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Explosion In Telephone

By on April 25, 2019
telephone tester











I received this telephone that was not functioning properly. I plugged the telephone to my telephone tester some of the dialing numbers were not functioning as well as ringing problems. I opened the set by 5 screws.

telephone repair

telephone repair and fix

Inside I faced with a very terrible picture of an explosion of the protection circuit that was responsible of  stopping the 220 volt AC and protected the set by opening the sensible fuse by the act of explosion and stopped the rush of 220 volt inside the telephone. By experience I knew that the component that responsible for protection of telephone is a VDR  this is (Voltage Dependent Resistor) or a Varistor.


Voltage Dependent Resistor

The code number is 07D201K  or 130volt  VDR this VDR up to 130 volt AC act like a wire but if the voltages goes more act like a fuse and by blowing itself protect the telephone from 220 volt AC .

But my question was if the protection circuit has done the job on time why the telephone is not working properly. By close looking on the solder side of the motherboard I found many bad cold solder joints. I resoldered all of the bad joint and after that I tested the set with a telephone tester, The tester showed ok which mean the telephone was working fine.

telephone tester

I told the owner of the telephone that he must take special care and do not plug the telephone to the AC outlets any more. Of course I installed a new VDR in the same place of the exploded one.

new varistor

The telephone came back to life again after replacing the VDR and resoldering the board.


NOTE- This telephone tester is a local made machine that can test the ringing of the telephone by sending the ring pulses. It also test the dialing function by showing the number on display as well as checking the microphone and speaker of telephone. I did not found something like this in Amazon.


Update: Since many of you asking for the telephone tester diagram, so I opened it and found that the ICs part number was erased. Due to this, I’m not able to provide  the complete diagram to you. Below  is the photo of the telephone tester:

telephone tester diagram

beh from iran

This article was prepared for you by Mr Beh from Iran.

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  1. Albert van Bemmelen

    April 25, 2019 at 10:15 pm

    I had never heard about a Telephone tester device before if it is a device that exists for testing Telephone sets, Beh. I guess it was not a DIY project you made? Replacing the VDR with the right matching type is an excellent fix. And the owner indeed better never makes the mistake to connect it to the 230VAC power lines again. In old telephones they used 2 diodes to protect the analogue circuits, and even after a struck by lightning the phone operated fine after both diodes were replaced.
    In the past I made a Telephone exchange device from a project in Elektuur magazine somewhere in the 80s that also can be used for testing the old Pulse Dial Telephones. It can also be used to connect to 8 phones in total and be used to make a connection by dialing to any one of the 8 numbers.

    • beh

      April 26, 2019 at 2:15 pm

      ...yes this is a local made telephone tester and as i told you is not found in AMAZON and thanks for your supports.

  2. Ghashaan

    April 26, 2019 at 2:10 am

    hi Beh

    I like your article!

    Is there any way that we can get that kind of telephone tester?

    • BEH

      April 26, 2019 at 2:20 pm

      You are welcome . Let me ask the market if there is available i will tell you soon.

  3. Robert Calk Jr.

    April 26, 2019 at 6:36 am

    Good job, Beh. Nice to see that you are still kick'in.

    • BEH

      April 26, 2019 at 2:23 pm

      you are welcome and thanks for supports.

  4. dicksy

    April 26, 2019 at 12:34 pm

    good job .how did you read VDR voltage

  5. Parasuraman Subramanian

    April 26, 2019 at 9:32 pm

    Good job there! My elder son is touring Iran and I am glad to see an article posted by someone from Iran! Yes, like the others asked, where can we get a telephone tester? Do you have the circuit diagram?

    • beh

      April 26, 2019 at 11:09 pm

      Unfortunately this model of tester is not available in our market but i may open the cover of this tester and take photo and diagram for those that want to make it should not very complicate . the cover of this tester is metal and is very heavy and post also is very expensive but i think all the parts of this tester must be available in your stock and any one can make one for own job i will send this as article to all i hope so.

      • Albert van Bemmelen

        April 27, 2019 at 4:53 pm

        Beh, that sounds very promising if you are able to re-cover the original design of the circuit by reverse engineering! I hope the device also is able to test tone-dial phones?

  6. beh

    April 26, 2019 at 10:40 pm

    ok i will ask from our market and i will tell you .Also i invite you to come to IRAN and see my country


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