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Hacking an Indetel with SC35A-1 DTMF

Started by markosjal, November 19, 2016, 03:00:02 AM

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I am taking on a project and want to know if anyone here knows the specifics of the Stromberg Carlson SC35A-1 DTMF pads .

My subject here is an Indetel phone from Mexico, as seen in the first image. These dial pads produce pulses.

This dial pad is not like those found on most N American phone as there are no letters. So in an attempt to preserve that, I want to use the SC DTMF Electronics keeping the white numeric only dial pad.

Image 2 shows the front of the dialpad as removed from whe phone when I cleaned it.

Image 3 shows the front of the SC35A-1 that I actually do not yet have in hand as it is being delivered internationally

Image 4 Shows the rear of the Indetel Pulse pad

Image 5 Shows the rear of the SC35A-1 (not yet in hand)

Image 6 Shows the wiring diagram for the SC35A-1

As you might notice these pads are very similar in form factor . I could easily replace the Indetel pulse pad for the SC DTMF pad ,however that is ugly to not preserve the white buttons and trim, not authentic "looking" and not enough fun anyway.

My first question is that from what I can tell and what I have seen many dial pad parts are interchangable and this appears to be the case here . I would like to know is the SC35A-1 mechanical contacts or membrane ? I can see one mechanical contact (to mute the ear) on the rear of the SC35A-1 but tyat does not mean that all contacts are mechanical.

I am hoping to replace the front section of buttons on the SC dialpad with the Indetel in order to preserve the white appearance while making it DTMF

Is anyone here familiar with the SC35A-1 ?

Image 6 also shows something very interesting . The SC35A-1 dialpad uses only 4 wires.

Thanks for any help


Phat Phantom's phreaking phone phettish


Adding pic of partially disassembled Indetel Pulse pad keys
Phat Phantom's phreaking phone phettish


I finally got the SC35A1 Touch Tone Keypads . I found an odd surprise if anyone knows anything about it. WHen the Pad has no button pressed I have 0 Ohms across 3 of the 4 wires which seems quite odd.

Anyone know anything about touchtone pads?
Phat Phantom's phreaking phone phettish


They almost look interchangeable, or at least the face plate can be swapped.


The More People I meet, The More I Love, and MISS My Dog.  Dan Robinson


I wanted to update this thread so that other may benefit from it...

Using the SC35A1 DTMF dial I now have had complete success

The key to this was

1) Red/Orange is basically to one side of the line through the hookswitch so it is protected from ring voltage. (if one side of the line does not work then try the other inside the phone set, as often times both are switched.)

2 Red/Green wire to common connector of 3 wire handset (this may be "R" on many WE/SC/ITT models

The above gave me tones with 2 330 (using only one gave me the sound of overdriven/distorted tones) Ohm resistors in parallel with red/green and Orange/Black or a total of 165 Ohms. This pair serves as voltage suppy and tone output.

upon further investigation the White/Blue should have worked in conjunction with the red/Green placing additional resistance across the line when a key is pressed thereby attenuating the DTMF tones hears in the handset. I discovered however in the Indetel, that even  completely disconnecting the common from the handset had no effect on output volume.

I lifted a resistor and and a dark blue wire that connected these both to the same solder pad that the red/green connected to, and then connected them together and connected a yellow wire to it. I now had an "isolated" switched resistor to connect to the earpiece and connected it inb series with the other earpiece wire , using a spare screw as a terminal.

While I was there the Red wire should completely disable the mic when a key is pressed.  It too was wired also to the same solder pad as orange/black by way of a switch that broke the mic connection when a button was pressed , with a yellow/violet wire. I removed this yellow/violet  from solder pad and  I extended the yellow/violet wire with a green wire (maybe it was blue). I could now use this as an "isolated" switch to open the mic circuit when a button was pressed using another unused screw.

I now get mic muting an ear attenuation when a button is pressed. Now to find the white buttons to see if they fit......

By the way the SC35a1 has a big HONKIN colorburst crystal 3.579545 Mhz which is too big for the bell. I am inclined to think that maybe I can swap or move the crystal if anyone has ideas there???

If anyone needs it I sent the SC35A1 docs to the TCI library.


Phat Phantom's phreaking phone phettish


In my earlier post the red wires is not for the mic AT ALL rather I now believe it's function to be completely bypass the keypad when no key is pressed. Meaning It need not stay in series with the phone as a resistance. No key pressed TDTMF pad bypassed by way of red wire.

This may be noteworthy for those wanting to add DTMF keypad to a candlestick or other phone as this Magic red wire would connect to one side of the phone so that bthe DTMF pad "removes itself" when no key is pressed. 
Phat Phantom's phreaking phone phettish