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Old 10th Feb 2008, 10:39 pm   #1
Dave Moll
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Default Ericsson Ericofon (“Cobra”) telephones

I have recently acquired three Ericofon “Cobra” telephones in various states of disrepair:

1: A so-called “German Chassis” which was complete, but the standswitch (The Ericofon’s equivalent of a cradle switch) was jammed solid and the dial was stiff and failed to return when turned.

2: A K7 Chassis which had clearly got wet at some time with evidence of rust on the transmitter insert and the parts of the chassis. Also, the bodyshell, which started off as Persian Grey has badly yellowed, giving it a rather unattractive greenish tinge.

3: What appears to be another “German”, but without the PCB or transmitter insert. Annoyingly, the leads from the receiver had been cut for some unknown reason. Admittedly, this was sold as “parts”.


No. 1 turned out to be a relatively easy challenge to start the restoration project. After dismantling, it was possible to lubricate where the tip of the standswitch passes through to operate the contacts. Once this had soaked through the switch was moving freely. And applying further lubrication to the workings under the red plastic cap got the dial working again.

So everything was now working except the buzzer – which had me scratching my head for a while until I unscrewed the top. Once loosened off, it burst into life – having been screwed down too tightly to allow it to vibrate. I did find, however that the wiring instructions didn’t work for my chassis:

The white lead (black on the Erico) needed to be connected to the second (not the first) terminal, with a strap (I used a 3.3K resistor) to the first terminal to connect the buzzer. As shown, the red lead (yellow) was connected to the third terminal.

Having got it fully working using its internal ringing capacitor, I made a (reversible) modification by disconnecting the 1μF where it goes to the buzzer and wiring the latter to the unused fourth terminal, connecting this to the blue lead (red on the Erico). It now works in the accepted UK mode via the capacitor in the master socket.


No. 2 was more of a challenge. In fact most of it ended up being made up from parts of no.3. At first I was failing to get a dial tone, despite having everything wired up correctly. All I was getting in the receiver was a click when going on- or off-hook. The fact that I was hearing a click in the receiver threw me off the scent. Eventually, I solved the problem by swapping to the bodyshell from no. 3, whose receiver worked fine.

The next problem was that the dial was not creating the correct number of pulses. This appears to be a known problem, but I decided that the simplest solution was to swap the switchgear and electronics to the chassis of no. 3. I now had a working chassis.

This unit does not have any form of ringer, so the spare terminal (which would have been connected to the buzzer if there had been one and should then have been strapped to the white (black) lead) made a convenient place to park the blue (red) lead and relocate the lead which had previously been connected to the internal ringing capacitor.


Creating a third working telephone from the remaining parts may prove to be a challenge too far – at least for me, anyway.
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Old 12th Feb 2008, 12:40 pm   #2
geofy
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Post Re: Ericsson Ericofon (“Cobra”) telephones

Hi Dave

We tend to take phones for granted but they are made to a high standard with high quality components. Well done in getting these going again

Regards

Geof
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Old 14th Feb 2008, 11:15 am   #3
Dave Moll
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Default Re: Ericsson Ericofon (“Cobra”) telephones

Quote:
Originally Posted by Dave Moll View Post
Creating a third working telephone from the remaining parts may prove to be a challenge too far – at least for me, anyway.
Actually, it has proved easier than I expected.

What I omitted to mention in the original post was that I had ordered a replacement transmitter insert from Richard Rose at www.ericofon.com, which I have used in the K7 chassis (no.2 above). I have found, however, that the slightly rusty insert is in fact working fine. I was having problems with it because its shape is such that the contacts will short it out unless bent to exactly the correct angle.

By a stroke of luck, I found that a modernish (but dead) BT telephone I have contained a receiver insert which fits quite nicely in the bodyshell of the Erico, so it was just a matter of matching up the switchgear with the wiring diagram and adding the resistors and capacitors - fortunately, the original induction coil is still attached, otherwise I would have been stuck. The components I've used are not exact values but the nearest I had available.

I mentioned above that this dial was not working correctly. In fact in the current configuration the dial "off/normal" switch was permanently in the "off" position resulting in the line being shorted out. Following the instructions given under "My phone miss-dials phone numbers", I bent out the loop of metal until the contacts opened correctly. Thereafter the dial was fine.

I now have a working telephone, but the existing wiring (which I haven't disturbed) of the dial "off/normal" switch doesn't appear to have contacts available to implement the switch which disconnects the receiver (substituting a 220Ω resistor) while dialling. Bypassing this results in the dialling pulses going through the receiver giving a loud clicking while dialling. I shall revisit this at a future stage, but for the time being have made a totally non-authentic circumvention by installing a no.205 rectifier element across the receiver. But then the innards of this 'phone could hardly be described as authentic, anyway.

I attach below a wiring diagram showing the existing wiring in black and that which I have added in red. The contacts shown below the dial contacts are those of the "off/normal" switch. Where a number is shown, this relates to the appropriate connection of the induction coil (and the colour of its wires).
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