Relative old phone rings very silent

[ro0ter]

Hello everyone,

It`s been a while since I posted anything here, mostly reading, but here I come over some problem with no solution on this whole internet.

I have a phone, "Romtel 215" which I guess was an office phone. All good, the phone is in good shape, but the problem is that it won`t ring very loud. The ring is very pleasant to the ear, it uses UTC31002 integrated circuit. My uncle and aunt got some internet+tv+phone package from some company and they were gifted a very cheap telephone which has a very high-pitch sound that my uncle cannot hear anymore (we all grow old...).

So, I tried all that I knew regarding this circuit, but nothing helps. The IC (UTC31002) receives ~27V when the line "rings", the x cap is not harmed, the rectifier diodes are also healthy, the IC configuration is exactly as the one in its datasheet ( http://www.unisonic.com.tw/datasheet/31002A.pdf ) but the phone rings VERY QUIET... The speaker has no problem, if I switch on speaker, it is VERY loud... (its speaker almost killed my ears since I had my cell next to this phone).

The IC`s output is decoupled from the speaker through a 22uf cap and after through a transformer (which has the primary > 20Mohm and the secondary at about 1.5ohm).

Please, anybody that has any clue / idea help me out.

If you need schematics, I`ll draw them (I`ll do my best).

Thank you!

 

[Audioguru]

The phone ringer IC is powered from the telephone line.
It takes power to drive the ringer IC to be loud. Maybe the phone is too far from the telephone company so it does not receive enough power to be loud. Any other phones that ring at the same time use some of the available power which reduces the power to this phone.

 

[crutschow]

You could try an added external ringer such as this to the phone line.

 

[BradtheRad]

The IC (UTC31002) receives ~27V when the line "rings"

I was about to suggest your ringer is under-powered because typical ringing voltage is 90 VAC. However the datasheet states 30V is max supply V, so cancel my idea.

The IC`s output is decoupled from the speaker through a 22uf cap and after through a transformer (which has the primary > 20Mohm and the secondary at about 1.5ohm).

Page 5, line 10 shows the primary value being 1300 ohms. If you measured it as 20M ohm, then that will severely restrict current.

A less likely possibility is in the 22 uF. Is it an old electrolytic? These occasionally dry out and drop in value. Current would be restricted.

 

[ro0ter]

Hi everybody,

Thank you so much for all these replies in less than 5 hours.


Audioguru - that should not be a problem. I have tried this phone in 2 separate places: at my uncle`s and at my home. My apartment building has incoming fiberoptics, so there must be a medium translator inside the apartment building, therefore it should not be the case of large distance from phone operator central. Secondly, at my uncle`s, there is some sort of 10cm x 10cm x 3cm box which has RJ45 input (WAN), RJ45 output (LAN), wifi (WLAN) and RJ11 output (phone line), also having a transformer connected. The phone cable length is less than 2m, other phones ring very loud (like the high-pitched one killing our ears which my uncle cannot hear).

crutschow - honestly, that would be a great idea. But the price is too high, not including the shipping. This phone was like $10, so it`s not worth it to invest $29 in that device, plus shipping from US to EU (RO).

BradtheRad - brad, you have so far the best idea (electrolytic cap + transformer). I have changed the x cap (greater value makes the phone ring longer), I tested the diodes, the resistors, everything is just fine, but the only things that one cannot be tested in a system (while soldered) are the caps. Will do that as soon as I get home today (in like 9 hours...). I was wondering if it would make any sense to attach an electrolytic capacitor in parallel with the existing one while the phone rings or would that have no effect since (let`s say) the existing cap is totaled? Also, the transformer`s primary resistance (IC side) is way greater than 20Mohm.. I fear that it might be toasted.. but should it be toasted, no actual ring should occur...

Anyway, thank you all. This is my last chance of making the phone ring, so I really hope this forum will drive me into the right direction.

Best regards,
ro0ter

- - - Updated - - -

One more update... actually a thought:

Since it was an office phone, I guess it was designed to ring low, so that it won`t bother anyone else in the office // the person next to it (although it has a RING VOLUME HIGH/LOW switch which just bypasses the resistor in series with the IC decoupling cap, but with no effect).

I am now thinking of somehow amplifying the signal with a npn transistor (class b amplifier). There are other circuits which feed the speaker (the speakerphone circuit, the "onhold" music circuit), I hope I won`t bother any other component feeding amplified sound in the speaker.

Still, I don`t get it why there`s need for a transformer between the decoupled IC and the speaker. If anything else fails, I`ll plug in a simple amplifier.

Thanks!

 

[BradtheRad]

The transformer appears to be the problem. Nothing will get through it.

Here is a fix. The datasheet talks about "driving a pizeo-ceramic transducer type load". This means a piezo beeper like we find in countless household gadgets. They are inexpensive and easy to use.

You could install a piezo beeper and eliminate the transformer.

 

[Audioguru]

The transformer on the datasheet has impedances of 1300 ohms to 8 ohms. The windings are copper wire so the resistances should be about 162 ohms to 1 ohm. Your transformer has a broken 1300 ohm impedance winding.

The transformer is needed because a phone line has a high voltage but low current. The transformer increases the current to the speaker by reducing the voltage.

A piezo transducer operates properly only at high frequencies above 1kHz. The frequencies from the ringer IC are much lower.

 

[ro0ter]

Well well.. hello again

The transformer was the issue... First of all the wire was disconnected from the transformer`s leg, but it was the inside wire... i unwounded it, not a kill, but the wire was then interrupted two more times (some copper sulfur, you could predict when the wire won`t unwind anymore and you could tell where to search for its end).

Unwounded, soldered, measured resistance: ~57 ohm (57 / 1.5 = 38 ratio; 1300/8 = 162.5 ratio). Anyway, there was no more wire and the soldered parts are not touching anything, nicely wrapped in small piece of kapton.

Now the RING VOLUME HIGH/LOW switch has some effect, you can make a difference in the sound when it`s in a different position.

------------

But unfortunately... here comes another problem: whenever I pick the phone up (answering/dialing), the speaker is also in use, although it is not turned on from the phone.

what the hack.. can`t do a thing about this, the circuit is too d@nm complicated.

Should anyone have any guess, I`m all eyes and ears. But finally, the phone`s ringtone is now audible, and it won`t be that bad if the speaker is always on.

Thank you again for all your efforts!

- - - Updated - - -

....aaaaaaaaand FIXED!!!

The breaker (switch that closes the call / answers the call) was somehow not fully connected, while I was debugging the phone it somehow got a couple of wires broken, but I did not realized since the ribbon cables are old and hard.

Anyway, fixed now.

Thank you all for your help.

Topic may be closed.

Before closing, should I add any pictures of the phone / board ?

 

[BradtheRad]

Congrats, you've gained a personal triumph.
I'll bet it seems almost like a miracle that it's working now.

And part of the satisfaction is that you persisted, you tracked down the root of the problem, and solved a mystery.