crackle from at least one side (very quiet, so not sure if both sides were doing it, I might be a bit deaf!)

I'll probably wire them to a little box with a 1:1 transformer in to help protect them

Cheers for any advice

Martin

[Station X]

There'll be one receiver in each earpiece and they may be wired in series or parallel.

I'd try a simple resistance test across the wires of the cord first.

A common problem is rusting of the earpiece diaphragm, but it can be slid off and cleaned with VERY FINE glass paper.

[Dave Moll]

... and doing the resistance test should also result in a crackle in each earphone as you connect or disconnect the meter - somewhat louder than any generated from (presumably) the electric charge of your hand. If unsure whether both are crackling, listen to one at a time.

[Martin G7MRV]

Cheers Graham,

I read about a 'hum' test, which I've just tried, holding one connector and touching the other to a screw on a light switch! And yes, I got hum, but there's no insulation on the plugs so I couldnt hold it long enough! (ie, I was holding the cord which was too floppy!) I still couldnt tell if only one or both earpieces were working.

How do I discover if they are wired parallel or series? Are they 2k per earpiece, or overall?

It looks like the earpieces screw apart, but I dont want to try too hard! Is there a suitable lube that will ease the threads on something of this age and material?
(My knowledge of Bakelite is limited to its synthesis!)

And, is there a diagram about that will show me what to expect on opening them up?

[Martin G7MRV]

Originally Posted by Dave Moll

... and doing the resistance test should also result in a crackle in each earphone as you connect or disconnect the meter - somewhat louder than any generated from (presumably) the electric charge of your hand. If unsure whether both are crackling, listen to one at a time.

Very good point!

[Station X]

Careful examination of the cord will show whether the receivers are connected in series or parallel. That's assuming there's no wire across the headband.

It was common to connect two 2kohm receivers in series to give 4kohms for use with a crystal set.

[Martin G7MRV]

Where the cord splits to the two receivers on these the wire is overlapped and bound with thread, but from the shape of the resulting bundle it does look as though one of the wires in each receiver pair are a through connection, which suggests to me they could be wired in series

More 'crackle' testing shows that both receivers are working, albeit one possibly a bit louder than the other, and that my meters on the blink! So, no resistance reading as yet!

But its looking hopeful. I'm also surprised that they are actually quite comfortable!

[greenstar]

Good info here;

http://oldheadphones.com/crystal/info/repair.htm

and

http://oldheadphones.com/crystal/info/restore.html

http://oldheadphones.com/crystal/index.html

Problems may often be - open circuit lead (not your's)
open circuit coils (fortunately not in your case)
weak magnets possibly, but your's sound pretty good.
Sometimes an improvement may be had by adding or removing spacers between diaphragm and case, and magnets can be re energised.
I am a beginner in this.

[Martin G7MRV]

Thanks, some great info there. I've now had the cups and the diaphragms off (very carefully!) and apart from being a bit dusty they seem in good condition inside considering their age. I'm guessing these were made in the 1920's?

I still can't get an actual resistance reading! Must be my meter. My spare little meter only reads to 1k so I'm not getting anywhere with that either.

I might try the trick mentioned in those links about turning the diaphragms over in case theve gone a little dished over the decades. I'll wait until I've had some audio through them before thinking of remagnetising.

Which brings me to the question of putting some sound into them! I've acquired these for use with crystal sets and 'minimalist' transceivers such as Pixies, where I'd normally have to use a crystal earpiece. So I'm wondering what to connect to them to try them on real sounds! I'll use a transformer to keep any excess DC off.

[Radio Wrangler]

The BBC marking suggests they were intended for speech/music broadcasts.

There are also phones with resonant diaphragms on the go which were made to filter Morse signals. Morse is usually a lot less entrtaining thatn what the beeb put out, though not always. I learned Morse because a friend made tapes for me of lots and lots of risque rag magazine jokes. I played cassette copies in the car for practice. Hearing morse coming from a car with a driver gassing himself laughing may have seemed slightly odd...

David

[Herald1360]

Some DMMs go a bit loopy trying to measure inductive resistors. Try a series circuit of 1.5V cell, headphones, meter on mA and extrapolate from there.

Does sound like they're probably fine, though.

[Martin G7MRV]

Ah! Yes, never considered the fact its a coil!!!

My DMMs a bag of *** anyway! I suspect the works Fluke 77 would have no issues...

[paulsherwin]

They are normally 1k per earpiece, wired in series.

[Phil G4SPZ]

Don't expect high audio quality... proper bass and treble will be lacking, and there is usually a pronounced resonance around 900Hz. But don't worry about DC passing through them. Many early receivers placed the headphones directly in series with the valve anode.

[PJL]

and by now 50% are open-circuit due to corrossion of the wire...

However, they are easy to rewind by hand and quite often the fault is at the top of the winding.

I got some very pretty gecophone ones recently but all 4 windings are open circuit.

[AC/HL]

Even a crystal set produces some DC.
If there is any, perhaps it can usefully assist the almost certainly weak magnets, if they're connected the right way round.

[Herald1360]

Originally Posted by Martin G7MRV

My DMMs a bag of *** anyway!

Time to get an AVO

I believe Maplin no longer do the pretty good value ProsKit analogue MM that a few members, myself included, bought.

[greenstar]

PJL, I am interested to know how you go about rewinding. Many say the wire is so fine it's virtually impossible by hand.

[Station X]

My experience is that it's only possible by hand. If you try to use any form of powered winder, the wire snaps at the instant you switch on.

[PJL]

I can't remember the SWG I use but it is very fine but surprisingly strong. I use a very simple hand operated jig. They are rough wound so no special skills required other than patience! Something to do when watching the TV.