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Old 16th May 2020, 11:30 am   #1
Sparks
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Default 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.
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Old 16th May 2020, 1:09 pm   #2
pip5678
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Default 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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Old 16th May 2020, 1:10 pm   #3
WayneL74
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Default 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
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Old 16th May 2020, 2:21 pm   #4
Sparks
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Default 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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Old 16th May 2020, 7:24 pm   #5
Sparks
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Default 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.
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Old 16th May 2020, 8:57 pm   #6
Pellseinydd
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Default Re: Quiet or no dial tone in a GPO 706

Quote:
Originally Posted by Sparks View Post
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.
On a Tele706CB with a Mark 1 base (plastic one) the dial leads - Pink and Orange should clamped under the screw on top of the raised block encircled in red and the Blue, Slate and Brown leads pushed separately into the three holes. There is no need to strap T8 and 10 as it is done by the Pink & Orange leads.

The 1959 diagram N806 from the days when there were plenty of Tele706CB's in the GPO !!
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Old 16th May 2020, 9:19 pm   #7
Sparks
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Default 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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Old 16th May 2020, 11:36 pm   #8
WayneL74
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Default 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.
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Old 17th May 2020, 7:41 am   #9
Sparks
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Default Re: Quiet or no dial tone in a GPO 706

Yes I have a meter.
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Old 17th May 2020, 12:14 pm   #10
WayneL74
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Default 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
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Old 17th May 2020, 12:36 pm   #11
Sparks
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Default 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.
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Old 17th May 2020, 1:23 pm   #12
WayneL74
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Default 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
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Old 17th May 2020, 1:29 pm   #13
Dave Moll
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Default 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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Old 17th May 2020, 1:57 pm   #14
Sparks
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Default 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.
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Old 17th May 2020, 2:21 pm   #15
pip5678
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Default 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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Old 17th May 2020, 2:32 pm   #16
Dave Moll
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Default 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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Old 17th May 2020, 3:58 pm   #17
Sparks
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Default 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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Old 17th May 2020, 4:01 pm   #18
Sparks
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Default Re: Quiet or no dial tone in a GPO 706

Induction coil or something else ?
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Old 17th May 2020, 4:04 pm   #19
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Default 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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Old 17th May 2020, 4:26 pm   #20
WayneL74
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Default 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
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