Another Telephone 706 problem.

[russell_w_b]

Originally Posted by henry.crun

Measuring across T18 and T8, I see 50V off-hook & 47.5V on-hook.

Sheesh!

OK, then. Has your new meter got a DC milliamperes range? If so, disconnect one of the wires going to the microphone (either at the telephone-end or by unscrewing the plastic mouthpiece on the handset) and, with your multimeter set to 'DC milliamperes', measure the 'off-hook' current flowing through it.

Then report back. On a UK system, the current should be something in the range of 30 - 200mA. Anything more and you'll 'fry' your mic if it's a carbon one - don't know if it'll damage a type 21A electret insert though...

[henry.crun]

Originally Posted by russell_w_b

If so, disconnect one of the wires going to the microphone (either at the telephone-end or by unscrewing the plastic mouthpiece on the handset) and, with your multimeter set to 'DC milliamperes', measure the 'off-hook' current flowing through it.

Assuming I was measuring the correct location (T8 & T18?), I got a result of 34mA.

[AlanBeckett]

Originally Posted by russell_w_b

On a UK system, the current should be something in the range of 30 - 200mA. Anything more and you'll 'fry' your mic if it's a carbon one - don't know if it'll damage a type 21A electret insert though...

In the UK the limit is (usually) 40mA. 34mA looks OK, but the off-hook voltage is far too high - you'd expect it to be around 9volts. Meter problem?

Alan

[Station X]

Assuming I was measuring the correct location (T8 & T18?), I got a result of 34mA.

You're measuring in the wrong place.

Set your meter to the mA range.

Disconnect one of the wires from the microphone.

Connect one of the meter leads to the disconnected wire.

Connect the other meter lead to the terminal the wire was removed from.

Note on and off hook readings.

[henry.crun]

Originally Posted by Station X

You're measuring in the wrong place.

Sorry for being such a clot ...

With the meter set to 200mA, it's ~30.9mA off-hook & nowt on-hook.

Thanks & Merry Christmas

[Station X]

That's better.

Recheck the VOLTAGE on and off hook T8 to T18. It should drop when you go off hook.

[henry.crun]

Originally Posted by Station X

Recheck the VOLTAGE on and off hook T8 to T18. It should drop when you go off hook.

On-hook it's 49.8V (varies a bit as I wobble the probes). Off-hook about 5V.

[Station X]

Make sure that the line polarity is correct. ie:-

Phone on-hook.
Meter set to volts.
Red lead of meter to T18.
Black of meter to T8.
Confirm that meter reads forward.

If that works all I can suggest is that the problem is in the house wiring. Try inserting links as described in post #19 and see if you can get the bell to ring.

If you can get the regulator out, reverse it. It probably won't cure the howl round, but at least it can be eliminated from enquiries.

Do you have details of how the US plug and socket system is wired?

[DAVEHALL]

Unfortunately it's been that long since I worked on 706's that I've forgotten what's connected where (and no N diags to hand), but seem to remember that it was possible to trace the terminals on the 706 to the individual components (the major ones anyway) - so that might be the way to go .

[Station X]

Diagram N806 can be found here and at dozens of other places on the net:-

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

Remember it shows an unconverted phone though. ie it's hardwired to the line and not connected to a modern plug.

[DAVEHALL]

Originally Posted by Station X

Remember it shows an unconverted phone though. ie it's hardwired to the line and not connected to a modern plug.

Only ones I ever worked on ,though it's over 35 years ago -was trying to help if no wiring diag available .i got to stage where I could wire one up for most of thecommon plans without N diag .

[henry.crun]

thanks to all for your helpful replies. not being much on soldering & such, i asked the vendor for help. they sent a new microphone for the telephone. that reduced the sidetone feedback to a tolerable level.

i still haven't got the phone to ring - so far i've been unable to locate anything that i could use for bridging the terminals as instructed in much earlier postings. radio shack is a disappointment in that regard.

again, thanks for all the help & best wishes.

[dagskarlsen]

Try a paperclip for brigding the terminals.

dsk

[nickn05]

For making use of the phone's internal bell capacitor I would have thought it should be necessary to disconnect the blue line cord wire from T6 and attach it to spare (i.e T14 or T15), just leaving Red (T8) and White (T18), with straps between T8 & T9 and T6 & T7? Or is that not the case?

Also, a thought - to 'rob' a couple of straps from a converted phone in order to do the T6-T7 and T8-T9 thing, maybe attach the bell ringer wire to T18 instead of T16, leaving T18-T19 strapped, which gives you two unused straps between T16-T17-T18 which can be taken off and utilised? Not perhaps ideal but it would get things working possibly.

As far as I can tell, US phone system uses only paired wiring throughout - the primary pair being Green and Red (or the Christmas Pair) now CAT5 Blue/White and White/Blue which go to the middle two 'pins' of the jack and plug, or 3 and 4 in a 3-pair cable, then the secondary pair Yellow and Black (or the Halloween Pair) now CAT5 White/Orange and Orange/White which if used go to the outer pins, or 2 and 5 in a 3-pair cable.

Not sure therefore how the OPs phone plug from the 706 matches up with the line jack... but if he's managing a conversation then it sounds ok - it just needs the phone's bell capacitor to be utilised.

[DAVEHALL]

Originally Posted by henry.crun

According to my meter, the off-hook DC voltage is 70V. Got this result from poking Terms 5 & 18 & setting the meter to its 20V range. Does that sound like a plausible value? Given Google says AT&T line voltage is 48V I must be poking the wrong things ...

Thanks

Old Strowger systems /crossbar /etc were all 50v nominal ,only System X (Gec digital ,from memory was 60 v).As far as I'm aware the transmission bridge in the exchange ( in each line circuit ) has not changed substantially over the years - possibly been modernised ,but functionality stil the same .How do you get 70v reading from meter on 20v range ?

[AlanBeckett]

If that was 7 volts off hook it would be entirely plausible.
In a past lifetime I often had to connect American 'phones to our PSTN and vice versa. There were some difference in side tone and levels, but nothing that stopped them working.
Alan

[henry.crun]

i've not yet got my phone to ring but it's otherwise working to my satisfaction. a huge thanks to all who educated me in how these phones work & how to hack them. it was absolutely wonderful. you folks are great!

cheers