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Old 16th Jan 2015, 4:02 pm   #1
only1carra
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Default GPO 782 Won't Dial Out

Hi guys, not posted on here for a long time, hoping someone can help.

I have 5 GPO 782 phones (I think this is the model, just going by images on google)

I believe these were once part of an office phone system.

The problem is I can not dial out on the phones. When you dial a number the tone goes quiet but does not dial the numbers. You can still here the dial tone really quiet as you begin to dial a number. I have removed the button what I think is a hold button? Just in case this was causing the problem but the issue still remains.

The wiring looks OK to me and I have converted GPOs in the past but I am no expert.

The bell rings and you can receive calls no problem, tone and volume is fine , just cant dial a number.

Any advice welcome, have attached some pics

Many Thanks
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Old 18th Jan 2015, 7:03 pm   #2
mike_newcomb
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Default Re: GPO 782 Wont Dial Out

Hi Carra,

I am not an expert on this, but could it be the phone is set for pulse dialling and the line is set for only tone dialling?

Regards - Mike
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Old 18th Jan 2015, 8:09 pm   #3
dagskarlsen
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Default Re: GPO 782 Wont Dial Out

I do not know this one, but some US telephones was depending on line polarity.
dsk
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Old 18th Jan 2015, 8:11 pm   #4
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Default Re: GPO 782 Wont Dial Out

I've had this happen and it turned out to be the DIL DTMF chip on the keyboard PCB. Fitting another from a scrapper got it going, but they're hard to come by these days.

BTW, I think this is a DTMF-only phone, designed for PABX use. The circuitry is very simple with no memory or pulse-lengthening, so you have to hold down each key press for 1/2 second or so. "Stacatto" dialing doesn't work.

If these are GPO phones, rather than the Plessey own-brand ones, the model number will be stamped in white ink on the base.

Nick.
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Old 18th Jan 2015, 8:12 pm   #5
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Default Re: GPO 782 Wont Dial Out

Quote:
Originally Posted by dagskarlsen View Post
I do not know this one, but some US telephones was depending on line polarity
I think all our British ones had a bridge rectifier - the 4 diodes on the bottom left of pic 4.
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Old 19th Jan 2015, 6:28 pm   #6
only1carra
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Default Re: GPO 782 Wont Dial Out

Thanks for your replies, My rotary dial phones work fine on my line so im pretty sure it is actually the keyboards that are the isse.

Many thanks
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Old 19th Jan 2015, 6:34 pm   #7
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Default Re: GPO 782 Wont Dial Out

PB phones which use rotary dialling generally have a reed relay to generate the dial pulses. If listen carefully after dialling '0' you may be able to hear the relay operating.

As stated though this is most likely a DTMF/MF4 tone dialling phone as shown here:-

http://www.samhallas.co.uk/repositor.../0000/N882.pdf
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Old 19th Jan 2015, 7:09 pm   #8
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Default Re: GPO 782 Wont Dial Out

I really do suspect the DTMF IC on this model.

My budget-conscious school bought about 100 of these in the early 1990s as surplus. They worked very well, but several failed exactly like this, and subbing the chip proved that it was responsible.

Nick.

Last edited by Nickthedentist; 19th Jan 2015 at 7:17 pm.
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Old 19th Jan 2015, 9:38 pm   #9
only1carra
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Default Re: GPO 782 Won't Dial Out

So is there any way I can fix these economicly or is it not worth it?
Thanks
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Old 19th Jan 2015, 10:41 pm   #10
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Default Re: GPO 782 Won't Dial Out

One thing we haven't established. When you press a button do you hear a DTMF/MF4 tone in the earpiece?
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Old 19th Jan 2015, 10:43 pm   #11
only1carra
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Default Re: GPO 782 Won't Dial Out

When you begin to dial a number you can still here the tone you normally hear when you lift the handset but its very quiet.
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Old 19th Jan 2015, 11:01 pm   #12
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Default Re: GPO 782 Won't Dial Out

If the phone is wired up correctly, then the fault is almost certainly on the Printed Circuit Board (PCB) to which the keypad is attached.

As Nickthedentist suggests the fault is most likely the Integrated Circuit (IC) on this board. You could change this assuming you have the skills to do so but the phone still might not work.
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Old 19th Jan 2015, 11:49 pm   #13
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Default Re: GPO 782 Won't Dial Out

Changing the chip is easy, it's finding a suitable replacement that's probably impossible.
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Old 20th Jan 2015, 1:53 am   #14
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Default Re: GPO 782 Won't Dial Out

In post 1 you mention 5 phones, are they all faulty?
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Old 20th Jan 2015, 10:23 am   #15
only1carra
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Default Re: GPO 782 Won't Dial Out

Yes they are all faulty ,well at least all have the same issue on my line, they may not be faulty at all. They are all from the same office, all exactly the same phone.
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Old 20th Jan 2015, 10:35 am   #16
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Default Re: GPO 782 Won't Dial Out

There is an outside chance that they were all faulty when you got them, for example, they might have been put aside one-by-one when they went wrong, and then forgotten about.

But I hope there's another explanation...
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Old 20th Jan 2015, 8:47 pm   #17
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Default Re: GPO 782 Won't Dial Out

I'm pretty sure that DTMF-synthesis ICs are pretty readily available (I've come across them in recent catalogues), but how much modification might be needed to make them work in these phones I don't know.
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