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Old 6th Dec 2021, 5:06 pm   #1
SjjBeard
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Default 746 no dial tone. Inexperienced repairer hoping for help.

Good afternoon. This is my first post as a new user so thanks in advance for taking the time to look at this query.

I bought this phone for spares or repair and after a bit of research, I think it's a GPO 746(f?) from the late 70s. There are no markings on the underside of the base unlike most models I've seen. They've most likely been worn away after a few decades' use.

I had to source some spare parts (mostly for the handset) to get it close to working and it's pretty much there but I can't get a dial-tone and I can't hear the other end of the line.

The phone can make and receive calls and the person at the other end can hear me okay but I can't hear them at all.

When I lift the handset there is a quiet hum where there should be a dial-tone and If I blow into the mouthpiece I can hear it through the speaker. I can also hear the relevant clicks when dialling.

If anyone is able to take a look at my wiring or help in any way, it would be much appreciated!

Thanks again.

Stuart

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Old 6th Dec 2021, 6:24 pm   #2
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Hi Stuart and welcome.

I would agree that it's a 746 (or non-GPO equivalent) from 1979, made by Plessey.

At a quick glance, your wiring looks fine.

What parts have you changed, please?

Are you definite about the blowing into the microphone and hearing it in the earpiece bit? Does it stop doing that if you press down the black buttons which the handset would normally sit on?

In the absence of any better ideas from others here, a few things to check:

(1) Check that the dial contact which short the earpiece when dialling aren't stuck closed. You could temporarily disconnect either the grey or blue wires from T1 or T2 to see if that makes a difference. Similarly, temporarily remove the click supressor (the square black thing) from T1/T2, just in case it's gone short circuit.

(2) Failing that, remove the PCB (one screw in the centre) and look under good light for any broken tracks.

(3) Check both the "bulbs" in the black holder have continuity, using a multimeter on its low ohms range.

(4) Also see that the fine copper wires going from the coils on the ASTIC (the thing just above the screw, and slightly to the right) haven't broken away from their solder posts; this can happen if the phone's dropped.

N.
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Old 6th Dec 2021, 7:08 pm   #3
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

In addition to Nick's suggestions above, if you have a multimeter, set it to ohms and touch its probes to T1 and T2 with the 'phone unplugged. You should hear a crackle in the earpiece as you do so - and note the reading given by the meter.

Another check (plugged back in to the line) is with said multimeter set to an appropriate DC volts range, check the voltage between T8/T9 (-ve) and T16/T17/T18/T19 (+ve). It should be about 9 or 10V with the receiver off-hook and 50V (ish) on-hook.
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Old 6th Dec 2021, 7:30 pm   #4
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Thanks so much, Nick and Dave

With regards the multimeter, I have just ordered one so will have to wait until that arrives before I can report back.

I replaced both the line and headset cables as well as the 4T earpiece speaker and the no. 16 mouthpiece transmitter (replaced with a 21A electret transmitter)

Iíve checked through these steps with the following results.

1) After removing the click suppressor, the line level is greatly increased and I definitely hear myself blowing in the earpiece - this stops when I press the black buttons for the handset cradle. After temporarily disconnecting the grey wire from T1 there is no change.

2) The PCB looks good to me!
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3) See above re: multimeter.

4) It's really quite hard to see but I think there could well be some broken/missing copper wires. If so, is this something that one might be able to fix themselves?
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Thanks again
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Old 6th Dec 2021, 10:07 pm   #5
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Quote:
Originally Posted by SjjBeard View Post
It's really quite hard to see but I think there could well be some broken/missing copper wires. If so, is this something that one might be able to fix themselves?
Are not the connections to the ASTIC via the PCB rather than there being wires connected to the opposite side?
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Old 7th Dec 2021, 9:11 am   #6
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

You should be able to see fine wires coming from the coil, soldered to the pins embedded in the plastic, which then go through the PCB and are soldered to its pads.

It's difficult to see from a photo, but it looks a bit iffy in that area.

When you get your meter, you can do some resistance checks to see if the ASTIC is OK. The expected DC resistance of its windings are printed on the ASTIC, and its terminal numbers are moulded into the top of its plastic former.
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Old 7th Dec 2021, 11:48 am   #7
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Thanks again for the replies.

I will update as soon as possible with readings.
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Old 7th Dec 2021, 4:22 pm   #8
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Quote:
Originally Posted by Nickthedentist View Post
You should be able to see fine wires coming from the coil, soldered to the pins embedded in the plastic, which then go through the PCB and are soldered to its pads.

It's difficult to see from a photo, but it looks a bit iffy in that area.

When you get your meter, you can do some resistance checks to see if the ASTIC is OK. The expected DC resistance of its windings are printed on the ASTIC, and its terminal numbers are moulded into the top of its plastic former.
Hi Nick

The readings are as follows

1-2 = 10.8 (11 expected)
2-3 = 14.1 (7 expected)
4-5 = 45.3 (23 expected)

Of course, I don't understand the significance of these numbers but obviously the second two readings are around twice that of what's expected which can't be good?

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Old 8th Dec 2021, 1:03 pm   #9
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Quote:
Originally Posted by Dave Moll View Post
In addition to Nick's suggestions above, if you have a multimeter, set it to ohms and touch its probes to T1 and T2 with the 'phone unplugged. You should hear a crackle in the earpiece as you do so - and note the reading given by the meter.

Another check (plugged back in to the line) is with said multimeter set to an appropriate DC volts range, check the voltage between T8/T9 (-ve) and T16/T17/T18/T19 (+ve). It should be about 9 or 10V with the receiver off-hook and 50V (ish) on-hook.
Hi Dave,

Regarding the first point, unplugged I get the crackle and a reading of 22 ohms.

Regarding the second, this has thrown some... interesting findings into the mix and makes me think the problem might be more complex. There appears to be no voltage coming from the BT phone line. I didn't get any DC voltage readings on the phone between T8 and T18 so I dug down to the master socket and there was still no voltage coming from the test line.
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I took the socket off the wall and looked at the board inside the red rectangle where the two IDC connectors bring the main BT line inside the house. When I touch the probes at these two point, again no voltage.

Is it even possible that my ADSL internet could work fine through a microfilter and my phone could ring/dial out despite no voltage or am I just being a complete n00b somewhere?
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Old 8th Dec 2021, 1:48 pm   #10
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

To answer your last question first, yes, ADSL & VDSL do not require the 50v supply on the line.

But I think that's a red herring because without the line voltage you wouldn't hear side-tone (blow-back) or be able to make or receive calls.

Maybe you have an open circuit winding in the ASTIC coil.

P.S. have you tried measuring a known voltage source on your new meter, say a 9 Volt battery?

It might be that your meter probes are too fat to make contact to through the IDC connectors in the photo.
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Old 8th Dec 2021, 4:27 pm   #11
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Quote:
Originally Posted by rambo1152 View Post
To answer your last question first, yes, ADSL & VDSL do not require the 50v supply on the line.

But I think that's a red herring because without the line voltage you wouldn't hear side-tone (blow-back) or be able to make or receive calls.


Maybe you have an open circuit winding in the ASTIC coil.
I missed the fact that calls can be made in and out and that the other party can hear you which, as rambo1152 says, confirms that the line is working.

It does appear that the problem lies within the ASTIC in that the audio on the line isn't reaching the receiver, but looking at diagram N846, I would expect an open-circuit winding to give a much higher resistance reading than twice the expected one, given the external components through which measurement would be made. Incidentally, I assume the readings are with the handset off-hook. On-hook there shouldn't be any external path between 4 & 5 of the ASTIC, given that the hook-switch isolates terminal 4. But then o/c winding between 4 & 5 would mean that there was no circuit between the transmitter and the line, so no way for sound to be transmitted to the other telephone.

Did you say you were hearing sound in the receiver when you blow into the transmitter? I can't find this stated - but I've already proved that I sometimes miss things! If not, it seems likely that the problem lies in the winding between 2 & 3.
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Old 14th Dec 2021, 3:42 pm   #12
SjjBeard
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Hi all, thanks for your help and advice.

I've now been able to test with a fully working board from a different 746F and everything works perfectly so I think you are all correct with the ASTIC coil diagnosis.

My next question is, would it be possible to buy an ASTIC separately and replace the defective one? If so does anyone know where might I get my hands on one?

Thanks again, Stuart.
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Old 15th Dec 2021, 7:55 am   #13
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Just a thought from someone who is not much more experienced than SjjBeard him/herself, and way less experienced than almost all the other contributors on this forum, but ...
Whilst admittedly this observation might have been more useful before money had been spent on the multimeter etc., for the amount of time, trouble, surgery and potential expense involved in replacing parts, wouldn't it be easier either to just ( a ) buy another known-working phone of the same type, or if ( like me ) you're particularly fond of the green colour, buy another known-working phone of the same type and replace the whole of the innards lock, stock & barrel? This would only require changing the line and receiver wires, which is not a big deal even for someone with absolutely no experience.
As I say, just a thought, or am I just being too simplistic ...?
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Old 15th Dec 2021, 11:49 am   #14
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Telephone guy... you're right of course, but where's the fun in that? And the sense of satisfaction that comes with correctly diagnosing and then rectifying a fault is great. And nice to keep things as original as possible by repairing rather than making yourself a "bitsa".

But on that note, I have to admit that I'd probably swap the whole PCB rather than just the ASTIC, simply because complete PCBs often turn up for very little, whereas the ASTICs alone virtually never do. These people sometimes have them in stock, so might be worth ringing: https://telephonelines.net

Or ask in the "wanted" section of this forum.

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Old 15th Dec 2021, 1:02 pm   #15
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

You can get a working 746 board from Telephone Lines for a fiver with free postage - https://telephonelines.net/index.php...roducts_id=774
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Old 15th Dec 2021, 4:34 pm   #16
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

A fiver for a 746 board with postage sounds a very reasonable repair expense and a bit of fun swapping the board.
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Old 16th Jan 2022, 8:59 pm   #17
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

op- have a look at http://www.samhallas.co.uk/repositor.../0000/N846.pdf
which gives the circuit for the board. This may help you to fault the PCB.
The complete list of N diagrams for GPO phones adds 100 to the number of the phone ( i.e a 746 has diagN846. I've no idea of the numbers of push button phones as this was prior to my time on subs app).
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Old 17th Jan 2022, 6:59 am   #18
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

I guess that the tough way to get your phone working, not perfect but surprisingly well will be to move the wire from the handset slightly.
The wire terminated to terminal marked T1 should be tried to move to Terminal 19A or 19B (Not only 19, it Has to be A or B to be sure to break dial tone when going on hook)
No harm is done if it does not work.
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Old 17th Jan 2022, 7:32 am   #19
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Seems to be several versions of this board

The wire terminated to terminal marked T1 should be tried to move to Terminal 19A or 19B if you have those terminals. If not try to clip it to one of the legs on the induction coil (transformer) marked 4 or 6.
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Old 17th Jan 2022, 8:08 am   #20
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Default Re: 746 no dial tone. Inexperienced repairer hoping for help.

Looking at the image in post 1, this board doesn't have T19A or T19B.

I think these were added when push button variants became available.
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