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Old 18th Jun 2015, 4:05 pm   #1
andy1702
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Default Need help tracking down 746 dial out problem

Hello all. I'm new to the forum and hoping someone might be able to help with a problem I'm having with a couple of recently purchased 746 phones. They both seem to be suffering from the same problem of not being able to dial out. I have checked my line with a more modern phone set to pulse dial and that works ok. On one of the phones I can also dial by tapping the receiver pegs.

So from what I've read here, I'm thinking my exchange (Utility Warehouse line) might be picky about the dial speed.

However one thing I have noticed with both phones is that when I begin to dial a number there is a click but then the dial tone continues. Am I right in thinking once the first digit is dialled the line should go silent?

I'm new to restoring old phones, so any help gratefully received.

Andy
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Old 18th Jun 2015, 7:03 pm   #2
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Default Re: Need help tracking down 746 dial out problem

With the 'phone disconnected from the line, try connecting an analogue multimeter on resistance setting between terminals T8 and T10. Whilst the dial is returning from dialling (say a zero for longest pulse train), the pointer should quiver with the sequence of disconnection pulses and finish at zero ohms when the dial returns to normal. If this is not happening, it may be worth checking that the dial pulsing contacts are connected correctly: D4 (pink) to T10 and D5 (orange) to T8. The remaining dial contacts - D1 (blue) to T2, D2 (slate, i.e. grey) to T1 short out the receiver whilst dialling; D3 (brown) to T3 and D4 (as before) short out the transmitter.

If you are able to dial using the cradle switch, your line can't be particularly fussy about dial speed!
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Old 18th Jun 2015, 8:54 pm   #3
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Default Re: Need help tracking down 746 dial out problem

Okay, I've done this test now. Unfortunately i only had a digital meter available, but it certainly jumped around while the dial was returning and ended at zero. I also checked that T1 & T2 and T3 & T10 were shorted. T1 & T2 seemed OK, but there was a small resistance between T3 & T10 which seemed to vary a little as the dial returned.

I also tried both phones again. They both have a dial tone, which goes silent when I dial. However when the dial returns to it's rest position the dialling tone comes back. The one exception to this is when I dial 1 on the yellow phone. That sometimes seems to work and remains silent, waiting for the next digit. However once the next digit is dialled and the dial returns to rest then the dialling tone comes back. The other (ivory) phone even the 1 does not seem to catch and the dial tone returns immediately the dial returns to rest.

I can confirm that on both phones I can successfully dial out by tapping the cradle switch.

I have also checked the line cables between where they terminate in the phone and the contacts on the 'new style' plugs. these seem fine with no breaks in evidence.

The dials seem to return at a normal speed.

The only thing I can think of now is maybe some poor contacts in the switch on the back of the dial. Could this be it or have the tests I've done so far effectively eliminated this? I've looked on Rotatone's website and noticed they supply a 'cleaning strip' with their kit to clean these contacts, so I'm thinking they must be a known problem.

I've noticed neither phone has the extra resistor fitted to the ringer, despite both being 746 units rather than 8746. Am I right in thinking this only matters for ringing and would not have any effect on this dialling problem?

I'm a bit stumped.

Andy.
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Old 18th Jun 2015, 9:05 pm   #4
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Default Re: Need help tracking down 746 dial out problem

With the meter connected, rotate the dial as if dialling '0'. Restrain the dial and allow it to return very slowly. The resistance should alternate ten times between 0 ohms and infinite resistance. You'll see some wavering of the reading as the contacts make and break, but this can be ignored.

An analogue meter is ideal for this test, as the position of the needle during dialling shows the make to break ratio of the contacts.
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Old 18th Jun 2015, 10:31 pm   #5
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Default Re: Need help tracking down 746 dial out problem

The problem described on the Rotatone website is not normally relevant in the normal use of rotary dials owing to the much greater current passing through the dial contacts when making and breaking the loop on the telephone network, breaking down any non-conductive deposits on the contacts. Because the Rotatone only draws a minute current by comparison, its operation can be disrupted by very small deposits on the contacts.

I can confirm from personal experience that a 'phone that was working fine when performing loop-disconnect dialling (albeit on a PABX rather than directly on the telephone network) failed to create pulses recognised by the Rotatone until I had dismantled the dial and cleaned the contacts very thoroughly.

And yes, you are correct that the absence of a series resitor with 500Ω bell coils will make no difference whatsoever to dialling. The worst it can cause is to reduce the number of (on-hook) telephone instruments that can be handled in parallel on a single line.

One "silly" thought that occurs to me is that I am assuming that the "B" leg (red wire) of the line cord is connected to T8 and not T10 (while the "A" leg (white wire) is connected to T16/T17/T18/T19).
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Old 19th Jun 2015, 10:33 am   #6
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Default Re: Need help tracking down 746 dial out problem

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With the meter connected, rotate the dial as if dialling '0'. Restrain the dial and allow it to return very slowly. The resistance should alternate ten times between 0 ohms and infinite resistance. You'll see some wavering of the reading as the contacts make and break, but this can be ignored.
Thanks for the tip Graham. I tried this and everything seems to be working as it should from the various tests I've done.

Attached is a photo if the wiring, in the hope that someone out there can spot something I've missed.
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Old 19th Jun 2015, 10:49 am   #7
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Default Re: Need help tracking down 746 dial out problem

The wiring looks correct to me.
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Old 19th Jun 2015, 11:03 am   #8
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Default Re: Need help tracking down 746 dial out problem

Looks fine to me too. Some line cords were/are wired differently to others, although this seems unlikely in your case. Just to be sure can you post a picture of the plug showing the wire colours.
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Old 19th Jun 2015, 11:29 am   #9
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Default Re: Need help tracking down 746 dial out problem

I can't see the colours inside the plug, but from testing the continuity as you look at the plug with the prong end in the air, it's wired white, green, blue, red. Testing between the plug and the terminals in the phone, I get a small resistance on each cable and a slightly larger one when green at the plug is tested with blue at the phone and also blue at the plug with green at the phone, so I guess these are joined together some way in the circuitry as the resistance wasn't infinite.

I've also found a thread on here going way back (on page 11 I think) where someone seemed to have a similar problem with four old trimphones. He said he couldn't break the dial tone, which sounds very similar. Unfortunately the thread just peters out without the problem being resolved.

I'm not sure what to try next.

Andy
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Old 19th Jun 2015, 11:54 am   #10
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Default Re: Need help tracking down 746 dial out problem

That's odd as the green wire is connected to T16 which doesn't go anywhere.

Did you get both 'phones from the same source? It seems odd for two to have the same fault and I'm wondering if they've been deliberately modified at some time. I'd have a look under the circuit board and check that nothing has been added to T16, or anywhere else.

- Joe
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Old 19th Jun 2015, 12:00 pm   #11
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Default Re: Need help tracking down 746 dial out problem

No, they didn't come from the same place, which is what makes it really puzzling. I did get another phone from the same place as one of them, but it's an earlier model and has some parts missing, including the line cord.

What should that green wire to T16 actually do?

Andy.
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Old 19th Jun 2015, 12:31 pm   #12
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Default Re: Need help tracking down 746 dial out problem

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That's odd as the green wire is connected to T16 which doesn't go anywhere.
The green wire is actually connected to T15 which is a spare tag on which the green wire is parked.

andy1702.

Disconnect the line cord and see if there's any continuity from T15 to T6 where the blue wire is connected. If there is, you have problem with the phone, a modification perhaps.

While you're at it check for continuity between the wires of the line cord. There should be none.
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Old 19th Jun 2015, 12:33 pm   #13
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Default Re: Need help tracking down 746 dial out problem

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The green wire is actually connected to T15 which is a spare tag on which the green wire is parked.
Oops! I only just noticed that. from reading around I understand this wire is not used, so is normally parked on an unused tag.
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Old 19th Jun 2015, 12:35 pm   #14
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Default Re: Need help tracking down 746 dial out problem

My mistake, Graham.

My remarks still stand as if the green wire is parked on a spare terminal it should read infinite resistance to the blue one.

- Joe
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Old 19th Jun 2015, 1:46 pm   #15
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Default Re: Need help tracking down 746 dial out problem

You're right! It should indeed (I would have thought)

Let me go and check again what I did. I've done so much poking about with a meter I might have got that bit wrong
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Old 19th Jun 2015, 2:10 pm   #16
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Default Re: Need help tracking down 746 dial out problem

Right... I've checked again, so here are my findings.

What I've been doing is measuring resistance between the plug ends and the terminal blocks in the phone. I get about 3 ohm resistance in each wire and infinite resistance if I touch any of the other terminals connected to the line cable EXCEPT between white and blue. If I touch the white wire at the plug with blue at the phone I get around 65 ohms, and if I touch blue at the plug and white at the phone I get a similar 65 ohms. Doing this I also notice a crackle in the receiver (speaker)! Obviously the phone is disconnected through all of this.

I was wrong about the green line wire. This does absolutely nothing, which (I believe) is correct.

So my next question is... Can I put this 65 ohm between the blue and white as something to do with the circuitry in the phone (as I said, I'm a very green novice here) and hence correct, or is it a potential fault?
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Old 19th Jun 2015, 2:34 pm   #17
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Default Re: Need help tracking down 746 dial out problem

The bell coils sit between the white and blue wires, so it's normal to see continuity there. The "500Ω" or "2kΩ" marked on the coils is an AC impedance, the DC resistance will be smaller than that.
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Old 19th Jun 2015, 4:36 pm   #18
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Default Re: Need help tracking down 746 dial out problem

Well I'm completely baffled now! Every test I do seems to check out ok, but still the phones do not dial out. I can't say for sure they are not dialling, but what I can say is that when I dial a digit the dial tone comes back on the line when the dial returns to it's normal rest position.

So my next question is where does the dial tone actually come from? I had always assumed it was generated at the exchange, but now I'm thinking the phone itself generates it. The reason I came to this conclusion is the dial tone on the two (supposedly identical) phones is slightly different.

I think I could do with a crash course in phone basics to help me chase this problem. Until last week I hadn't touched a dial phone for about 20 years, so I'm finding the learning curve a bit steep. All good fun though!
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Old 19th Jun 2015, 4:39 pm   #19
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Default Re: Need help tracking down 746 dial out problem

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The "500Ω" or "2kΩ" marked on the coils is an AC impedance, the DC resistance will be smaller than that.
Actually, that IS the d.c. resistance! The a.c. impedance is typically higher than that.
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Old 19th Jun 2015, 5:03 pm   #20
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Default Re: Need help tracking down 746 dial out problem

Dial tone comes from the exchange. It is removed from the line as soon as any digit is dialled.

The photo indicates that the phone is wired correctly. Assuming everything we've been told is true (I'm not suggesting you're lying) then, if the fault is in the phone, it can only be down to the wrong dial speed or make to break ratio.

Does the dial tone disappear if you dial digit '1'?

Can you actually dial numbers when tapping the switch hooks, or merely "tap" digit '1'? Be extremely careful when trying this lest you tap out 112.

Does the dial take about one second to return to rest when you dial digit '0'?

Quote:
Originally Posted by andy1702 View Post

So my next question is where does the dial tone actually come from?
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