Doro 1455 cordless phone

[Vesa]

I just got free phone, Doro 1455 Walk&Talk codless phone from 1996. This phone use analog 900MHz radios with full dublex channel scheme. Exat standard is European CT1. Phone itself is in mint condition, no need for eny fixing for casing or exterior of the phone.

The catch is: the phone is broken. Handset has power, base charges handset and it connect to base. But if you call the phone it rings and that's all what it do. Short investication lead to the result: The base analog hybrid has external remotely controlled hook switch. Switch is allways open but control works. The switch is single transitor or fet based on functionality. Measurement of voltages lead solution: it is PNP transitor. A91 is quite commonly used on phones as line hook switch. After replacement phone rings and you can answer the call. But more issues: caller voice can be heard from handset but no audio from handset to caller. Also no functional number dialing. Opening line and dialtone works.

Phone is base TEA1110 analog hybrid chip. Datasheet is available and there is reference schematics. Based on this information chip functionality can be tested. After some amount of testing end result is clear: handset transmits DTMF and it is received correctly on base and it is audible on hybrid DTMF input pin. Same for audio, it is wroking ok on hybrid mic input. But hybrit do not transmit that to the line. So TEA1110 is broken. There is several on sale on ebay and now new chip is coming. Part is coming china, so there is some level uncertainy of how genuine that chip is. There is no market for these chips anymore so hopefully this is genuine part.

Doro 1455 reprecent firts generation CT1 phones with out external antenna. Display shows also caller phone number, supporte caller info is etsi dtmf. Caller id display function also works. I see this unit as worth of fixing. CT1 phones are also allowed to use in Finland, so no issues to use it. This model should has good voice quality, it use analog compressor-expander chip-pari for 900MHz FM radio channel.

[Vesa]

More testing, this tima radio path functionality. I fired up SDR dongle with SDR++ software. I recorder all radio transmission from basestation and handset. On first image is handset radio transmission. Bottom part of image is waterfall display of spectrum. From bottom to up firts there is data burst whan hook off event happens. Next there is 6 digits. Doro seems to transfer dialed digits by data bursts, not as DTMF over voice. After digits there is some audio when I talk to handset mic. So far so good, handset is complete functional.

Next is base radio transmission. First item is data burst, so handset-base seems to do somekind handshake to verify handset id. This protects phone line from other user, you can not just dial usin any radio. I did not start decode data so I do not know how good this protection is. Next there is dialtone and 6 data burst for digit transmission confirmation. So data path seems to be somehow protected, includes error correction/data confirmation. After digits dialtone continues, base is unable to send anything to line.

Audio between base and handset is just standard FM. SDR++ was able to play both direction audio channels with out issue. So no anykind privacy on this phone. Euro CT1 standart spcifyes only FM modulation so it is upto manufacturer add crypto for audio channel. Doro do not do that and my understanding is that situation is same for US, UK and Euro CT1 phones.

So situation looks still positive, just waiting analog hybrid chip for replacement. Base should work all ok after that.

[countryman]

A very interesting repair, I keep my fingers crossed.
For our American friends it may be remarkable that this CT1 standard was first established 1984 and there were no legal cordless phones before in many countries, such as Germany. People started to import sets on their own and used them illegally. The first legal sets distributed by the phone companies were considered luxury items for the prices charged, so the "grey" imports continued well into the 2000 years (my observation). By then affordable digital sets (DECT standard) appeared.
For curiosity I collected 2 early "illegal" sets which both work after I replaced the NiCd batteries. CT1 phones were declared obsolete and their use was forbidden in Germany when the frequencies were assigned to be used with mobile services. It is unlikely to cause interference, though.

[Vesa]

Still waiting component to arrive, but here is images of the base station circuit board. On the bottom side picture: A is an electronic hook switch. Original is SOT-23 type package but now it is replaced by TO92 package A92 PNP transistor. I did not have right type on my spara part stock. Do not worry, right type is also coming, but this works ok for testing. Just visual mismatch. B is the analog hybrid chip TEA1110TA, it is broken and only one way audio is working. C is the analog compressor/expander chip DBL5015V for FM audio path. It support also data path over FM audi channel but needs a microcontroller control for it. Last chip on right is the microcontroller.

On top side E is the analog FM radio module. It supports multiple CT1 standard radio channels, channel selection from the microcontroller. During first call base and handset do channel scan to found free channel. Radio module has also trimmer cap for crystal frequency fine tuning (white round with screw head). That can be use for frequency tuning. Yep, I spend some time with RF spectrometer for tuning base and handset redios to just right freqyency. The handset has just same radiomodule. Mismatch is now just -0.8kHz to standard and around 0.03kHz to each other. D is the DTFM decoder for caller id receptions, chip deliver caller id to microcontroller. Microcontroller transmit caller id to handset over digital handshake. Yellow screw head components are TX (VR1) and RX (VR2) audio gain trimers.

The base use two voltages from an external power supply. 12v DC voltage is for the base itsel and on the board there is 5v linear voltage regulator. Separate 9v AC voltage is for handset. You can place the handset buttons up or donw on crandle and AC voltage works on both positions. The handset has bridge rectifier and voltage regulator for battery charging.

TEA1110 package is in Finland, I should get it in a few days now.

[Vesa]

First step on chip change is to remove old chip. For this I used hot air de soldering. Best place to do it is under microscope. It is easy to see when solder joints are liquid. On the board I used capton tape heat shielding for other components. Original manufacturing process for the base station board has been wave sodering and chip has glued on the board. Hot air melts also the glue but there is still on the board some amount of the glue. After solder pad cleaning component footprint looks ok on the board.

Still waiting new chip. But now it is easy to solder new chip on the board.

[SUnset2]

If anyone else needs to replace a device like this, and doesn't have fancy tools, I've replaced many SOIC type packages without a hot air desoldering station.  The brute-force way to remove these is to clip off all of the leads close to the IC body using fine tip nippers.  Then remove each pin one at a time with a soldering iron.  Of course, this destroys the original part.

[5415551212]

Nice repair. Thanks Vesa and keep it up, it is nice to have someone like you with lots electronics experience on here.
Cheers
--Stephen

[Vesa]

SOIC can be remove with clipping just legs. But on this case chip was glued on the board and that will need heat. With out you can damage tracks below chip.

New chip arrived and it is now installed. Funnu detail is that TEA1110AT has been out of production around 30 years, but there is still old new stock available. Phone is now completely functional and sound quality is good. When handset is in good radio reception area sounds is same as wired phones. Doro's selection for voice compressor-expander chip seems right for FM radio path.

[Vesa]

If you do not test it, it is broken. So here is test results after repair. There is three images from same sequence. BASE_TX is RF caputer of the base station radio transmission. Doro_line is osciloscope graph from phone line. Handset_TX is RF capture of the handset radio transmission. Numbers on the pictures are:

1: Off hook event
2: dialtone from PBX
3: 4 digits, in this case 800#. It is Asterisk echo test number.
4: Call on going with echo test
5: On hook event

1: Handset opens RF cahannel to base statio and sends data burst for handshake verification and off hook command to base station.
2: Base go to off hook state and pass dialtone to handset
3: Handset sends dialing digits to base station as data burst and base repeates them as DTMF to the line
4: Echo test answers the call and start read instructions
5: Handset sends on hook command data burst to base station and base station go to on hook state for the line

Echo test repeates voice back to handset and volume for both directions are also now on good level. Seems to work as it should. Handset also rings and display caller ID when I call to it.

[countryman]

Wow, caller ID (CLIP) on a 1996 phone! Wikipedia says it was introduced here in Germany 1998 for analog lines (ISDN had it before). When did it come in Finland or other countries?

When I moved into this house in 2000 I ordered ISDN because it would allow 2 lines for the price of 1. Telekom pushed their first digital standard really hard. I never got digital phones though but an analog PABX instead. It is still up and running. CLIP was an awesome feature back then - one of the last true amendments added to landline phones while ISDN flopped.

You can be rightfully proud of the repair!

[Vesa]

Caller ID is supported by Nokia DX200, same as all phone switch based services like conference call, answering machine, call diversion etc. First DX200 activated 1982 and 1984 around 40% of subscription lines in Finland was DX200 based. From first version, DX200 is 100% voip/software based phone network platform. Current version of DX200 supports also upcoming 6G. Funny thing is that most of case landline upgrade was from electromechanical pulse dial switch to DX200.

On phone networks 1980-2020 all technologies introduces first in Finland, after that it vent to standard and other companies followed up. 1971 Finland has 100% land coverage for mobile phone network (ARP) and it evolved 1980 100% coverage for Nordic countries based on NMT technology. 1995 GSM land area coverage was around 80% and it was cheaper than land line phone. Even university students has GSM phone late 1990 in Finland.

[countryman]

I've not yet been to Finland unfortunately, but when I visited Sweden in the mid eighties that was the first time that I saw cars equipped with mobile device in noticeable numbers 
As well for the first time I saw people working on laptop computers in a public park.

[Vesa]

Final post. Now also the hook switch transistor is the correct type one. Exact modern part is MMBTA92LT1G, Digikey part number is MMBTA92LT1GOSCT-ND. Allmost all analog DTFM phones use this same type of transistor for the electronic hook switch. Typical circuit use mechanical switch on hook grandle but this controls analog hybrid chip. The hybrid chip then drivers transistor. Most of cases old good looking phone is not used long time due lightning damage. And the damage is burned electronic hook switch transistor. 80% of my broken phones has just this same issue.

[5415551212]

Final post.

Thanks for the update Vesa I do sure enjoy deep dives into phones new, old or really ancient and its great to have someone like you on the forum that understands the electronics, so hopefully this is not the final post about this interesting piece of history.
Cheers
--Stephen