SA 4258? Wiring question

[Henmond]

I have a push button 746 style phone which seems to be called the SA 4258, fitted with the 7A push button unit.

I was just wondering if my wiring is correct, as it functions fine, but I don't seem to get any "confidence tone" when pushing any button. Does anyone know much about this? I have included how it is currently wired

[Roger Ramjet]

Some of the earlier push button keypads were made for pulse dial [loop diss] so would not give any confidence tones. Not sure if yours is same but the presence on a reed relay on the keypad pcb would confirm.

Rog

[Henmond]

It definitely uses DTMF, because it works fine on my VoIP without using a pulse converter.

[Roger Ramjet]

Originally Posted by Henmond

It definitely uses DTMF, because it works fine on my VoIP without using a pulse converter.

Alas, this is unfamiliar territory to me because the phone was designed to work on a copper PSTN line.

Rog

[Henmond]

I would expect that the confidence tone isn't a PSTN only feature, unless it was fed back from the exchange, but if it was produced by the push button unit, you would surely expect that there would be some way to hear the tone as you dial.

[Henmond]

I have also decided that it is more correct to refer to this model as the 782, with a 7A PBU

[pip5678]

Does this push button unit short circuit the receiver as a dial off normal spring would? It's hard to tell off the photo.

[Henmond]

Originally Posted by pip5678

Does this push button unit short circuit the receiver as a dial off normal spring would? It's hard to tell off the photo.

It has the standard silver on T1 and blue on T2 like a rotary dial phone would have for shorting the receiver, so my best guess would be that is how it operates on this model.

I have traced the blue line back to the PBU, it connects to a microswitch that is held on until a button is pressed. From my experience, connecting microswitches at the end terminals means a normally closed circuit, and in this case, the normally closed contact is held open until a button press. I imagine this kind of action is equivalent to the shorting operation of the rotary dial.

The other wire attached to the microswitch is white with orange stripes, and I've traced it across the circuit board, but I'm not really sure where it goes.

If all this does is short the receiver, would it be safe to remove either the blue or silver line and connect them to one of the spare terminals (T12-T14)?

[Roger Ramjet]

I have tested an old 746 with PBU56A/4 keypad & I can hear the dial tones in the receiver. However connections are different to yours i.e.

Pink > T10
Blue > T2
Brown > T19A ?
White > T19B ?
Orange > T8

From the piccy's your phone does not have T19A nor T19B.

It would not hurt to diss on of the wires you suspect shorts out the receiver.

Rog

[Graham G3ZVT]

Before you do that, check if side-tone is still present when pressing the buttons.

ie. blow into the microphone when dialling and listen to see if the earpiece is muted at any point.


Incidentally does the keypad have * & # keys?

[Henmond]

When holding any button down the earpiece is totally muted, no side-tone.

And yes, it does have * and #.

[pip5678]

If all this does is short the receiver, would it be safe to remove either the blue or silver line and connect them to one of the spare terminals (T12-T14)?[/QUOTE]

I would try removing the silver/grey/slate colour wire from T1 first especially as Roger Ramjet's doesn't have one and just tape it up unless you know which terminals are actually spare (I am pretty sure some of them have no circuitry connected to them though!). I am not sure the black rectifier element between T1 and T2 would be of any use as that is supposed to prevent accoustic shock on loop disconnect dialling.

[Henmond]

I've removed the silver wire from T1, and I can hear the tones as I dial them in. I think several of the terminals on the second row have nothing electrically connected, I've included a photo of the underside.

I do wonder if maybe it was the intention of this design of PBU for you to not hear the tones as you dialed?

That rectifier element was there when I got the phone, so I just left it in.

[Pellseinydd]

Originally Posted by Henmond

I've removed the silver wire from T1, and I can hear the tones as I dial them in. I think several of the terminals on the second row have nothing electrically connected, I've included a photo of the underside.

I do wonder if maybe it was the intention of this design of PBU for you to not hear the tones as you dialed?

That rectifier element was there when I got the phone, so I just left it in.

I've looked at my copy of the BPO's SA(L)4528 but unfortunately I only have the Issue A dated 1975 rather than the first issue dated 1972 and because there were a number of variants of the circuit by 1975, the BPO removed the diagram of the circuit and just left the ways of connecting the telephome up.

However Bob Freshwater's website has a copy of the BPO's Technical Instruction for the SA4528
and it mentions -

"CONFIDENCE TONE
This is a combination of audio tones that can be heard in the receiver when a push-button is depressed. Its intention is to give the user an indication that the call is proceeding satisfactorily during the keying period. The tones will vary in frequency, depending on the button pressed."

Ian J

[Roger Ramjet]

Originally Posted by Henmond

I've removed the silver wire from T1, and I can hear the tones as I dial them in. I think several of the terminals on the second row have nothing electrically connected, I've included a photo of the underside.

I do wonder if maybe it was the intention of this design of PBU for you to not hear the tones as you dialed?

That rectifier element was there when I got the phone, so I just left it in.

This is only a theory... but I would assume the tone keypad evolved from the initial loop diss one's which were matched to the legacy exchanges & obviously needed the receiver mute feature to prevent dial clicks being heard. I recall having a Trimphone with an early loop diss keypad that impressed visitors. However, here in the UK changes were happening very fast & it was not long before BT STAR SERVICE's became available [signified be a different dial tone]. To access BT STAR SERVICE's required the use of the HASH & ASTERISK keys which were only available on the evolving tone keypads & my much loved trimphone became partially obsolete.

So to answer your query, maybe the receiver mute feature was retained on the tone keypad because of a redesign oversight ?

Rog

[Henmond]

This is how I've got it wired up now. I've removed the grey from T1 and moved the white from T10 to T11, how it was wired initially, as I was having trouble dialling out.

[Pellseinydd]

Originally Posted by Roger Ramjet

This is only a theory... but I would assume the tone keypad evolved from the initial loop diss one's which were matched to the legacy exchanges & obviously needed the receiver mute feature to prevent dial clicks being heard. I recall having a Trimphone with an early loop diss keypad that impressed visitors. However, here in the UK changes were happening very fast & it was not long before BT STAR SERVICE's became available [signified be a different dial tone]. To access BT STAR SERVICE's required the use of the HASH & ASTERISK keys which were only available on the evolving tone keypads & my much loved trimphone became partially obsolete.

So to answer your query, maybe the receiver mute feature was retained on the tone keypad because of a redesign oversight ?

Rog

Not correct. A confidence tone was provided - see my quote from the PO's Technical Instruction for the 'Telephone SA4258' in message No 14

And the specification quotes -
"S.9 Confidence Tone. When any button is operated with the handset lifted and normalised, the transmission circuit shall be rendered inoperative and the appropriate tones sent to line.
A 'Confidence Tone' composed of the same frequencies as those sent to line shall be ac coupled into the receiver. The combined levels of these tones measured across a 150 ohm resistor inserted in place of the Receiver, and
with the transmitter replaced by a 68 ohm resistor, shall lie between -40 dB and -52 dB relative to 1 volt. This requirement shall be met with any length of O.Smm artificial cable between zero and 6 Km.

Ian
ex Technical Officer 'back in the day'

[Roger Ramjet]

Thanks for the extra info Ian, however you may have misinterpreted what I said because I was not querying the confidence tone feature. I was querying as to why there was a mute feature on this particular phone's keypad.

Rog
ex 'TTA back in the day'