Need help tracking down 746 dial out problem

[Station X]

65 ohms DC resistance for the bell coils sounded all wrong to me. To get that up to 1K impedance you'd need about 1K of inductive reactance. A quick in the head calculation showed that the bell coils would need an inductance of about 6.5 Henries to achieve this and they don't look anywhere big enough to be that inductive.

So I measured the bell coils in one of my own phones and measured..............

50 ohms.

After a bit of head scratching I realised that the phone was OFF Hook. Placing it ON Hook gave a reading of 1050 ohms, which is much better.

Studying the circuit for a CONVERTED phone showed that when OFF Hook the bell coils are shunted. Hence the 50 ohm reading.

[ThePillenwerfer]

Great minds, Graham: I went and tested one too with the same result.

- Joe

[Station X]

About 50 years ago I used to install these phones for a living. They're very simple devices and even though they're now 40 to 50 years, I'm always puzzled as to why it's sometimes so difficult to get them to work on modern plug and socket systems. I've investigated a few problems for friends and neighbours and in all cases it was down to faulty (incorrectly installed) DIY extension wiring, often with more than one master jack installed.

I pulled a converted 746 out of my attic where it had been languishing for the last 30 years. When plugged into a BT socket, served by a BT exchange, it worked straight out of the bag.

I took a few resistance and voltage readings and here are the results for reference.

PHONE UNPLUGGED

T4 to T16 (Bell Coils)
Bell coil resistance On Hook. 1050 ohms.
Bell coil resistance Off Hook. 50 ohms.

T9 to T19 (Across Line)
Resistance On Hook. Infinite (open circuit).
Resistance Off Hook. 42 ohms.
Resistance with dial off normal 32 ohms.
Resistance with dial contacts open. Infinite (open circuit).

PHONE PLUGGED IN.

T9 to T19 (Across Line)
Voltage On Hook. 47.5V
Voltage Off Hook 7.5V
Voltage with dial off normal. 1.3V
Voltage with dial springs broken. 47.5V

[Dave Moll]

Originally Posted by Station X

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.

According to post #3, doing this does have the desired effect. This was why I concluded that the line can't be too fussy about dial speed and ratio.

[Station X]

I concluded that too, but there's something we're all missing here. Everything says that the phone should work, but it doesn't. We're missing something or misunderstanding something the OP has said.

[andy1702]

I just read all your posts since my last attempt at getting these phones going this afternoon. Thanks for all the tips. I think my next step will be to go through all the same tests Graham did with his known working phone. For the plugged in tests I'll isolate my home wiring and do it from the hidden socket in the old BT master where it comes into the house. I do have a few extensions, all with filters for broadband, so getting that out of the equation will help a lot.

To confirm, I can dial any number on both phones by tapping the hook switch. I actually did 123 for the speaking clock quite easily. However when I use the dial normally, on the yellow phone I can occasionally get "1" to work and the dial tone is correctly stopped. However as soon as I dial another digit the line stays silent until the dial returns to it's normal rest position (I understand the speaker is shorted out while the dial is moving), but once the rest is reached then the dial tone returns, even if dialling "1" previously worked to kill it.

On the ivory phone, I can't even get the digit "1" to kill the dial tone.

It's like the dial coming to rest also activates the hook switch somehow, although I'm by no means saying that is what actually happens.

Another thing I want to try is plugging the phones into someone else's line. The trouble is finding someone around here who is still on BT is not easy!

[andy1702]

Originally Posted by Station X

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

Sorry, I missed this question earlier. The dials on both phones seem to be running at somewhere around the correct speed. I've used the test of saying "one thousand one hundred and one" and the dial comes to rest just as I finish.

I wonder if my exchange wants the pulses quite fast? When I tap them out on the hook it only works when I tap them really fast. From memory of doing something similar as a kid, I don't think I used to tap them as fast as I have to now. I'm clutching at straws at this stage

Andy.

[ThePillenwerfer]

Originally Posted by andy1702

Another thing I want to try is plugging the phones into someone else's line. The trouble is finding someone around here who is still on BT is not easy!

I don't know for sure about Chesterfield but Virgin exchanges in Sheffield work with dial 'phones OK.

- Joe

[Station X]

I think that the limitations of the human body and the inertia of the switch hooks would make it extremely difficult to tap out digits at the correct rate of 10 pulses per second. It would even more difficult to get the correct make to break ratio.

When working in a strowger exchange at a quiet time it was always easy to hear and see when someone was "tapping up". The slow clunky irregular sound and movement of the selectors would give the game away.

[russell_w_b]

Do you have a push-button telephone with 'loop disconnect' (pulse dialling) you could try? This will give a precise set of pulses and may act as a guide as to what is required.

[andy1702]

Yes, I've tried a push button phone set to LD for dialling and that works without any problem. That, coupled with the fact there doesn't appear to be any visible fault with the phones, is what makes it puzzling.

Andy

[russell_w_b]

I see... (I also see that you'd done that if I'd read your original post correctly! )

If your dial contacts (pulse and 'dial off-normal') are in order and you haven't got any contact 'bounce' which may be cutting you off after pulsing, then I suggest (as you've said yourself) trying your telephones on a different exchange to see if they'll dial out there. Otherwise you might just be chasing your tail.

As Graham says, an analogue meter is way better at checking dial pulses and you'll get an indication of the mark/space ratio from the position of the pointer.

[dagskarlsen]

You may take a look at this thread:
http://www.classicrotaryphones.com/f...7436#msg147436

Usually the make brake ratio is less important, but the dial speed might be of great interest.

dsk

[andy1702]

I've now completed Graham's tests on both phones. Here are the results...

T4 to T16 (Bell Coils)
Bell coil resistance on hook....Graham's phone 1050 ohms .... Yellow phone 1073 ohms .... Ivory phone 1042 ohms.
Bell coil resistance off hook....Graham's phone 50 ohms .... Yellow phone 63 ohms .... Ivory phone 62 ohms.

T8 (not T9 as in Graham's post) to T19 (Across Line)
Resistance on hook....Graham's phone INF (open circuit) .... Yellow phone INF .... Ivory phone INF
Resistance off hook....Graham's phone 42 ohms .... Yellow phone 450 ohms .... Ivory phone INF
Resistance with dial off normal....Graham's phone 32 ohms .... Yellow phone 35 ohms .... Ivory phone 31 ohms
Resistance with dial contacts open....Graham's phone INF .... Yellow phone INF .... Ivory phone INF

All the above tests were done with the phone unplugged.
Next I plugged the phone into the hidden test socket in the master, where it comes into the house, and measured the following voltages across the line T8 to T19.
Voltage on hook....Graham's phone 47.5V .... Yellow phone 51.2V .... Ivory phone 51.3V
Voltage off hook....Graham's phone 7.5V .... Yellow phone 2.1V .... Ivory phone 6.5V
Voltage with dial off normal....Graham's phone 1.3V .... Yellow phone 0.9V .... Ivory phone 0.8V
Voltage with dial springs broken....Graham's phone 47.5V .... Yellow phone 51.4V .... Ivory phone 51.3V

I've highlighted the suspect readings I got in orange and red above.

While doing these tests I noticed the two phone's are not quite identical, so I'm not sure if the slightly different models should cause any of the readings to be different. See the attached photos to see the differences between these phones.

[ThePillenwerfer]

The ivory one is two years younger, 1981 against 1979. The only difference is that the rectifier element is permanently fitted in the former whereas in the latter it's screwed to T1 and T2.

I'll leave our more learned brethren to interpret your readings, at least you seem to have found SOMETHING to investigate.

- Joe

[Station X]

If the ivory phone had infinite resistance across the line when off hook, it wouldn't work. I suspect this reading is in error because the off hook VOLTAGE for the same phone looks OK.

The 450 ohm off hook resistance of the yellow phone is high and the voltage low. Most likely culprit is the microphone which is short-circuited when the dial is moved off normal. You could prove this by short-circuiting the mic by putting both wires on the same tag on the rear of the mic.

I'm afraid nothing so far ties up with your problem dialling out. Sounds like a speed or make to break ratio issue, but the ability to "tap up" is in conflict with this.

[andy1702]

The infinite resistance off hook measured across the line of the ivory phone is only infinite when the phone isn't plugged it. When it IS plugged in it measures around 300-400 ohms. I traced back the wires on the circuit board and found I eventually ended up at one side of the microphone. I put the meter across the mic (while still connected to the circuit board and got inf resistance (not plugged in). I did the same on the yellow phone and got about 250 ohms.

I've also disconnected the dial from the ivory phone and checked the switch contacts actually have continuity when closed. They all appear to be ok.

Interestingly the resistance off hook reading as per the original test of the yellow phone is now 250 ohms, not 450. This seems to be different every time I check it, so I'm wondering if there is a capacitor somewhere along the line doing something funny?

Also I don't seem to be able to unscrew the mic part of the ivory phone's handset. Are these sometimes a sealed unit? The speaker end unscrews easily.

[andy1702]

Also I've disconnected the dial from the ivory phone and checked the three switch contacts do actually have continuity when closed. They all check out ok.

[Station X]

A phone which is plugged in is powered up from the 50V DC line voltage.

As a general rule you should NEVER use a meter to measure resistance in a powered circuit. The reason is that the meter measures resistance by applying a voltage to the circuit, measuring the current which flows, then effectively calculating the resistance by using Ohms Law. If the circuit is powered up the external voltage will affect the reading and render it useless. Apart from that, if the equipment being worked on has high voltages present (the HT in a valve radio for example) it could blow up your meter.

It sounds like you have a problem with the microphone. You could prove this by swopping the mics between the two phones.

What's probably happening is that, with line voltage applied, the open circuit in the mic breaks down, which is why you're measuring the correct voltage when off hook.

The problem with the mic won't be anything to do with capacitors on the line. The resistance of carbon microphones does vary. That's how they work. They can sometimes "fry" causing a sound like frying bacon on the line. The way to check for this was to dial say digit '1', whereupon the line should be quiet with no noise in the receiver. If noise was present, the mic would be short-circuited (generally with a pair of thin nosed pliers). If the noise disappeared the mic was at fault.

700 type handsets were never sealed units, so you should be able to unscrew the mouthpiece. Gentle tapping all round, or gentle heat may help.

[andy1702]

I'll go try a receiver swap right now and get back to you.