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Old 9th Nov 2020, 5:25 pm   #1
Mikey405
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Default GPO 746 Dialling Out Issue

Evening everyone.

I recently purchased a GPO 746 from eBay and am having all sorts of hassle with dialling out. The phone appears to have had some love - it has had a 3.3K resistor connected in series with the 500 ohm bell and a new cord with a standard BT plug on it.

The phone receives calls fine and the bell works nicely but I am unable to make any calls out. Occasionally it will dial a wrong number (last time I tried the speaking clock on 123, I ended up with the emergency operator on 112) but mostly the numbers it dials just go to an N/U tone.

The phone also appears to exhibit rather a lot more "clicking" and "crackling" immediately after the dial has come to rest although I've cleaned the two leaf-switches on the dial (Type 21) to within an inch of their life and they both seem quite nicely tensioned.

The dial speed seems to be roughly the same as my kitchen 741 (I've tried speeding it up and slowing it down by hand with no apparent change to the issue).

The make / break ratio looks to be similar to the 741 too but of course it's difficult to tell exactly. I've reduced and increased the gap slightly by gently bending the fixed part of the "dial" leaf switch but again no real change. If I bend it too far then it doesn't dial at all.

I'm definitely not an expert on telephony matters so any help would be much appreciated.

Thanks all.

Kind regards.

From Mike.
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Old 9th Nov 2020, 5:48 pm   #2
Nickthedentist
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Default Re: GPO 746 Dialling Out Issue

Hi Mike,

Has it got an old-style carbon transmitter (mic.)? If so, try temporarily shorting it out.

You could also test the dial by swapping it with the one in your 711.

Are you with BT or an alternative provider?

Nick.
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Old 9th Nov 2020, 6:01 pm   #3
Mikey405
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Default Re: GPO 746 Dialling Out Issue

Hi Nick.

Thank you for your very quick reply.

It does indeed have a carbon mic (which rustles and crackles as you move the handset) but I will indeed try shorting it out.

I've just swapped over the dial from the 741 - it's a different kind (labelled "54A/EMT 83/2" in case that means anything to anyone) but it works fine and all the nasty "clacking" noises are also much reduced in the handset - just a soft purring as the number is being dialled.

I'm not with BT - I'm using a Grandstream HT701 VoIP analogue adapter - which normally works fine with these old phones as long as the mark/space of the dialler is slightly altered by slightly bending the fixed part of the leaf switch.

Thanks again Nick. I'll try shorting the mic and see what happens.

Kind regards.

From Mike.
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Old 9th Nov 2020, 6:09 pm   #4
Mikey405
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Default Re: GPO 746 Dialling Out Issue

Ni Nick again.

Well, I've just shorted out the carbon mic and there are definitely less crackles but the dialler still doesn't want to give me the correct number.

Thanks Nick.

Kind regards.

From Mike.
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Old 9th Nov 2020, 6:14 pm   #5
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Default Re: GPO 746 Dialling Out Issue

OK Mike,

Just to clarify... shorting the mic reduces the crackling but doesn't improve the dialling-out.

But do you mean that fitting the No.21 dial from the 711 (in place of the No. 54) resolves the dialling-out problem, or not?

Nick.

P.S. the Dial No. 54 was a cheapie plastic dial popular from the late 1970s at a guess. Like quartz clock movements they're great until they go wrong, but they weren't really designed to be repaired, just replaced when they failed. Some of their components are now rather fail and prone to cracking: https://www.britishtelephones.com/dial1.htm#d54
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Old 9th Nov 2020, 6:18 pm   #6
Mikey405
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Default Re: GPO 746 Dialling Out Issue

Hi Nick.

Yes - Shorting the carbon mic on the 746 reduces the crackling but doesn't make any difference to the dialling.

The wall-mounted 741 in my kitchen (which works nicely) has a "54" dial. The new 746 has a "21" dial and that doesn't seem to work properly.

Taking the "54" dial from the 741 and putting it in the 746 cures the dialling problem and I can make calls okay.

Thanks Nick.

Kind regards.

From Mike.
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Old 9th Nov 2020, 6:28 pm   #7
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Default Re: GPO 746 Dialling Out Issue

Great.

In that case, I would clean the "dial off normal" contacts on the "21" dial. Just get a strip of thin, non-glossy card (e.g. cut from a non-pitcure postcard), moisten it with Servisol Super 10 switch cleaner, then drag it back and forth between the closed contacts a few times. You'll need to twist the dial from its rest position and hold it like that to get the contacts to stay closed under their own springiness, so that the cleaning strip can do its stuff.

There will be black streaks on the card where the oxidation has been removed. Don't use anything fiercer than that or the contacts' plating will be lost.

Pic is from an older dial, but you get the idea, I'm sure.

Good luck,

Nick.
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Old 9th Nov 2020, 6:30 pm   #8
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Default Re: GPO 746 Dialling Out Issue

Quote:
Originally Posted by Mikey405 View Post
Hi Nick.

Thank you for your very quick reply.

It does indeed have a carbon mic (which rustles and crackles as you move the handset) but I will indeed try shorting it out.

I've just swapped over the dial from the 741 - it's a different kind (labelled "54A/EMT 83/2" in case that means anything to anyone) but it works fine and all the nasty "clacking" noises are also much reduced in the handset - just a soft purring as the number is being dialled.

I'm not with BT - I'm using a Grandstream HT701 VoIP analogue adapter - which normally works fine with these old phones as long as the mark/space of the dialler is slightly altered by slightly bending the fixed part of the leaf switch.

Thanks again Nick. I'll try shorting the mic and see what happens.

Kind regards.

From Mike.
Dial 54A' is a later cheaper version that replaced the Dial 21. See here for all about dials with the No 54A at the very bottom.

I've been in discussions with Grandstream and got them to add the 'reverse' New Zealand dial option and the ability to add tones below 301Hz to allow creation for the old UK tones for those old enough to remember them. But had no luck getting then to add any pulse break/make ratio other than the 60/40 US one. They don't seem to understand that half the worlds uses 66.6/33.3% ! Would open a bigger market up for them. They've only updated the firmware for their current HT8XX range of ATAs.

Ian J
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Old 9th Nov 2020, 6:40 pm   #9
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Default Re: GPO 746 Dialling Out Issue

Hi Nick.

That all sounds great - I have to say though that the contacts look spotless to me now and I couldn't get anything much more off them. I did clean them earlier (with paper and Servisol) and they were rather grotty.

Apologies for my ignorance but, referring to the picture below - which one is the "dial off normal" switch?

Thanks Nick.

Kind regards.

From Mike.
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Old 9th Nov 2020, 7:00 pm   #10
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Default Re: GPO 746 Dialling Out Issue

Hmm. I have the two dials side by side here with a beeper connected to contacts 4 and 5 - and the one which doesn't work seems rather faster than the one that works. I'm going to try slowing the not-working one down a fraction again.
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Old 9th Nov 2020, 8:06 pm   #11
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Default Re: GPO 746 Dialling Out Issue

Quote:
Originally Posted by Mikey405 View Post
Apologies for my ignorance but, referring to the picture below - which one is the "dial off normal" switch?
1 and 2 are.

They should both be open when the dial's in its rest position, and both be closed when the dial is rotated (or "off normal"). Check that their closed resistance is a fraction of an ohm.
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Old 9th Nov 2020, 8:08 pm   #12
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Default Re: GPO 746 Dialling Out Issue

Quote:
Originally Posted by Mikey405 View Post
Hmm. I have the two dials side by side here with a beeper connected to contacts 4 and 5 - and the one which doesn't work seems rather faster than the one that works. I'm going to try slowing the not-working one down a fraction again.
Yes, I would do that.

If you have a pushbutton phone with pulse dialing, you can use this as a reference as they're usually controlled by a ceramic resonator (poor man's crystal!) and so are pretty accurate.

Nick.
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Old 9th Nov 2020, 8:15 pm   #13
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Default Re: GPO 746 Dialling Out Issue

Or you can use the (free) sound recording program Audacity apparently: http://www.classicrotaryphones.com/f...?topic=17011.0
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Old 9th Nov 2020, 8:20 pm   #14
Mikey405
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Default Re: GPO 746 Dialling Out Issue

Hi Nick and Ian.

Nick - Okay, thank you for the explanation. They both measure 0.1 ohm so I think they're making okay.

Ian and Nick - Looking at the mechanism on this particular kind of dial, I suspect that because of the way the "21" mechanism "snaps" to close the contacts during the dialling process, the 60/40 make/break may be rather more awkward (and possibly nearly impossible) to achieve than the 741 with its "54A" dial. I can see that bending the fixed contact position won't really affect the ratio (except in the wrong direction).

I think I need to be on the lookout for a "54A" dial (or just fit a Rotatone of course...).

Many thanks for all your help though Nick and Ian. It's much appreciated and I've learned a lot today.

Kind regards.

From Mike.
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Old 9th Nov 2020, 11:50 pm   #15
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Default Re: GPO 746 Dialling Out Issue

I've mentioned this before, but we used to use a simple test out in the sticks, where we had no access to any dial test kit. Dial "0" and let the dial operate. Then say slowly "100 /1000 and one", and at the "one" the dial should have returned to rest.
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Old 11th Nov 2020, 3:25 pm   #16
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Default Re: GPO 746 Dialling Out Issue

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Originally Posted by Oldcodger View Post
I've mentioned this before, but we used to use a simple test out in the sticks, where we had no access to any dial test kit. Dial "0" and let the dial operate. Then say slowly "100 /1000 and one", and at the "one" the dial should have returned to rest.
This works in 99% of the cases, we used the same in Norwegian

If you want a mor spessific investegation of the dial pulses it is possible to use a recording from audacity and analyze it well.

http://www.classicrotaryphones.com/f...5673#msg175673
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