Old 700/PYE telephone system help

[beko1987]

Hi All.

I'd better introduce myself before my first post. My name is Sam, and I have started collecting old telephones (without meaning to).I have a Trimphone (forget the date and model), 8746L from 1983, 8746L from I think 1984, and an 8755 (forget the date again). Didn't mean to start collecting, but I reckon 4 warrants as a collection.

However, today I got 4 more!

I have no idea about this at all, wondered if anyone could shed any light on it? It needs a fair bit of restoration, the voice volume is hardly there on the phones (although one connector has a snapped wire), the buzzers are faint and prone to cutting out mid buzz and all the handsets need a good polish.The power supply must hold some juice as I can buzz each phone even when unplugged.

The cord was cut to the power supply, so I chopped the end off a spare pc kettle lead and wired that up. The unit vibrates softly when powered, is that good?

Pictures are here (along with my 8755: https://picasaweb.google.com/1006131...eat=directlink

The phones seem to be a bit Frankenstein inside, like someone's project, and it would be nice to restore it to full working order. Also, would the power supply for the unit be man enough for me to rig up a bell'd normal phone? I'm sure I could knock up a split connector on one of the leads to plug a home made bt plug adapter.

Going to take the handsets apart tomorrow, anyone mind if I post up more pics?

[AC/HL]

It appears to be a simple intercom, each phone able to ring the others but probably a common speech circuit. The bells must have been replaced by buzzers, as the 10V (on load) from the PU53 will not ring the originals. If the buzzers cut out they possibly need a bit more voltage, 10 is quite low. The hum is probably just transformer buzz. Can you measure the output voltage (assuming you're comfortable with the cover off)
The only restoration possible is to do a bit of tidying, there will be nothing to copy.

[kernowcam]

I might try to get my dad to join this forum. He was a GPO / BT engineer for 40 years and worked on a lot of this kit.

[beko1987]

That would be great if your dad could shed some light on them, and thanks for the info above as well!

I don't think its genuine GPO apparatus, unless it's had some recent work. The phones have RS Components resistors, which look fairly new. I think they all need some receivers as well, as speech is very faint. Would the red modern ones from 8746s work? Can get those fairly easily at car boot sales.

My main aim with these is fairly long term. We live in a 2 bed flat at the moment, and with the Missus about to give birth to our first child, need to expand. When we get a place with a garage or shed, I will wire them in as intercoms, maybe even convert to a nicer handset if I find one, either way I've been reading the vaious threads on here about making an intercom out of old phones, I now have one ready made!

I will clean up all the contacts and take the rest of the bell bits out to clean up inside, will post pictures when I do.

Cheers

Sam

[AndiiT]

Originally Posted by beko1987

...Would the red modern ones from 8746s work?....

Hi,
They might, but the red 21A transmitters like a bit more voltage than the original carbon granule ones so you might have to increase the voltage to around 15 volts instead of the 10 or so that the PU53B power unit produces.

On the subject of poor speech this may be caused by the speech circuit itself not being correctly wired, although if you obtained the system "as is" so to speak there is no reason that there should be any dodgy wiring unless you suspect this is the case.

Depending on how the speech circuit is arranged the speech "battery" (for this read 10 volt dc power supply) will be with in series with the 'B' legs of the two communicating telephones with the 'A' legs used as a common return path or, alternatively, there will be a transmission bridge somewhere usually consisting of a choke and a capacitor of around 2uF in value, this capacitor may be internal to one (or more) of the phones themselves.


Regards

Andrew

[Dave Moll]

In field telephones I have used a 21A transmitter successfully with a 4½ volt battery, so it may depend upon how much voltage drop there is between the supply voltage and the transmitter.

[AC/HL]

six posts moved to a new thread: https://www.vintage-radio.net/forum/...203#post641203