Fixing DTMF on a WE 2500

Rurorii · March 04, 2023, 05:56:03 PM

Hello!

I recently joined the forum in hopes of getting help with an issue with a WE 2500 I bought recently.

(Side note: I don't actually have a telephone service. Instead I am using an Xlink Bluetooth gateway, but i'm pretty sure this is not the issue as another phone worked fine with it)

It can ring, receive calls, and works fine audio wise. The issue is that I am unable to make calls.
It has dial tone, and pressing a key makes a tone, but dial tone never cuts out nor does it make a call.

I've tried recording the tones and playing them into this website: https://unframework.github.io/dtmf-detect/, and it is able to detect the tones that I input correctly.

What could the issue be and what could I do to fix it?

Thanks for any help, Attachments below:

TelePlay · March 04, 2023, 05:59:20 PM

Have you tried reversing the line cord connections (red and green conductors) to see if it's a polarity issue?

Rurorii · March 04, 2023, 06:15:09 PM

I just tried that.

I still have dial tone but pressing the buttons no longer generates a tone. I'm going to switch them back for now.

TelePlay · March 04, 2023, 06:23:55 PM

Well, that's out of the way. It might be your DTMF pad. Others who work with those can help you further diagnose your issue.

Rurorii · March 04, 2023, 06:37:59 PM

Well thank you anyways!

poplar1 · March 04, 2023, 07:25:30 PM

Do you hear one tone or two when you press a button?

If hearing dual tones, one or both oscillators may have drifted off frequency

Rurorii · March 04, 2023, 07:39:06 PM

As far as I can tell it is only one tone

Audio sample: https://youtu.be/Waxn6ApN-F4

TelePlay · March 04, 2023, 11:22:58 PM

Recording the frequencies generated in your YouTube file, I see the 7 tones or frequency peaks. I can't tell exactly what the frequency of each peak is with my iPhone app but it seems the tone generators are working, maybe not on frequency spec but working - may be out of tune.

The 1st image below is the frequency layout of the DTMF dial pad.

The 2nd image is the pad frequencies in table format. The combination of 2 frequencies yields a distinctive tone recognized by the central office as being a number, 1 to 0 plus * and #.

The 3rd image shows the frequency peaks for a working dial.

The 4th image shows the frequency peaks from the YouTube video.

The frequency analyzer picks up the tones separately, the tones generated by the dial are not combined into one new frequency when a button is pressed. Both frequencies are sent together to the central office and their equipment recognizes them and then knows what number was pushed.

Both the 3rd and 4th images show 4 peaks on the left and 3 peaks on the right corresponding to 4 tones generated on one side of the dial and 3 tones on the other side of the dial pad. Distinct tones are there, are they in spec is the question. It does look like the left side tone generator is not uniform in the the 1st and 4th peaks look weaker than the 2nd and 3rd. I don't know if that is enough to not work.

If everything is in spec, then there is another issue with the dial. I don't know what instrument you would need to check the frequencies to see if they are in spec.

The height of the peaks, the difference, is a function of speaker volume and not an indication of the phone's tone generator performance for each spectrum.

A pdf file in the TCI library detailing an Automatic Electric DTMF dial which is not the dial pad you have but it shows how they work.

The spectrum analyzer that acquired the plots is available in the Apple App store and is called "Audio Spectrum Analyzer."

Contempra · March 05, 2023, 11:55:30 AM

Excellent TelePlay.... however, it's not a really simple job to do (adjusting frequencies), you need to have some knowledge in electronics and with some devices for frequency adjustment. It's the same thing with radio/television. Fortunately for me, I have several touch-tones, but everything is fine on these phones, but I am not fond of touch-tones. But I know that we have some experts on this forum , and it's a reason why i love this forum

.

Rurorii · March 05, 2023, 07:27:40 PM

I tried going through and measuring all of the frequencies online. Below are my results.
I'm not sure if the difference between what I measured and what they are supposed to be is enough to cause issues.

TelePlay · March 05, 2023, 09:01:51 PM

I know little about fixing or adjusting DTMF pads. I tried to find the specification range for each frequency but only find the whole numbers, no +/- figures, so hard to know what "off frequency" enough to cause an issue might be.

There are members who know this or have worked with pads so hopefully they can help out.

IIRC, the pad has two tone generators and the dialing frequencies (4 from one generator and 3 from the other generator) are produced by crystals.

The tone generators can be adjusted but the adjustment of one generator changes all 4 dialing frequencies and adjustment of the other generator changes all 3 dialing frequencies.

Looking at your readings, they are all below spec except one. Adjusting the 4 crystal generator up would push the 852 frequency even higher pout of spec. The 3 crystal generator could be adjusted upward and all would be closer to spec.

Lot of stuff going on in a DTMF pad, switch contacts, crystals (7), 2 basic tone generators and the wiring of the pad in the circuit. Toward the bottom of this article on working with DTMF pads, the author goes into adjustment a little bit but not much detail.

This topic on the forum:

http://www.classicrotaryphones.com/forum/index.php?topic=20848.0

. . . also discusses issues similar to yours with DTMF pads, has some tips that might work and includes several links to supporting information found by that author. I didn't look at them but they may be useful to you.

As many say, if the pad is bad, replacing it is probably the only solution. I don't know if crystals can be changed out or how to do it.

Then there is the other possibility that there is something else that is not allowing your phone to break dial tone. Even if the frequencies are a bit off, it seems strange to me that out of 12 buttons, not one of them breaks dial tone.

GLadstone provided a lot of links to different stuff related to your issue. And, you can search the web, the Forum and the TCI Library for more information. That's about all I can help you with.

poplar1 · March 06, 2023, 01:23:15 PM

Any frequency deviation less than 1.5% should not be a problem.

https://www.infineon.com/dgdl/Infineon-AN2122_Analog_Standard_DTMF_Detector-ApplicationNotes-v08_00-EN.pdf?fileId=8ac78c8c7cdc391c017d0736b28259bd

TelePlay · March 06, 2023, 04:06:04 PM

Seems all frequencies are well within spec.

Rurorii · March 06, 2023, 06:42:11 PM

Assuming there is no issue with the frequencies, what could the problem be? Maybe a wiring Issue? The tones can be heard through a calling party but don't seem to work with automatic systems.

5415551212 · March 07, 2023, 01:03:29 AM

Quote from: Rurorii on March 06, 2023, 06:42:11 PMAssuming there is no issue with the frequencies, what could the problem be?

Quote from: Rurorii on March 04, 2023, 05:56:03 PMI've tried recording the tones and playing them into this website: https://unframework.github.io/dtmf-detect/, and it is able to detect the tones that I input correctly.

When you recorded the tones for dtmf-detect did you use the same wiring?
What about the other phone that works did you use all the same wiring with that?

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Fixing DTMF on a WE 2500