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Old 19th Apr 2019, 12:41 pm   #1
OscarFoxtrot
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Default 706 Intercom Circuit

Further to this thread
https://www.vintage-radio.net/forum/...d.php?t=147174
which has been closed

I have now got these working as original.

1. Handset cord as 706 PCB Model.

https://www.britishtelephones.com/t706.htm

2. Regulator removed. Bell motor removed. Dial cord rewired to T numbers not PCB tags. Unused dial wires removed.

3. Strap: T18-19, Wire link: T13-19, T9-16, T8-10 (that is link 13 and 19 only, not 13-14-15-etc)

4. Insert Res 1k 5% (brown-black-red-gold) T4-T16.

5. Dial Grey (Slate) T11, Blue T12, Brown T13.

6. Buzzer Orange (+) T12, Black (-) T4.

7. 9 volt + T19, - T4 (master only)

8. Line cord Green T4, Blue T6, Whi T8, Red T18.

The two phones are connected with 4 way line cords to matching colours. Only the master has a 9v battery or power source. Both phones are otherwise identical.

I shall be pleased to receive comments/errata.
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Old 19th Apr 2019, 1:25 pm   #2
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Default Re: 706 Intercom Circuit

Erratum no 1: insert link T6-T11
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Old 19th Apr 2019, 1:33 pm   #3
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Default Re: 706 Intercom Circuit

As shown, that added 1k resistor doesn't seem to do anything (it goes to a capacitor, the other end of which is not connected). Should T8 and T9 be strapped?
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Old 19th Apr 2019, 1:57 pm   #4
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Default Re: 706 Intercom Circuit

No, T8-9 not strapped. The only strap is T18-19, and wire links T6-11*, T8-10, T9-16, T13-18. *Not on dwg.

Oh well, although the speaking isn't great it does work. I'll have another look later.
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Old 19th Apr 2019, 6:45 pm   #5
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Default Re: 706 Intercom Circuit

Erratum no. 2: T8-9 is strapped. Apologies. Amended dwg attached.
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Old 19th Apr 2019, 7:47 pm   #6
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Default Re: 706 Intercom Circuit

Some photos.

I note that both my grey ones (obtained from different sources at different times) have the 'test plastic' label. They're moulded from much thicker than normal plastic - maybe rigid PVC?
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Old 19th Apr 2019, 11:14 pm   #7
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Default Re: 706 Intercom Circuit

Video (not mine) here
https://www.youtube.com/watch?v=T6NjF_G-RYo
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Old 20th Apr 2019, 12:39 pm   #8
Oldcodger
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Default Re: 706 Intercom Circuit

OF- sounds like a similar idea to the kit used on a planset n625 with internal extensions, but using a button on the top of the 706 to call the other handset. Mentioned it as you may not have seen this . remembrance from my Muirfield days( or was it Muirhouse , somewhere in a rough partof the city)
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Old 20th Apr 2019, 12:53 pm   #9
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Default Re: 706 Intercom Circuit

It might be interesting to compare the circuit.

The video mentioned above shows one of the switchhook contacts has been cut from the PCB, I didn't notice that on mine.
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Old 20th Apr 2019, 5:04 pm   #10
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Default Re: 706 Intercom Circuit

I also don't see where brown (D3) connects to, as the only wires connecting to the dial are Brown, Blue and Grey (Slate).
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