Struggling to convert GPO 706.

[only1carra]

Hi
I have an old 706 phone and opened it up to convert. I have converted a few successfully in the past with no problems but this one seems to be different.

When I follow the standard instructions for converting there is no dial tone. After returning the phone to how it was when I got it ( see how it is wired in pics) there is now a dial tone but the phone won't ring. It rings for a split second when you lift the handset.

The pictures show how it is currently standing, any help would be appreciated
Thanks

[Station X]

Are you sure it's a 706? I can't see a regulator and I've never seen a single bell coil like that before. What does it say on the underside?

[only1carra]

It looks like a 706 but the base seems from a later era? Doesn't say anything on the bottom so can't be 100% sure.

[Station X]

It's probably best to assume that although it isn't a 706, it's probably wired similarly to one.

That resistor doesn't look right to me, perhaps it's been fitted to overcome a fault in the circuit?

Have a look at diagram N806:-

http://www.samhallas.co.uk/repositor.../0000/N806.pdf

and check the continuity of the bell coils. Forget about a series 3.3K resistor for the moment and bridge the tags it's connected to. If your master jack is wired correctly, there should be no difficulty getting the bell to ring, even if nothing else works. It's only a metter of two wires from the bell coils to the jack after all!

[Nickthedentist]

It also has a microswitch gravity switch, which the 706 never had. It seems to be a hybrid of a 746 and 706. Probably an updated version of the 706 made by one of the companies who supplied them to the GPO, for use on a PBX.

First impressions are a lot of verdigris/damage around T8 and the microswitch. What does the PCB look like underneath? Do the gravity switch contacts test OK with a meter?

Nick.

[AC/HL]

It'll be a PAX phone, designed for internal use with very short lines, hence the lack of a regulator. The regulator does nothing for short lines and is omitted for economy.
It should still work, though, the basic design of phones didn't change until the electronic era.
There seems to be a lot of corrosion, that may be a clue.

A few crossed posts here!
Check the bell motor coil resistance, it may be high impedance and won't need the resistor.

[only1carra]

Thanks for all the replies so far. Unfortunately I am not very technical and do not have any testing equipment however I have successfully converted GPO phones in the past

I am just after a bit of guidance to wire this one up. Like I said I have managed to get it to ring when using the standard wiring instructions but it only rang when the handset was lifted and not when it was down. I wired it back to how it was originally wired before I touched it so you can see how it was originally in the pictures

Thanks again

[Nickthedentist]

The "standard" conversion wiring should be fine for your phone BUT it looks water-damaged.

Can you remove the screws holding the circuit board in place, so that we can see the copper tracks on its underside?

[only1carra]

I will have a look and see if I can.

The strange thing is that with the standard wiring it rings with the handset off the hook and there is no dial tone or anything

But wired the way it was originaly it rings for a second when lifting the handset and the dial tone works fine

Thanks

[Station X]

Does it need an incoming call to make it ring with the handset off hook? If no incoming call is needed then this points to the phone having a DC bell in which case you'll never get it to work on a normal line. If it's an AC bell, then as I've said before, you should have no dificulty in getting it to work.

All you can do is convert it as if it were a 706. Also check for corrosion between tracks on the circuit board and corroded away tracks. Carefully scrape away any corrosion and bridge any cut tracks with a piece of wire soldered across the break.

If that doesn't work and you're unwilling or unable to carry out any testing, then I'm afraid you're stuffed.

[only1carra]

Yes it does require an incoming call to make it ring

Thanks

[only1carra]

Here is an image of the bottom. I have scraped off some corrosion.

[Station X]

That's bad. You need to scrape off all that corrosion.

[julie_m]

I'm with AC/HL, I think the phone is made for use with a PABX, hence no regulator. But at least it has an induction coil, so it should work on a normal exchange line -- particularly if the line is short. (If your Internet is nice and fast, that means the line is short.)

Can you get a shot of the underside with all the PCB traces, and all the components on the top side? This will enable us to work out what connects to what, and therefore get an idea of how to wire it.

[only1carra]

Please see the image I added a couple of posts ago. Thanks

[julie_m]

The earlier image of the underside cuts off some of the traces.

[only1carra]

This image should be clearer.

Thanks

[only1carra]

Have now had some success!

Wired it up the standard way with the resistor and it rings!

However it makes no difference whether the handset is up or down and also there is no dial tone or sound through the handset

I guess the phone is not activationg the switch for it being off the hook?

Cheers for the help so far

[Nickthedentist]

At least it's a very simple design, with hardly any components! You could draw the circuit diagram on the back of an envelope in a few minutes.

First thing I would do is to check the continuity of those corroded tracks. They may be fine, or they may be totally open-circuit.

If you haven't got a meter, buy one (less than a fiver on many market stalls, or even in Maplin when on offer, or from eBay), or you could improvise: A doorbell (the sort a traditional bell and built-in batteries) would do at a pinch.

Once you've checked those, the next thing I'd do is check the switch contacts in the same way. We can give you guidance on where to connect your tester.

Nick.

[only1carra]

right, have now re attached the large resistor that was there already between 1 and 10, and the phone is ALMOST working as it should.
Only problem now is with volume. The incoming call volume is very low, and the person calling the GPO cant here the GPO voice at all.

Also the bell rings in time to the pulse dialling, and occasionally there is a deafeningly loud crack as you place the handset down, bit confused now!