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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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16th May 2020, 11:30 am | #1 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Quiet or no dial tone in a GPO 706
Good morning everyone. I've this morning finished converting a dial-less 706 to plug and socket. I have strapped T8 and T10 together (pink and orange) accordingly, removed completely the grey dial wire and the brown and blue dial wires are disconnected, apart from their connection to the gravity switch. The phone rings beautifully, however, there is either a quiet dial tone or no dial tone at all.
- With the original type 16 carbon mic, the dial tone is quiet but blowing into the mic produces a significant echo, more than the usual sidetone. - I changed the earphone element in case it was faulty. It made no difference. - I tried another carbon mic. Same echo and quiet dial tone. - I then tried an all red type 21A mic. The dial tone disappeared completely and the sidetone was louder than before! I have also cleaned the gravity switch contacts with a sliver of cardboard. Obviously I can't dial out on this phone so the dial tone issue probably isn't that important but I would like the phone to be as functional as possible. Any advice would be most useful. Pictures herewith. Many thanks. |
16th May 2020, 1:09 pm | #2 |
Pentode
Join Date: Jan 2019
Location: Manchester, UK.
Posts: 163
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Re: Quiet or no dial tone in a GPO 706
Have you tried without the acoustic shock device across the receiver T1, T2 on the circuit board?
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16th May 2020, 1:10 pm | #3 |
Triode
Join Date: Jun 2015
Location: Ipswich, Suffolk, UK.
Posts: 49
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Re: Quiet or no dial tone in a GPO 706
Hi Sparks.
It does sound like you have got high resistance somewhere - possibly across the switchook cotacts. Check them carefully that they are making & breaking correctly - you can adjust the switch position slightly. Also a couple of other things to try: 1. Removing & re-insert the regulator board a few time times. (The board standing up between the switch hooks). Also try inserting it the other way around (upside down as it were). 2. Try without the shock suppressor fitted (the device across T1 & T2) Regards, Wayne |
16th May 2020, 2:21 pm | #4 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: Quiet or no dial tone in a GPO 706
Thankyou pip and Wayne. I'm grateful but your suggestions haven't made any difference. I've cleaned the switch contacts again and the regulator contacts as well.
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16th May 2020, 7:24 pm | #5 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: Quiet or no dial tone in a GPO 706
I made a call to my mobile answering service with this phone this afternoon, using another phone to dial out. The results weren't encouraging.
- I could scarcely hear the "This is the Vodafone recall service" introduction. - I could scarcely hear the message I left on my mobile. Very quiet, muddy unintelligible. To summarise, the bell works fine, the sidetone works too well (echoey) and both transmitted and received speech and the dial tone are underperforming. If anybody can shed any light I'd be pleased to hear from you. On the plus side, the plastics have polished up quite nicely. |
16th May 2020, 8:57 pm | #6 | |
Heptode
Join Date: Jul 2007
Location: Flintshire, UK.
Posts: 707
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Re: Quiet or no dial tone in a GPO 706
Quote:
The 1959 diagram N806 from the days when there were plenty of Tele706CB's in the GPO !! |
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16th May 2020, 9:19 pm | #7 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: Quiet or no dial tone in a GPO 706
Thanks Pell and you're quite right. I already have a purpose built 706CB with the pink and orange clamped in this spot. I didn't realise this when I bought it a few weeks ago and therefore strapped T8 and T10 anyway. This particular phone works well in every respect. It is the one that arrived today that is giving me aggro. It's as if too much power is being given to sidetone, the right amount for the bells and not nearly enough for the other functions.
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16th May 2020, 11:36 pm | #8 |
Triode
Join Date: Jun 2015
Location: Ipswich, Suffolk, UK.
Posts: 49
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Re: Quiet or no dial tone in a GPO 706
Hi Sparks.
Do you have a multimeter? If you do we need to take a few measurements. |
17th May 2020, 7:41 am | #9 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: Quiet or no dial tone in a GPO 706
Yes I have a meter.
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17th May 2020, 12:14 pm | #10 |
Triode
Join Date: Jun 2015
Location: Ipswich, Suffolk, UK.
Posts: 49
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Re: Quiet or no dial tone in a GPO 706
OK then lets check a couple of things.
Unplug the phone. Set to meter to the lowest rersistance (normally 200 ohms). Make sure the phone is "off hook" (The handset "pips" are up). Measure between T2 & T5. This should read a low number - hopefull less than 1 ohm. Measure between T19 & the brown dial wire. This should also read a low number. Also one other thought, Is this a known working line cord? Have you tried swapping with a known working phone? Regards, Wayne |
17th May 2020, 12:36 pm | #11 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: Quiet or no dial tone in a GPO 706
The line cord is brand new Wayne. I've done a continuity check and it's all good.
T2 to T5 gives 0.4 ohms T19 to brown dial wire gives 1 ohm. Let me stress, this phone has no dial and didn't when it arrived yesterday. So, the BDW is only connected to the gravity switch. |
17th May 2020, 1:23 pm | #12 |
Triode
Join Date: Jun 2015
Location: Ipswich, Suffolk, UK.
Posts: 49
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Re: Quiet or no dial tone in a GPO 706
OK that's good - we can discount the gravity switch as the issue then.
The next thing I would check is the handset wiring. You could do a continuity test with your meter or as you have a few other phones simply swap the whole handset over. Regards, Wayne |
17th May 2020, 1:29 pm | #13 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,130
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Re: Quiet or no dial tone in a GPO 706
If you are disconnecting and reconnecting handsets, you could check that T1 to T2 is open circuit while the handset is disconnected, just to confirm that nothing in the telephone is shorting out the receiver.
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17th May 2020, 1:57 pm | #14 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: Quiet or no dial tone in a GPO 706
The handset cord is also brand new and fully functional. I am getting continuity in all four wires to the respective earphone and transmitter connections. There is also continuity across both connections of the earphone when it isn't connected but I think this is normal for a rocking armature device.
T1 to T2 is open circuit with the handset disconnected. |
17th May 2020, 2:21 pm | #15 |
Pentode
Join Date: Jan 2019
Location: Manchester, UK.
Posts: 163
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Re: Quiet or no dial tone in a GPO 706
Looking at diagram N806 shouldn't you see the value of R1 in series with part of the induction coil across T1 and T2 with handset disconnected and switch hooks open (handset down)?
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17th May 2020, 2:32 pm | #16 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,130
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Re: Quiet or no dial tone in a GPO 706
Of course, Pip is quite right that R1 (150Ω) in series with the 156Ω (ish) between tags 2 and 3 of the induction coil should give a total DC resistance a little over 300Ω. It may be worth checking out the induction coil for continuity.
My apologies for forgetting about the components in parallel with the receiver.
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Mending is better than Ending (cf Brave New World by Aldous Huxley) |
17th May 2020, 3:58 pm | #17 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: Quiet or no dial tone in a GPO 706
Sorry pip and Dave. This is the bit where my lack of tech savvy comes in. Whereabouts do I need to be placing my probes to look for these values and continuity ? Also, is the induction coil the thing in this picture below ? Thankyou.
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17th May 2020, 4:01 pm | #18 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: Quiet or no dial tone in a GPO 706
Induction coil or something else ?
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17th May 2020, 4:04 pm | #19 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,289
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Re: Quiet or no dial tone in a GPO 706
That's the Induction coil.
Anti Side Tone Induction Coil (ASTIC).
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17th May 2020, 4:26 pm | #20 |
Triode
Join Date: Jun 2015
Location: Ipswich, Suffolk, UK.
Posts: 49
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Re: Quiet or no dial tone in a GPO 706
Hi Sparks.
Set your multimeter to 2000 ohms (may be marked 2k) and measure T1 to T2 again. Do this with the hookswitch pushed down |