UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Telephony and Telecomms

Notices

Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment

Reply
 
Thread Tools
Old 27th Jul 2018, 1:59 pm   #1
OscarFoxtrot
Heptode
 
OscarFoxtrot's Avatar
 
Join Date: Apr 2011
Location: Edinburgh, UK.
Posts: 559
Default Payphone 500 / 600 handsets

I have some BT handsets with the armoured cord as used on Payphone 500 / 600 (the 700 style handsets, not the flat angular ones)

They are of course riveted shut preventing internal examination. They have 6 wire cords (red, grn, blu, whi, org, blk)

Are these standard 700 style inserts and on the standard colours red+grn for receiver, blue+white for xmtr? as I am having difficulties getting them working.

On one of them red+grn makes the mouthpiece work as a receiver.

I want to repurpose them for the Pratical Wireless 741 op-amp intercom requiring 700 type circuitry.
OscarFoxtrot is offline   Reply With Quote
Old 27th Jul 2018, 2:36 pm   #2
Dave Moll
Dekatron
 
Dave Moll's Avatar
 
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 3,634
Default Re: Payphone 500 / 600 handsets

If the mouthpiece works as a receiver, I assume that means it's a coil type rather than a carbon one.
__________________
Mending is better than Ending (cf Brave New World by Aldous Huxley)
Dave Moll is offline   Reply With Quote
Old 27th Jul 2018, 6:47 pm   #3
OscarFoxtrot
Heptode
 
OscarFoxtrot's Avatar
 
Join Date: Apr 2011
Location: Edinburgh, UK.
Posts: 559
Default Re: Payphone 500 / 600 handsets

I have drilled out the mouthpiece rivet and can report:

* blu/whi are used for transmitter

* transmitter is a Receiver 4T in black rubber mouldings. Not enough space for the mouldings and a Transmitter 16, and presumably they wanted to reduce handling and wind noise. There is a purple plastic sheet to spit-proof the transmitter

* org/blk are joined together and sleeved in the transmitter compartment, presumably to provide a cut cord detection circuit

* red/grn go up the handset to the receiver. I haven't drilled open the receiver as the damage would be noticeable there, but it clicks nicely on a PP3 battery and I assume it's also a 4T.

*there appears no way of undoing the armoured cord entry and the wires are very reluctant to pull through any longer.

I was a bit surprised the mouthpiece was only riveted, and not glued shut as well, I don't think it would be possible to re-rivet it closed after drilling out, so the entire handset must have been treated as non-repairable.

Having now verified the handsets are all intact I can get to work on the circuit. I'm hoping that simply removing the resistor providing battery voltage to the carbon transmitter will do for starters ...

The circuit I'm building is here
https://www.vintage-radio.net/forum/...8&postcount=20
"741-field-telephone-system-intercom-PW1980.pdf"
OscarFoxtrot is offline   Reply With Quote
Old 7th Aug 2018, 7:19 pm   #4
OscarFoxtrot
Heptode
 
OscarFoxtrot's Avatar
 
Join Date: Apr 2011
Location: Edinburgh, UK.
Posts: 559
Default Re: Payphone 500 / 600 handsets

Despite drilling out the rivet, the receiver does not want to open. There's definately something in there other than a standard 4T. I suspect possibly an amplifier and coil for hearing impaired users.

It clicks quite loudly with one polarity from a PP3, but only faintly with reverse polarity, and gives extremely (unusably) low volume on the field telephone circuit (which works nicely on standard handsets).

The circuit also, incidentally, gives quite astonishing volume into a 330 ohm (30 watt 100 volt line) PA horn speaker.
OscarFoxtrot is offline   Reply With Quote
Old 10th Aug 2018, 1:45 am   #5
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 11,977
Default Re: Payphone 500 / 600 handsets

A PA horn will give as much audio from 10mW as 100mW into a typical cone speaker. Adequate listening power from a tranny in other words.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline   Reply With Quote
Old 15th Aug 2018, 8:50 pm   #6
OscarFoxtrot
Heptode
 
OscarFoxtrot's Avatar
 
Join Date: Apr 2011
Location: Edinburgh, UK.
Posts: 559
Default Re: Payphone 500 / 600 handsets

I can now report that

- wrapping doublesided sticky tape round the earcap provides sufficient grip to remove it.

- inside there is a Receiver Inset 2/3T and made by Hosiden-Besson (Code DAE). Splitting this open shows a small moving armature receiver (a normal 3T?) connected *in series* with a large coil round the outside for inductive coupling.

see bottom of
http://www.samhallas.co.uk/collectio...3_variants.htm

So I guess my question now is why won't a 2/3T work on a circuit where a 4T does? There are no transistors or anything else active in the 2/3T so it can't be needing a DC bias voltage.

Any ideas anyone? I'd really rather avoid drilling out all the receiver caps to change the inserts.
OscarFoxtrot is offline   Reply With Quote
Old 15th Aug 2018, 9:13 pm   #7
Dave Moll
Dekatron
 
Dave Moll's Avatar
 
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 3,634
Default Re: Payphone 500 / 600 handsets

Quote:
Originally Posted by OscarFoxtrot View Post
There are no transistors or anything else active in the 2/3T so it can't be needing a DC bias voltage.
The only other reason for needing a DC voltage there that I can think of is to energise a field coil if a receiver uses one of these instead of a permanent magnet.
__________________
Mending is better than Ending (cf Brave New World by Aldous Huxley)
Dave Moll is offline   Reply With Quote
Old 15th Aug 2018, 10:13 pm   #8
OscarFoxtrot
Heptode
 
OscarFoxtrot's Avatar
 
Join Date: Apr 2011
Location: Edinburgh, UK.
Posts: 559
Default Re: Payphone 500 / 600 handsets

Quote:
Originally Posted by Dave Moll View Post
The only other reason for needing a DC voltage there that I can think of is to energise a field coil if a receiver uses one of these instead of a permanent magnet.
Interesting idea.

Inside the capsule and with the induction loop removed there are two coils either side of a small magnet on the armature. Bypassing the induction loop gives a loud click on one polarity and no click on the other polarity, but I'll experiment further tomorrow.

Thanks.
OscarFoxtrot is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 10:39 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.