Speedcall DTMF Decoder Schematic

Discussion in 'Radio Circuits, Repair & Performance' started by K7KL, Sep 21, 2019.

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  1. K7KL

    K7KL Ham Member QRZ Page

    We have two Speedcall Model 527C DTMF decoders, rack mount type. 12 button decode, with optional holes in solder traces for adding the 'fourth column.'
    We could sure appreciate a schematic for this product, long since out of production.
    Thanks.
    K7KL, Portland, OR
     
  2. KB0MNM

    KB0MNM Ham Member QRZ Page

    See if this helps:
    http://www.electronics-diy.com/dtmf-decoder-using-mt8870.php
    There were also circuits ( in the very early days ) that simply used 567 Phase-Locked Loops. When you look at columns and rows, realize that DTMF ( Dual-tone ) codes are just a combination of two tones per digit on a telephone keypad. There are a number of circuits in a book by Rudolph P. Graf, and if your circuits do not include the 567 or 8870 chips, you might look at that encyclopedia. P/N 276-5001 Engineers Handbook by Forrest M. Mims has the 567 shown as a tone decoder on page 120-121. There were circuits built using the 8051 microprocessor, just google DTMF Decoder. Many of the newer commercial circuits are sold by Midian, and unfortunately, not every maker wants to publicize their schematics ( usually for fear of patent/copyright infringement ). The only other thing to do is look for the approval labels- sometimes the patent number is in plain sight. Then there is the issue of getting the schematic from the patent libraries...
     
  3. VE3CGA

    VE3CGA XML Subscriber QRZ Page

    I couldnt find any info on that device.
    is the 4th colum for the extra "A, B, C, D" digit tone decoding, if its just missing the individual digit decode driver circuitry, might be as simple as copying the other 12 channel outputs, if its part of the individual tone frequency decoding, like a 567 decoder then possibly the same as the other tone channels but with different rc networks
    have you taken the lid off it and what chip numbers are in there
    Is it a 567 type decoder or teltone dtmf chip, plessey or MT8870, is a picture of the board possible,
    like KB0MNM said some commercial units may not have published drawings, I found a counter that the manufacturer bent the IC pins on a intersil counter backwards and the only was to find out what the chip was by desoldering it
     
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Do you know what DTMF chip it uses ?

    The 567 was a good chip in the day.

    By today standards the 567 is not so speedy and is subject to a false or no trigger. The PLL is slow to lock.

    You need 1633 HZ to get the last column.
     
  5. K7KL

    K7KL Ham Member QRZ Page

    this isn't the 567 PLL style decoder. It's a base station style PCB, all discrete circuitry.
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    Does that one have a separate board for each tone that has a 10-20 turn pot on it ?
     

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