Slightly odd connection on a GPO 786 DTMF Trimphone

[noggin72]

Hi there - first post, please excuse me if I break any etiquette. Not intentional if I do.

I recently acquired an STC DTMF Trimphone (for use with a home-built Asterisk exchange). It came with a strange 10 way ribbon cable about 6 inches long, terminated in a 10 way IDC socket, instead of a line cable (or none) or a conversion to a current BT plug. The phone is labelled "1/786STC80/2"

Please see the attached picture.

The ribbon cable looks like it is a professional installation. Wondered if the DTMF Trimphones were ever wired into PABXs or similar, as it appears that the DTMF keypad and tone ring transducer are connected externally - and in fact the ringer transducer isn't obviously connected to the phone (or tone call generator PCB) at all, just the external ribbon cable. There is no recall switch fitted at the front.

Before I convert the phone for standard PSTN connection (for use with a VOIP ATA) I thought I'd see if anyone knew any more, particularly if there were any pitfalls I should be aware of.

The ribbon cable is curved through heat shrink and is held captive by a neat plastic attachment that screws under a spade terminal (again making me think this is an official conversion). The connector connects to the following terminals :

Ribbon Grey - Tone Ringer transducer (NOT tone caller PCB)
Ribbon Black - Tone Ringer transducer (NOT tone caller PCB)

Ribbon Blue - T18 (Strapped to 19)
Ribbon Purple - T1 (Also to Handset Red and DTMF Keypad Grey (I think))
Ribbon White - T2 (Also to Handset Green and DTMF Keypad Blue (I think))
(No Ribbon to - T3 but Regulator and Handset Blue)
Ribon Red - T5
Ribbon Brown - T6
Ribbon Yellow - T8 (Also to DTMF Keypad Orange)
(No Ribbon - T10 (DTMTF Keypad Pink and Handset White and Regulator)
Ribbon Blue - T18 (Strapped to T19)

T19A strapped to T19B and connected to DTMF Keypad Brown
Ribbon Orange - T2A

Ribbon cable colours appear to be : Brown, Red, Orange, Yellow, Green, Blue, Purple, Grey White, Black from Pin 1 end on IDC connector.

LK1 on the main PCB with the T1-T19 terminals has been cut.

There appear to be no connections to the PCB around the tone ringer transducer at all - there are empty PCB holes where I see wires connecting to the main terminating PCB in photos of other Trimphones.

If anyone has any idea what this might have been used for - I'd love to know.

Looks like it might take a bit of soldering to update - as the tone caller PCB looks never to have been used?

[AndiiT]

Hi,
It's possible that the keypad may be for DC code C signalling and not DTMF.
The only way you will be able to tell this is by checking the keypad for the presence of a number of diodes and very little else (i.e no DTMF IC or other components), also the DC code C keypads were very light in weight compared to the DTMF versions.

Regards

Andrew

[noggin72]

Hi Andrew

It has a 54A/STC80/1 keypad and the PCB has an Intersil ICM7206BJJE IC on it - and this appears to be a DTMF encoder chip. The PCB is labelled : PG 7930 35YPB00001ABH ISS 4 51-1 ISS 01B

There are four link positions on the keypad. Links 1,3 and 4 are not made, Link 2 is made.

All quite strange! The short length of the ribbon cable, and the fact that it terminates in an IDC connector AND the odd situation that the tone-call PCB is effectively redundant and disconnected, with the transducer connected straight to the ribbon cable, leads me to believe that the phone was integrated with something else it plugged into and worked next to. Maybe I'll never know.

Be useful to know what wires to solder where for the tone caller to work. The circuit diagrams for the Trimphone I can find online just treat it as a sealed blob (the same is true of the keypad modules)

[ThePillenwerfer]

Tone Ringer details attached.

- Joe

[noggin72]

Thanks ThePillenwerfer. I think I have a different Tone Ringer - it's labelled 8A - STC 79-2.

Reason I think it is different is that it has two transistors TR1 and TR2 not just one, and the resistor numbering goes up to R10, and the diode numbering up to D5 at least?

It appears that the 8A was in use on later Trimphones - but I can't find an N-diagram for it on Sam Hallas's (excellent) site. He has a picture of it here : http://www.samhallas.co.uk/collectio...one_caller.jpg

Which matches mine - apart from the lack of wires to the tone caller PCB from the main PCB or from the tone caller PCB to the transducer (which is connected directly to the bizarre ribbon cable!)

Have attached a picture of my tone caller PCB - but to be honest with the picture from Sam's site I suspect I have all the information I need, though it would be good to track down a circuit.

[noggin72]

Found out a bit more. It appears that this phone was salvaged from an old telephone exchange and was mounted in equipment racks. That would explain the odd external ribbon cable connector, and also why it doesn't have rubber feet but instead bolts that could fix it in position.

It appears that Tone Caller 8A replaced the original Trimphone tone caller to fix a few issues in the original design. Be great to track down a circuit if anyone has one.

[noggin72]

Just an update. All happily converted and working. Ringer board needed four connections to be soldered (back) on to it. Two to the transducer and two for connection to the main board. Amazingly the ringer PCB was totally redundant in the phone's previous incarnation.