Restoring DTMF decode capabilities to Kenwood TM-V71A and TM-D710GA radios

Discussion in 'Radio Circuits, Repair & Performance' started by N4NZ, Sep 9, 2021.

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  1. N4NZ

    N4NZ Ham Member QRZ Page

    In 2018 Kenwood stopped including DTMF decode capabilities in the TM-V71A and TM-D710GA radios. I believe the same happened in the E variants as well. I've read that the reason for this is the DTMF chip is no longer available from their supplier. Manuals in these newer radios have notes about wireless operation being unavailable in serials B8610081 and up for the V71A and B8610001 and up for the 710GA.

    In practice this means the radio can still operate in cross-band repeat mode but you can not send DTMF codes from your HT to change frequencies or functions. This pretty much neuters cross-band repeating.

    I dug around in the most recent service manuals I can find (TM-V71A and TM-D710 -not G, but similar?. I see where the DTMF chip should be. It's referred to as IC921 and the part number is LC73881M-E.

    I'm crazy enough that I'm thinking about ordering one, taking apart my radio, and soldering the DTMF chip in. I have two concerns, though.

    Firstly, I am unable to find exactly a LC73881M-E chip. I have found LC73881M, LC73881M-TL, LC73881M-TLM, and LC73881M-TLM-E. I have not deduced the differences in those parts yet. Looking at the data sheet for the base LC73881M the pin assignments look identical to the service manual diagram.

    Secondly, the menu options for configuring remote functionality are missing from the models without the DTMF chip. I'm concerned it will not be as simple as soldering in the chip and poof, there are the menus. I know the firmware versions work on all radios, so it's not exclusive firmware locking it out.

    Attached are some images showing the board and chip in the service manual. I also have found images of boards without the chip as well as variations with the chip. I note that there are some capacitor layout changes on the D/6 board but they appear to have similar values.

    I posted this on Reddit and some feedback there suggested contacting a Kenwood engineer might be helpful. I plan to call and ask around to see if they can offer any insight.

    Any additional thoughts on this project would be appreciated!

    Attached Files:

  2. KA5IPF

    KA5IPF Ham Member QRZ Page

    Kenwood is great about sticking oddball nomenclature on the end of normal parts.. If package and pinout are the same I would try it.

    Just don't buy the chips from china.
  3. W9WQA

    W9WQA Ham Member QRZ Page

    btw, mine are LC73881M. im late and following.!
    do wonder about menu item.
  4. N4NZ

    N4NZ Ham Member QRZ Page

    I spoke with Kenwood Amateur Radio Support and did not receive much help. The gentleman was unsure whether installing the DTMF chip would return the menu functions or not. He was not aware of any resistors or solder bridges blocking menus 522 and 523.
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    You should go for it. :)

    If all else fails you can use a RPi3 to decode/encode DTMF as well as controlling the radio making it a repeater.

    This is basically a repeater controller, It includes Allstar and Echolink connectivity.
  6. KN6JJC

    KN6JJC Ham Member QRZ Page

    Any update on this? If all else fails I can design a daughter board with a microcontroller that simulates the dtmf chip. Send me a message and we can get in touch via email
  7. N4NZ

    N4NZ Ham Member QRZ Page

    I did not find any US suppliers. I received quotes from a few different suppliers in Hong Kong and China. I figured they were all probably selling the same counterfeits so I ordered this one:

    I hope to receive it in the next week or two.
  8. KB0MNM

    KB0MNM Ham Member QRZ Page

    If you are inexperienced with SMT ( Surface Mount ) soldering, I would suggest that you ask other amateurs in an open forum about their experience and see if you can find one who would do the work for you. While some of us have used $1000.00 and greater-priced hot air guns for this work, others may say it can be done with a butane cigarette lighter-type tool. The problem with doing it yourself may be 'collateral damage' to nearby parts, if precautions are not taken. Even experienced techs can do this, yet they usually know enough to protect the nearby chips which are more than discrete capacitors, resistors, etc. Speed in heating and cooling can be critical. Please let us know if the chip is sufficient, as others may have similar thoughts about replacing the DTMF decoder part(s). The approval or lack thereof for 'Sky Command' may have been the reason for this. Some 'cross band' repeat chats include reminders about station ID each 15 minutes or so.
  9. N4NZ

    N4NZ Ham Member QRZ Page

    From photos of the area on the board I am hopeful some kapton tape will keep things safe. We’ll see once I get it opened up.
  10. N4NZ

    N4NZ Ham Member QRZ Page

    The DTMF chips arrived today. When I get some time this week to take the radio apart I'll start working on installation.
    KA9JLM likes this.

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