Autophone T7102/11 office loudspeaker telephone with four-valve amplifier

[Dave Moll]

When I first saw this advertised (three years ago!) I thought "I must have one of those." Other than the amplified version of the set "F" field telephone (another item I'd like to add to my collection some day), I had never come across a telephone with valves in it.


This telephone is housed in a veneered wooden cabinet approximately 52cm wide x 16cm high x 20cm deep with a plastic handset attached to the left-hand side. When the handset is lifted, it operates in the normal manner, but with it on-hook the speaker function can be placed "off-hook" by means of a switch to one side of the dial on the front panel. On the other side is a volume control for the speaker. In addition, the panel has a light to indicate that the set is off-hook. This has a push-button on either side, one red and the other black. There are three additional controls on the rear of the chassis - accessible through holes in the rear of the cabinet which are normally filled with rubber bungs. The speakerphone function is provided by a mains-powered four-valve (EY9*, N78, EL91, EL91) amplifier with a microphone on the left and a loudspeaker on the right. *EY9 is what it says on the glass - does this exist or could it be EY91 with the "1" rubbed off?



The unit is apparently designed to operate as part of a larger system, as shown by the number of connections at the end of the line cord. The push-buttons described above each earth one or other of a pair of these connections - "1" in the case of black (presumably to signal another unit) and "P" (any suggestions regarding the significance of this designation?) in the case of red. As "P" is strapped to "A", I take this to provide an earth-recall function.


The line connections are as follows:
P: (strapped to A) connected to E when the red button is pressed
A: telephone line "A" leg
B: telephone line "B" leg
EB: extension bell (for connection in series with the internal ringer)
E: earth
6.3AC: output from the heater winding of the transformer
1: connected to E when the black button is pressed
2: spare
3: spare

-24: -ve connection for an external 24V supply between this and E

Getting it working:

Having got the unit home after collecting it at the 2007 NVCF from the seller, my first challenge was trying to work out how to extract the chassis from the cabinet. I had clearly removed the correct retaining screws, but there was still something preventing the cabinet from coming away. It appears that at some time one of the pots on the rear of the chassis had been replaced with one with a shaft about 2mm too long, and it was fouling the cabinet as I tried to remove it. With some trepidation I managed to ease the cabinet over the end of this shaft. Thanks to my junior hacksaw, the shaft is now 2mm shorter and the cabinet comes on and off easily!

Neither the handset nor the speaker system gave any response whatsoever. As nothing was visibly amiss, my next mission was to seek service information or at least a circuit diagram. Unfortunately, neither through this forum nor anywhere else was any information forthcoming, so it was eventually a case of attempting to trace out the circuitry myself - the reason for the project going into limbo for three years.

The first function I identified was the ringer, which is wired to be in series with an external bell. As this was not present, it is not surprising that the ringer was not functioning. Having identified this and that the ringer has an impedance of 1.5K, I wired a 2.7K resistor in place of an external bell to give a functional ringer with a REN of 4, using the internal ring capacitor. I have not rewired the unit to operate from the capacitor in the master socket.

The on-hook/off-hook status is controlled by a relay operating from the 24V supply - at least it does now that I have identified that this voltage needs to be supplied externally. Originally, this would presumably have been supplied from somewhere else in the system. I have wired up a battery box containing five "flat" (in the physical rather than electrical sense!) 4½V batteries (of which I happened to have a job lot) giving a total of 22½V - which has proved adequate to power the relay. Discovering the need for the external supply was one of the key points in the progress of this project.

The relay has a total of three positions: first there is the on-hook condition with the relay coil unpowered, a second position (by powering the coil via a resistor) when the handset is off-hook, and a third position (with the coil at full voltage) when the handset is on-hook and the speaker system is placed off-hook. The two functions of the handset cradle are to set this relay to the second position described above, and to connect the line circuitry to the handset, thus disabling the speaker function - even if it has been selected from the front panel.

Having got this far, I found that I could get sound onto the line from either the handset transmitter or the speakerphone microphone, but was still getting absolutely no sound in the handset receiver, and it was impossible to get an acceptable sound level out of the speaker without howl-round. This was eventually tracked down to the handset receiver insert fouling the contacts and shorting out its connection. I have taken the simple approach to circumventing this by slackening off the receiver cover so that the insert presses more lightly on its contacts. The long-term solution will be to re-align the contacts within the receiver housing, but hey, it works!

Having sorted the handset, this just left the speaker function to get working satisfactorily. In addition to the control on the front panel to adjust the loudspeaker volume, there are the three above-mentioned pots on the rear of the chassis. The first of these adjusts the level of sound coming from the telephone line, the second and third both seem to adjust the level from the microphone. There does, however, seem to be a degree of crossover between the function of these three controls. With some juggling of them I have now achieved an acceptable sound level from the speaker just short of the point of instability. I haven't quite figured out the exact function of all these controls, but the following suggestion has been given by another forum member:

Originally Posted by brianc

I remember from my early days with the BBC that loudspeaker talkback units which worked on 2 wire circuits always had a balance pot for the hybrid transformer which was set to achieve minimum electronic crosstalk between the mic circuit and the input to the LS amp. The adjustment of these pots was very critical to remove acoustic coupling between the mic and loudspeaker. I'm sure that one of those pots will be the balance pot, wired directly between two tags on the hybrid.

After it having spent three years as a "display-only" part of my telephone collection, I am now pleased to have an unusual example of telephony as a functional unit.

I hope that at least some of you have stayed awake through this rather lengthy account!

[Steve_P]

Here's a thought. When Alexander Graham Bell invented the telephone, who did he ring?

Interesting bit of kit. I must admit that I know little about telephones - does it work on a domestic line and can an interface be made for it to do so?

Cheers,

Steve P.

[crikeyitslive!]

Thats an amazing looking peice of kit. Just when you think you've seen them all somehting like this crops up! Nice job.

[Dave Moll]

Originally Posted by Steve_P

does it work on a domestic line?

Yes, that is how I have it set up. As described above, the line cord is connected between the "A" and "B" terminals, and the ringer wire from the master socket is left unconnected as the internal ringer capacitor is being used.

As the unit uses separate capacitors for the ringer and the receiver, the latter would continue to use the internal capacitor even if the ringer were connected to the master capacitor.

[Dave Moll]

Originally Posted by Steve_P

When Alexander Graham Bell invented the telephone, who did he ring?

I don't know about "ring", but if I remember my history (or is it urban myth?) correctly, the first successful telephone call occurred when the aptly-named Mr Bell was heard by his assistant to call "Mr Watson, I need you."

... but that line (excuse the pun) of discussion would take this thread off course.