Adding an analogue Voltmeter to a set of Leich 901s set up as an intercom

[JacobCoffin]

Hello! My name is Jacob, thank you for letting me join the forum!

I've recently started putting together an intercom using two Leich 901 crank telephones using advice from these forums and the excellent writeup on Valhalla Tree Farm's website.

I've got them both working - they can ring each other and talk just fine. One will be a basic desktop unit with a battery box, the other is mounted to an oak display board with two cutouts to display the wiring and batteries and an analogue voltmeter (picture attached).

My goal is for it to work as a sort of physical wiring diagram/teaching tool.

[im g]https://i.imgur.com/2xJzty8.jpeg[/im g]



[im g]https://i.imgur.com/fCwMOxK.jpeg[/im g]



So far, so good. Where I ran into trouble was adding the voltmeter. I got the idea from Valhalla Tree Farm's website, under one of the photos they said "Here's a shot of the garage Leich phone. Note the small voltmeter used to monitor the DC Talk Voltage. When you speak, the needle moves rapidly like a VU meter!"

That seemed simple enough. I had assumed (yeah, I know) that with just three wires and an analogue system, the talk signal was transmitted by varying the voltage on one of them but I'm not seeing that when it's hooked up.

(The voltmeter I got is 0-5 volts, supposedly rated for over a thousand volts for a minute, so it seems to tolerate the ringer just fine.)

[im g]https://i.imgur.com/JUXL4fT.png[/im g]



[im g]https://i.imgur.com/RxbNqaF.jpeg[/im g]



Connecting the voltmeter to the yellow and red wires shows the expected 4.5V, and connecting it to green and yellow shows 3V which I'm not sure what that's about. I had expected that connecting it to red and green (often marked as T and R or L1 and L2 on the wiring diagrams) would show the talk signal but it sits at zero even when the phones are in use. The other two readings are completely steady even when talking. At this point I'm not sure what's being modulated to send the signal.

I'm very new to antique telephones (though I've been reading these forums) and though I can handle basic residential wiring I'm kind of past my understanding here. I guess my questions are: how did these old phones send the talk signal over the wire? and if possible, does anyone have any advice on how to hook up the voltmeter to work as described above?

[5415551212]

Welcome to the forum
I looked up the system you mentioned Valhalla Tree Farm has a page on it:
https://www.valhallatreefarm.com/magneto%20phone/phonemagneto.htm

So your volt meter is showing correct just not acting as a vu meter?
I would imagine you should connect the voltmeter across the + and - right at the battery and insulate it from ringing AC voltage.
The Valhalla setup appears to be using several old car batteries that probably put out a few more volts, they say their running at 6 volts and show 1 amp fuses on the batteries.
I am not sure what your AA batteries can put out but probably less than 1/2 amp (500mA)
I am guessing here but my guess is you need to up the voltage and possibly add some caps to the power supply.

 

[JacobCoffin]

Thank you for taking a look! I wondered about their car battery setup - another thread on here talked about Leich 901s and someone in the conversation said 4.5v was right about where their stock power output was supposed to be and recommended AAs, so I just went with that as it was easier and I'm not running these for any real distance. It hadn't occurred to me that that would impact this side of things.

I've got some space left in the display board, so I wouldn't be against adding a VU meter if that's the 'right' way to do this, but I'm just starting to read about them so I've got even more to learn before I can try to make that work. I'm guessing that's not a right-out-of-the-box solution.

[5415551212]

OK I think that for 2-wire phone audio to regular audio (like music on hold) a 600 ohm 1:1 audio transformer is used, so you'd probably use that for a vu meter, you'd probably want a on/off switch for the meter, you might also use a resistor to drop the voltage ~1.2 volts if your using a real vu meter.

[SUnset2]

I suspect that the audio modulation is much less than the battery voltage, and is not visible on the meter.  If you have access to an oscilloscope, you could measure this.  If this is the case, you would use a more sensitive meter, AC coupled (transformer as suggested above, or capacitors), and protected from ringing voltage (using the ground wire for one side of the ringer would make this easier).

[JacobCoffin]

Thank you both! I think that makes sense, I'm guessing the green and red where I'm reading zero is where the talk signal is going through. I'm going to read up on some of the stuff you mentioned and will probably come back with a few more questions when I have more of a plan. For now it looks like the voltmeter will just be measuring the batteries' charge. Thanks for your help!

[JacobCoffin]

So Dave Ingebright of Valhalla Tree Farms got back to me - he sent a very nice email with some more information - and I wanted to share a relevant excerpt as it seems like I was somewhat right in how it would work for the wrong reason, and my current setup is wrong for reasons I didn't expect!

"The voltmeter should be connected across the battery wires, and reads the "talk" voltage. It's the yellow and red wires. In my case, the battery is a couple of hundred feet away.  Don't get dazzled by the voltages on the other wires. You really just need to read the talk voltage. On my system, I note the other voltages, like the 3 volts you read on the other wires, but it does not mean anything. Sometimes I can identify which color is which by knowing the voltages on the other wires.
What's funny is that the voltmeter in the garage varies as I talk on that phone, sort of like a VU meter on a tape recorder. The power was so far away that the voltage drop on the wires caused the voltmeter needle to move with my voice."

So it sounds like I was right with where I connected the voltmeter but wrong because I hooked it up right next to the battery (I couldn't have made it much closer if I'd tried). And that this is more of an odd quirk of his setup rather than an 'official' sort of reading of the signal? I'm still not sure but it's cool it works that way. I'll have maybe a hundred feet of cable at most between the two phones, I could try a voltmeter on the other end and see if it behaves the way he describes - it's less distance but I'm also not using a car battery so the power is likely lower too.

Otherwise I'm open to other ideas for showing the talk signal (and I'll keep reading about VU meters - it sounds like they are probably the way to go, though I'll admit the first I really heard of them was when you folks started talking about them here!). I'd already planned to have a battery box on the other end, it might not need another battery but it'll give me space to convert from the three wires on the antique cord to RJ11 without cutting them (one of my goals is to (mis)use existing wiring and ports in my building, disconnected from the outside) so there's no reason I can't add other stuff while I'm at it. I haven't even cut the wood to make the box yet.

Thanks again for all your help!

[SUnset2]

Try adding some resistors between the battery and the meter.  You will probably have to experiment.  Try about 50 Ohms and go up and down from there.

[dsk]

This was not easy to understand, but a resistor of 147 ohms in series with the battery will have a voltagedrop of close to 5V at a current of 35 mA.  The battery voltage should be raised to 7.5V or maybe 9V just to compensate the voltagedrop in the resistor.
The voltmeter across the risistor should move pretty well together with the transmitter current. Be careful because the curent trough the transmitter capsule should not exceed 40-50 milliamps depending on the version.

[dsk]

Diagram:

[JacobCoffin]

Thank you for the diagram! That makes it much easier to picture!