GPO 706 in trouble.

[SeanStevens]

Originally Posted by russell_w_b

Have you tried doing the battery test with T8 and T10 shorted together yet?

I will do this tonight. If this 706 ever becomes useable it will be a miracle!




I will also test the wires from speaker to board.


SEAN

[russell_w_b]

Originally Posted by SeanStevens

I will also test the wires from speaker to board.

I'm surmising these will be OK, given that your telephone passed the music test. Even if the receiver was removed, the telephone should still 'work' (no sound, obviously!).

The microphone forms part of the DC loop, and - if the mic circuit is broken - the telephone will not function. Have you measured the resistances yet, as I suggested earlier?

[SeanStevens]

No, I have not checked the resistances yet - my meter is rubbish and I'm not too clever either.



But I'll try as many tests as I can tonight - I may have the house to myself, so I can concentrate.

SEAN

[SeanStevens]

OK - I've been given 30 minutes by the authorities!

Short out the microphone (terms: 3, 10) and measure the resistance, off hook, between terms: 8 and 18 (red and white connections, or 2 and 5 to your LJU - socket numbers!). You should see something like 25.5 Ohms with the regulator in-circuit, or 19 Ohms with the regulator shorted out (reversed).

Done, 38.3 ohms (according to my dodgy multimeter) and 1 when the buttons are depressed.

There is continuity from speaker to board (both blue and white)

SEAN

[russell_w_b]

Originally Posted by SeanStevens

Done, 38.3 ohms (according to my dodgy multimeter) and 1 when the buttons are depressed.

Hang on a minute... What buttons? You don't mean the handset cradle switch, do you? If you do indeed mean the handset cradle switch, and you're only measuring 1 Ohm, then you've got a dead-short across your incoming pair when you're 'on-hook'!

Originally Posted by SeanStevens

There is continuity from speaker to board (both blue and white)

Do you mean the mic? Blue and white to microphone (terms: T3 and T10), red and green to 'speaker' (terms: T1 and T2). Don't get them the wrong way around!

[Nickthedentist]

Originally Posted by russell_w_b

Hang on a minute... What buttons? You don't mean the handset cradle switch, do you? If you do indeed mean the handset cradle switch, and you're only measuring 1 Ohm, then you've got a dead-short across your incoming pair when you're 'on-hook'!

I think he does mean handset cradle switch. But by "1", I suspect he means that the DMM's reading open-circuit, displayed as 1- on many models.

Nick

[russell_w_b]

Originally Posted by Nickthedentist

'But by "1", he means that the DMM's reading open-circuit, displayed as 1- on many models.'

Ahhh... I'm used with the 'O.L' (Open Line) variety. Bit misleading, that; didn't know 1 was an option for open-cct. Guess I've been spoiled!

No such ambiguities with an AVO!

[russell_w_b]

There's a remote possibility that capacitor C1B is short-circuit and shunting your microphone via R2 and the regulator. C1B and C1A (the un-used bell-capacitor) are contained in the one can, but to check that it's OK, do the following:

• Remove the regulator from its slot.
• With your metimulter on 'Ohms', measure between Terms: T10 and T2. You should hopefully see the display increase over the Megohm range until, after a few seconds, it reads open-circuit (whatever 'open-circuit' on your meter is ).
• You should not expect to read a measurable resistance here. If you do, the capacitor is shot.

[SeanStevens]

Sorry for the confusion and my lack of correct terminology - I'm not trying to be difficult, but may look that way

I guess that the test I did with the meter is not showing anything wrong? The 38 Ohms - then (1) open circuit when the cradle switch is depressed is correct.... ? Feedback on these results would help my understanding immeasurably.

Anyway,

Originally Posted by russell_w_b

Remove the regulator from its slot.

That is the PCB under the cradle? If so, I'll do that tonight. There should be a in it for the person who sorts this phone out!

Thanks to russell_w_b and Nickthedentist for all your efforts so far.

SEAN

[Nickthedentist]

Originally Posted by SeanStevens

I guess that the test I did with the meter is not showing anything wrong? The 38 Ohms - then (1) open circuit when the cradle switch is depressed is correct.... ? Feedback on these results would help my understanding immeasurably

Yes, that sounds right to me.

Basically, the phone should have a very high resistance when the handset's "down", and a low one when the handset is picked up.

Keep at it, you can't be far now...

Nick.

[russell_w_b]

Originally Posted by SeanStevens

I guess that the test I did with the meter is not showing anything wrong? The 38 Ohms - then (1) open circuit when the cradle switch is depressed is correct.... ? Feedback on these results would help my understanding immeasurably.

Correct. But do remove the regulator (that little slide-in, reversible PCB) and check for an eventual open-circuit between terms: T10 and T2. I say 'eventual', because your meter, set to 'Ohms', will charge up capacitor C3 (0.9uF).

If you do measure a finite, steady resistance here, it's the capacitor at fault and it must be replaced.

Incidentally, who provides your telephone service? Have you either plugged your 706 directly into the master-box with no extensions connected or tried it on someone else's telephone line?

[Nickthedentist]

I have seen 706s with cracked print on the circuit boards; one had even been repaired with wire links by the GPO.

It might be worth checking continuity along all the tracks. You would need to remove the PCB for this, obviously.

Nick.

[SeanStevens]

Originally Posted by russell_w_b

Correct. But do remove the regulator (that little slide-in, reversible PCB) and check for an eventual open-circuit between terms: T10 and T2. I say 'eventual', because your meter, set to 'Ohms', will charge up capacitor C3 (0.9uF).

If you do measure a finite, steady resistance here, it's the capacitor at fault and it must be replaced.

Incidentally, who provides your telephone service? Have you either plugged your 706 directly into the master-box with no extensions connected or tried it on someone else's telephone line?

I have a standard BT line, I'm using the upstairs extension. Unfortunately this is the wrong side of the ADSL filter

The main box in the hall is difficult to test from, so I'm sort of stuck with the extension for the upstairs. Ideally the whole house would run off of the ADSL filter, but even so, the phones used upstairs are useable, but I notice the ADSL drops out and needs time to gets its ducks back in a row after a call is received/made from upstairs.

SEAN

[Nickthedentist]

Do quickly try the master socket; it would be frustrating to waste time looking for a fault on the phone if in fact, the fault were on your extension wiring.

[SeanStevens]

Well, all other phones work OK on the extension (althouth - as discussed earlier, the ADSL squealing is present in the earpiece)

I will try from the master socket - but for some reason remember that it was difficult to connect wires to for some reason.

SEAN

[russell_w_b]

Originally Posted by SeanStevens

'I will try from the master socket - but for some reason remember that it was difficult to connect ...'

Could you pop next-door with it and try it there? Such a move would eliminate any hitherto-undiscovered line faults you may have.

- Assuming your neighbours' line is OK, of course!

[Nickthedentist]

Originally Posted by SeanStevens

Well, all other phones work OK on the extension

Don't forget that modern phones are wired in a slightly different way; most only need 2 wires to function and polarity doesn't matter, whereas 700-series phones need a 3rd wire too, and won't work properly if the other 2 wires are wired "backwards".

Nick.

[SeanStevens]

I'm worried that I'll end up back at the exchange at this rate!!!



I have a 746, a modern phone and a 50's bakelite 310 and they all work fine from whatever socket I use.

I think the fault is with this 706, but I will try from the master socket.

SEAN

[AndiiT]

Originally Posted by SeanStevens

...............I have a 746, ......................

Hi,
Electrically the 746 and 706 are almost identical so you should be at least able to do some resistance comparison checks between the two telephones; if you do go down this route you need to make sure that the regulator is fitted the "right way up", as it were, in your 706 from memory this would be with the higher number of contacts towards the PCB.

Getting back to fault finding on your telephone, as well as what everyone else has advised on here, I would proceed as follows -

Forget the bell circuit for the time being and concentrate on getting the transmisison side of things working. Initially strap as follows -

T5 - T6 - T7
T8 - T9
T18 - T19
Red of line cord to T8
White of line cord to T18
Leave all other line cord connections disconnected for now and ensure no other straps are fitted than those mentioned above

Ensure that the regulator is fitted as mentioned above and that the handset cord is wired as follows -

White T10
Red T1
Green T2
Blue T3

Try the telephone connected direct to your master jack or to a line jack/master socket which has a 9 Volt battery (or power supply) connected across terminals 2 and 5. Dial tone should be present if connected to an exchange line and if using the "battery technique" you should have "Blow" (i.e. the ability to blow in to the micropone and hear this at the receiver end)

The next step would be to try reversing the regulator in its jack.

If this does not work short out terminals T3 - T10 at the telephone end, when connected to a known working exchange line and see if you then get dial tone in the earpeice. After trying these tests report back with your findings and I/we will (hopefully) be able to advise further

Most telephones of this type reasonably straighforward to fix providing that methodical approach is followed.

I must echo what Nick says about the possibility of the PCB being cracked so it may be worth checking this too.

Regards
Andrew

[SeanStevens]

OK:

Correct. But do remove the regulator (that little slide-in, reversible PCB) and check for an eventual open-circuit between terms: T10 and T2. I say 'eventual', because your meter, set to 'Ohms', will charge up capacitor C3 (0.9uF).

If you do measure a finite, steady resistance here, it's the capacitor at fault and it must be replaced.

There is an open circuit (1 on my ohm meter) between T10 and T2 with the removeable card in or out or upside-down. Now I'm hoping this will give me the naughty component to replace....??


SEAN