Advice on 'Intermains' Intercom units from 1950's?

[TonyDuell]

I am pretty sure M, MR and R are for the handset (Microphone/mouthpiece, microphone and receiver, receiver) and should be left wired as they are.

Try this :

Link RCTC on the 2 sets, and to the +ve terminal of a 4.5V or 6V battery

Link RZ to CZ on both sets, link them together (so that's a total of 4 terminals you have linked) and to the -ve terminal of the battery

Link HL on the first set to '1' on the other set

Link 1 on the first set to HL on the other set.

Have both handsets on their rest. Lift one handset and press button 1 on that set. Other set should buzz. Lift other handset, you should be able to talk between the sets.

As for the interconnecting cable, I'd replace it. You can use modern PVC-covered multi-core cable (it won't 'look right' but electrically it will work).

[Dave Moll]

Originally Posted by Woody Woods

I have just got my hands on the other 4 intact units, and pulled out two at random.... I suppose, with there being 4 of them, I could try to get two working, and rob spare parts off the other two where needed...

Could you not connect all four together such that each of the first four buttons calls the appropriate other terminal?

Incidentally, are the buttons latching (i.e. remain down once pressed)?

[Woody Woods]

Further to my earlier post, having just been and looked, one of the 2 spares appears to be suffering similar problems with sticking buttons, and the other one doesn't seem too bad.... Could just be dirt and crud buildup I suppose; I won't know until I take stuff apart for cleaning! Will try to start with whichever two appear to have the fewest mechanical issues, and use the remaining two as donors if needed.....

My brother-in-law seems keen to try and hawk these around some antique dealers or on online auctions in the hope of striking megabucks, (having poo-poohed my lowball opening gambit of offering him 20 quid for the lot), but I think he is being optimistic and assuming that just because they are old, they must be super-valuable..... The way I see it, one might be lucky to get two working ones out of the four, with a labour of love to clean everything up, replace bits and pieces, buy and rig up batteries etc (which I am willing to do as an interesting project, if I can get them cheap enough to start with!) and they are FAR from being in Mint Condition, plus being a fairly 'niche' market to find someone locally here in Ireland who would be interested in them.... ? What do you folks think?

Also, further testing may also reveal that the carbon mics, earpieces, or whatever other components no longer work reliably, etc, I won't know until I get that far! I'd be interested to see if he does get any other offers on them, and I'm sure he'll give me first refusal to beat whatever the best offer turns out to be!

Sorry, I hope it's not taboo that I'm asking about their possible value here, but I'd appreciate some guidance so that I can offer him a sensible price to take them off his hands, that we both consider a fair deal! If anyone has any opinions, please PM me or something....

I love to see if I can get them to work again for my own interest and amusement, even if he does end up flogging them to someone else!

Woody

[Woody Woods]

Originally Posted by Dave Moll

Could you not connect all four together such that each of the first four buttons calls the appropriate other terminal?

Incidentally, are the buttons latching (i.e. remain down once pressed)?

Hi Dave and Tony, cheers,

It would be great to connect all four together, but baby-steps! I am new to all this malarkey... I'd be chuffed to just get a pair of them working together, and being cautious, in assuming that some parts may have failed on some of them, and some swapping-about of bits may be needed.... Not sure until I get further into testing!

Yes, the buttons appear to stay down once pressed, until the receiver cradle is depressed, which makes them spring back up... Except the ones which stick in the down position, and nothing engages when the cradle is waggled back and forth!

I intend to collate all of the previous technical replies from folks in this thread into a single easy-to-read document which I can pore over at length and get my head around, as I am very new to wiring diagrams etc and on a steep learning curve at the mo.... Further questions will doubtless occur to me as I go along!

It's handy to know that there is a modern cable which will do the job... (Am I right in recalling that modern cable can be inserted into a braided shoelace, for a more 'period' look, assuming that one has far too much time on one's hands?!)

I am plotting to cadge a lift out to Maplins next week, so any suggestions for my shopping list would be helpful! I have a soldering iron, and some limited experience of basic soldering-of-wires-together, but it's by no means my area of expertise, hehe....

all the best, Woody

[julie_m]

Computer networking cable ("Category 5") contains 4 twisted pairs of single-strand conductors. You can get specialist comms and networking cables with up to 25 twisted pairs, though you probably don't need that many connections for just 2 - 4 stations.

I'll try and get my head around those wiring diagrams, see if I can work out how they connect to one another .....

[Woody Woods]

Thanks for chiming-in Julie, I appreciate it
I'll have to have a rummage in my electrical odds-and-ends box and see if I have any Cat5.... (Fiddlesticks, I used to work in tech support, I knew I should have pocketed a load from Stores whenever I left that job! I'm the Biggest Luddite Ever really.... long story!)
It's handy to know that Cat5 would do the trick, I was vaguely pondering along those lines but hadn't arrived at that conclusion yet, so thank you!! I'm sure it won't be hard to get hold of some from somewhere, on the cheap Please keep the suggestions coming...

Many thanks, Woody

[TonyDuell]

I suspect that although the buttons latch down (released when the handset is put on the cradle), they also make another contact when pressed (which opens when you take your finger off the button). On the diagram the first contact connects the line termianl to the upper horizontal line, the second to the lower line (as well).

Any wire will do for testing. The voltages and currents involved are very small. You could just use some single wires (preferably the stranded stuff as it bends more easily without breaking).

I am sure the sticking buttons can be repaired. Probably just needs the dirt and grease cleaning off the mechanism.

[Dave Moll]

OK, thanks for confirming that pressing a button makes a connection with the selected station until the handset is replaced. I therefore agree with Tony that the numbered terminal connects to the line (HL) of the selected station. I had become sidetracked by a system I have that uses a single speech path to all stations and non-latching buttons to signal individual stations.

The additional momentary contact Tony mentions above would make sense as the means of signalling the extension - unless it buzzes until the handset is lifted.

Yes, Cat5 cable should provide more than enough conductors for four stations: one pair for sharing the battery supply (which presumably also provides one side of the communication line) plus one conductor for each station, so you should have one pair to spare!

If you can get hold of flexible Cat5 all the better, otherwise it is all too easy to break the conductors. The number of times I've screwed a terminal down only to find the bit of wire round the screw is no longer connected to the rest of the cable!

And baby-steps are good! Get one pair connected up and working first. You should then find it pretty straightforward to add the other two.

[TonyDuell]

I have a French office telephone (combined exchange line telephone and intercom set) which has buttons that make one contact when pressed and another when held down. It therefore would not surprise me to find something similar here. If I am reading the diagram correctly, the latching position of the button connected the selected line terminal to HL (which we suspect to be the speech line of the instrument) and the momentary postion connects it to RZ (which I think is the -ve (Zinc) terminal of the ringing battery).

As for the cable, since there is no need for it to be twisted pair or screened, I would consider burglar alarm cable. It certainly exists in 4,6 and 8 cores. I think I have seen 12 core and 20 core as well. That tends to be multi-stranded and thus less liable to break.

[Dave Moll]

Burglar alarm cable sounds like a good idea.

[julie_m]

Burglar alarm cable is certainly not a bad choice for wiring up a simple intercom. There are usually a few part reels to be found in skips near office buildings being worked on .....

[Woody Woods]

Haha cheers folks.... I'm currently living up a mountain, no handy skips-next-to-office-buildings within easy pilfering distance, sadly!
I have however found some old bits of network cable squirreled away in my hoard, and robbed a 9v battery from a disused guitar pedal... Will have a tinker and keep you all posted!

PS - I expect I can get hold of extra cable either at the hardware store in the village, or next time we make a trip into the Big Smoke!
Brother-In-Law still seems reluctant to sell them to me, even though earlier I got out the soldering iron and fixed his car-diagnostic-plug-in-thing for him without so much as a murmur! Hehe...

Thanks again,

Woody

[Dave Moll]

By the way, collating the above comments about how to connect, I think it can be summarised as follows:

M - transmitter (microphone) in handset
MR (not used if M & R are connected in series within the handset)
R - receiver in handset
RcTc - battery/batteries positive
Rz - ringer battery negative
Tz - transmitter battery negative
(the three connections above are connected in parallel on all extensions, and a single battery could be used with Rz and Tz strapped together)
HL - line - connects to appropriate number in parallel on other extensions
1-10 - connects to HL on appropriately-numbered extension
EXB - ?

Best of luck!

[TonyDuell]

EXB is 'EXtension Bell' (or Buzzer). Being DC bells, you would connect them in parallel, if you want an extension bell at an extension you would connect it between EXB and RCTC. But normally you just ignore EXB.

The simpler (to remember) system of wiring is to link 1-10 in parallel at all units (that is, a wire between terminal 1 of each unit, a second wire between termianl 2 of each unit, and so on). And then to strap HL to the appropriate one of those at each unit (so at unit 1 you link HL to 1, at unit 2 you link HL to 2, and so on. HL is _NOT_ linked between units, unlike the numbered terminals and the battery connections). That will limit you to 10 units, in effect one button on each is 'wasted' as it corresponds to that unit. You can actually wire them up for 11 units, the wiring is easy in theory (and easy to work out) but a little harder to explain and as you only have 4 units, I don't think we need to worry about it.

[OscarFoxtrot]

'HL' is probably 'home line'

[dagskarlsen]

This system is made for pretty many wires to work, but vonncting HL's together, and R's(=Rc, =Tc) and putting in a battery in on phone negative to Tz and positive to R, the voice path should work between the phone with battery and the other. (You may choose to wire a wire no 3 between the T2 terminals, or having one battery in each phone.

To ring the other end, a battery of a higher voltage is needed between R (positive) and Rz (negative). Again, a common Rz wire, or one battery for each phone.

This is just a simple solution mainly for 2 phones. More advanced circuitry is will be needed for an individual ringer button to several other phones.

dsk