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Old 18th Dec 2019, 12:53 pm   #1
PhilDay
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Default BT 706 won't hang up after conversion

Hi Folks,

I've fitted a conversion kit to a type 706 phone that I found when clearing out my Mum's house. Still brand new and in the box - even the paper insert for the dial has never been fitted.

I've checked though the instructions for the kit several times and as far as I can tell its all connected correctly - photo attached.

Dialing out with it works fine - but it doesn't seem to hang up properly; If I try to call our number while its plugged in I get an engaged tone, and picking up the headset and holding the cradle buttons down I can still hear a faint dial tone.

I read somewhere that the microphone might be part of the problem - and If I disconnect either of T3 or T10 (which I think are the microphone) then I can call it and it rings fine, but then it carries on ringing even if I pick it up.

Can someone offer some tips on what I should try next please ?

Thanks
Phil
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Old 18th Dec 2019, 2:03 pm   #2
Nickthedentist
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Default Re: BT 706 won't hang up after conversion

Hello and welcome Phil.

The wiring looks fine to me, though I don't think you need the T8-T9 strap really.

How have you connected the other end of the line cord?

What happens if you temporarily lift the blue wire from T6?

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Old 18th Dec 2019, 2:16 pm   #3
PhilDay
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Default Re: BT 706 won't hang up after conversion

Hi Nick,

The other end of the line cord has a std BT plug on it and just goes into the wall socket (it came as a kit, so I'm assuming that end is wired up OK.

Removing the blue wire from T6 didn't make any difference

Phil
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Old 18th Dec 2019, 3:17 pm   #4
Dave Moll
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Default Re: BT 706 won't hang up after conversion

As long as there's no strap between T6 and T7, the strap between T8 and T9 is doing nothing. It is only necessary to disconnect one end of the capacitor that's wired between T7 and T9. The second strap is however a useful spare if you need one to put elsewhere.

Anyway, getting back to the original problem, it sounds as though it may be related to the cradle switch. First of all, are the cradle plungers moving freely so that they are depressed when the handset is in the cradle? If this is not the problem, it may be worth a visual check (with the telephone case removed) that the switch contacts are operating correctly when the plungers are moved up and down. This can be tested electrically by checking that continuity between the leftmost pair and between the rightmost pair is broken with the plungers down and restored with the plungers up. If so, the cradle switch is working OK and the problem is indeed elsewhere.
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Old 18th Dec 2019, 3:25 pm   #5
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Default Re: BT 706 won't hang up after conversion

Not related to your problem, but if you are plugging into an extension socket it is worth checking that contact 3 on the socket into which the telephone is connected is wired back to contact 3 in the master socket, otherwise the telephone will not receive a ringing signal on incoming calls (which it is expecting on the blue wire of the line cord). The reason this may not be connected is that many modern telephones don't use it.
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Old 18th Dec 2019, 3:29 pm   #6
PhilDay
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Default Re: BT 706 won't hang up after conversion

Hi Dave,

The plungers move fine, and with the case off I can see the sprung contacts inside the clear plastic case moving backwards and forwards fine.

When you say leftmost and rigthmost pair is that T10 and T19 ? That is around 130 ohms with the cradle up, and goes to open when its pushed down.

Phil
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Old 18th Dec 2019, 3:59 pm   #7
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Default Re: BT 706 won't hang up after conversion

Quote:
Originally Posted by PhilDay View Post
When you say leftmost and rigthmost pair is that T10 and T19 ?
I was actually referring to the wires emerging from the base of the switch, but the fact that there is no continuity between T10 and T19 with the plungers down seems to confirm an "on-hook" condition, so I am was puzzled by the "engaged" condition when attempting to ring the telephone, given that engaged implies a loop of 600Ω or less.

Following a re-read of your original post, a bit of experimentation with one of my 706s confirms that if I short out the left-hand two wires emerging from the cradle switch, I am able to re-create the symptoms reported, including the faint dial tone with the plungers depressed.
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Old 18th Dec 2019, 4:20 pm   #8
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Default Re: BT 706 won't hang up after conversion

So If I'm looking into the cradle switch from the side then I don't have wires as such but the contacts inside the switch bend out and go down into the circuit board

When the cradle is up then it looks like the first pair are connected together and last pair are connected together (0 resistance on a meter), and not connected when the cradle is down

When the cradle is down then three of the contacts are connected (2, 3, 4 inside the switch)
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Old 18th Dec 2019, 4:40 pm   #9
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Default Re: BT 706 won't hang up after conversion

Nice work, Dave
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Old 18th Dec 2019, 8:32 pm   #10
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Default Re: BT 706 won't hang up after conversion

According to Diagram N806, there is no connection back to the main terminals from the A side of the gravity switch (pin 4; 5 is connected to T19). So that's where I'd start probing with the multimeter. The contacts don't seem to be opening fully in the "on hook" position.

Disconnecting the microphone doesn't tell you very much. It will stop the phone from answering, since the loop current flows through the mic when the phone is off hook. If you held the dial off-normal while the phone was on-hook minus mic, it would appear engaged when dialled from a mobile phone, because the mic is shunted when the dial is turned away from its end stop. In ye olden days, the actual current to energise the selector solenoids and step the contacts around would have flowed right through your phone, and you want to minimise any unnecessary resistance in order to ensure a positive action. When you are done dialling, the contacts open and the mic is back in circuit.
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Old 18th Dec 2019, 8:39 pm   #11
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Default Re: BT 706 won't hang up after conversion

Quote:
Originally Posted by PhilDay View Post
I don't have wires as such but the contacts inside the switch bend out and go down into the circuit board
Yes, perhaps "wires" wasn't the best description, but I couldn't think what else to call the metal strips emanating from the bottom of the switch.

Quote:
Originally Posted by PhilDay View Post
When the cradle is up then it looks like the first pair are connected together and last pair are connected together (0 resistance on a meter), and not connected when the cradle is down

When the cradle is down then three of the contacts are connected (2, 3, 4 inside the switch)
Contacts 3 and 4 certainly shouldn't be making contact with each other. It sounds as though the rod from the cradle into the switch is pushing contact 3 too far in so that it is touching contact 4. Can you see anything untoward that could be causing this to happen?

I had expected the problem to be that contacts 1 and 2 weren't opening, which as I said earlier creates the problem, but the connection you describe also has the same effect, which I was able to simulate by wedging a piece of cardboard between the actuating rod and and the metal bar of the cradle.
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Old 20th Dec 2019, 8:50 pm   #12
PhilDay
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Default Re: BT 706 won't hang up after conversion

Hi Folks,

Thanks for all the help and advice. Attached are a couple of photos showing the switch in the off-hook and on-hook state.

If I number the contacts from left top right then:

#2 and #5 are the ones moved by the cradle

in off-hook #2 and #5 are sprung to let left so that #2 is in contact with #1 and
#5 is in contact with #4

In on-hook then #2 moves to the right, contacts with #3 and they both then go over and connect with #4. #5 is moved clear to the right.

If I put a bit of insulating tape between #3 and #4 so that they don't come into contact then I can call the home line and not get the engaged signal.

So it sounds then as if there is something not lined up properly in that switch; it does look a bit like its squished over to the left, so maybe I've got to do a bit of fettling there.

However even though its not now engaged I'm not getting it to ring properly;

- Most of the time it just doesn't ring at all, but if I pick up it seems to be working fine.

- A couple of times its made a single chime (so at least its trying the poor old thing ;-) - but If I pick up I head a dialing tone and one the phone I'm calling from I get a recorded voice that sounds like it's from the same era as the phone saying "please wait"

It does work if I plug it into the master socket so Dave it sounds as if you're almost psychic in pointing out the possible problem with contact 3 ;-)

But would I still have got even the occasional partial ring with that ?

Well actually it did make a single chime a couple of times, but that's all
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Old 20th Dec 2019, 11:15 pm   #13
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Default Re: BT 706 won't hang up after conversion

Lovely clear pictures. It looks like too much movement is being applied to the insulating piece which operates the contacts, causing them to over travel.

Can you move the switch further to the right by adjusting its mounting screws?
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Old 21st Dec 2019, 9:29 am   #14
Dave Moll
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Default Re: BT 706 won't hang up after conversion

Quote:
Originally Posted by PhilDay View Post
Dave it sounds as if you're almost psychic in pointing out the possible problem with contact 3 ;-)
No psychic powers needed. Your description of continuity between contacts 3 and 4 (which in correct operation should never come in contact), along with having an opened-up 706 in front of me, was the giveaway that the problem could only be caused in the manner described.

As said, somehow more distance needs to be (re-)introduced between the switch and the cradle.
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Old 21st Dec 2019, 3:02 pm   #15
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Default Re: BT 706 won't hang up after conversion

That sounds like "ring trip" -- something in the circuit is acting like a voltage-dependent resistor and becoming conductive enough to answer the line when the ringing voltage gets applied, but then returning to a high-resistance state when the voltage drops under load. Which the exchange sees as a hang-up. Since it's a modern, digital phone exchange, the progress tones you hear are generated by the caller's exchange (or even their handset, on the mobile networks!) and don't relate to the actual rings of the phone on the far end anymore, so you probably will get what sounds like one full ring at the calling handset.

If you turn round the regulator board so the regulator is disconnected and terminals B, C, D are strapped, do you still get this behaviour? What about if you plug the 706 directly into the test socket inside your NTE5?
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Old 28th Dec 2019, 1:57 pm   #16
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Default Re: BT 706 won't hang up after conversion

Looking at possible ring trip - at the calling end do you get any indication of a "ring". One or two ideas to look at. The usual cause of ring trip in the era of wired phones was excess capacitance, where some person wired a standard handset in parallel with the main one,the extra capacitance causing ring trip.
It also could be that you need more voltage than a standard meter will generate . I had ring trip at home and all looked well with a standard meter, but broke down to BT test volts ( circa 50v DC).
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