Deaf Paccomm Tiny-2 TNC

Discussion in 'Radio Circuits, Repair & Performance' started by KJ6HZH, Apr 14, 2018.

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  1. KJ6HZH

    KJ6HZH Ham Member QRZ Page

    How would one go about determining what's wrong in a Paccomm Tiny-2 TNC that's making it deaf? With the assistance of VE7GDH this morning, I was able to figure out what I'm supposed to be doing to do packet stuff. I have two TNCs, a Tiny-2 and an MFJ-1276, both second-hand. Prior to this help, I was able to pick up occasional ARPS packets from the MFJ, but not the Tiny-2 and I didn't know what the problem was. With VE7GDH's help, I could connect to the VE7SPR node on Salt Spring Island, just west of me, using the MFJ. When I tried with the Tiny-2, the node would note my attempt to connect, but the Tiny-2 didn't notice.

    So, I have a deaf Tiny-2. I figured that first a recapping would be in order, just because. What else should I look at?
  2. VE1IC

    VE1IC Ham Member QRZ Page

    what version is your board? Did you verify the obvious, like the cable being properly wired, jumpers set correctly for emphasis or de-emphasis. Depending on version, you have to jumper the terminal and the radio correctly on the Tiny2 (front of the pcb where the LED's are). What is your setup?
    The TCM3105 IC does the modulation and the demodulation on this TNC you could swap it out from a working one. Look at the manual and do the "Loopback Testing".
  3. KJ6HZH

    KJ6HZH Ham Member QRZ Page

    Board version is 1.7 (a Mk2). The cable is known good. It works with an MFJ-1276. I have the speed jumpers set for 9600 facing the computer and 1200 facing the radio.

    Digital loopback appears to be successful. Analog loopback failed (stuck a paperclip between pins 1 and 4 of the radio jack).

    The DCD LED is constantly rapidly flickering as to look dimly lit.

    I don't know what is proper for emphasis/deemphasis.

    I don't have a spare TMC3105.

    The manual scan I found is 4th edition 5th printing dated October 1991.
    Last edited: Apr 15, 2018
  4. KJ6HZH

    KJ6HZH Ham Member QRZ Page

    I beeped out the route from pin 4 on the radio jack (from radio) to pin 4 of the TMC3105. I went through R20, C7, and into pin 4 on the TMC-3105. R20 looks good. I don't have an ESR meter to check C7.
  5. W9GB

    W9GB Ham Member QRZ Page

    The Version 1.7 was the latest (last revision, as I remember).
    Changing Capacitors — “Just because?” — NOPE, not smart.

    Let’s focus on “most common” and “Easy”

    1.) CHECK DC Rails. In other words that you have GOOD +5 VDC.
    Gywn used Tantalum on earlier boards, but then switched back to Electrolytics.
    IF you have ripple (use scope) on +5 VDC after on board regulator (7805), then regulator and/or the 2 capacitors may have issue.

    2.) RS-232 output to computer is provided by Maxium 232 level converter.
    The non-ESD versions of MAX232 ICs are subject to destruction from rough handling (earlier owners, bad bench skills), static electricity, ESD events. This will appear if you can not get terminal output to computer from TNC, OR it does not respond to computer commands.

    3.) There are CMOS components within the TNC ....
    non-ESD bench work can damage without tech’s knowledge.
  6. W9GB

    W9GB Ham Member QRZ Page

    Photo of Pacomm TNC-2 board, Version 1.7
  7. KJ6HZH

    KJ6HZH Ham Member QRZ Page

    I found this mention of a factory mod from PacComm in several places. This one is most accessible: I'll try this once I get some chip sockets and 222 transistors.

    Here is the "Factory" fix for Pac-Comm Tiny-2's & Micro-Power 2's that have become hard of hearing. It seems that Pac-Comm has some boards out that work fine when new, but with age the tnc requires a huge amount of Audio input from the speaker jack or it won't decode properly. They are lost as to what is deterioating in the boards but offer this fix to its customers. It is quite simple and very neat, all parts are mounted on the under-side of a 16 pin IC socket, the Modem chip TCM3105 (U16) is removed, the modified socket is inserted into existing socket on TNC's board, and the modem chip is re-installed.

    Parts Needed:
    1 - 16 pin IC socket
    1 - 2222 Transistor NPN (EGC 123AP)
    2 - 10000 ohm 5% resistors (brown-black-orange-gold)

    Solder 1 resistor to Emitter, other end of resistor to pin 12
    Solder 1 resistor to Base, other end to pin 2
    Solder collector to pin 5
  8. W9GB

    W9GB Ham Member QRZ Page

    Your referenced “factory fix” was for the original Tiny-2 boards (TAPR TNC-2 clone)
    from WA2ISE on March 1989, via the legacy BBS “packet” topic forum.

    At that time, Gywn was still on the Tiny-2 ... PC board, version 1.1

    You posted a photo and stated you have the Paccomm version 1.7 board —
    that was Gwyn’s last version used in the Tiny-2 Mark II.
  9. KJ6HZH

    KJ6HZH Ham Member QRZ Page

    I still haven't fixed this TNC.

    The MAX232 is okay. I can talk to the TNC through the serial port and get feedback. It'll send audio through the radio, but won't recognize anything coming back.
    DC looks good.
    Hunting for the bad CMOS component is going to be a long task, so I figure that since the symptoms are identical to what's seen in earlier boards and the fix is cheap enough, I'll try that.
  10. W9GB

    W9GB Ham Member QRZ Page

    You may have an issue with your Squelch/Carrier detect OR
    the MODEM Chip (NRZI data).

    The Paccomm Tiny-2 TNC uses the Texas Instruments TCM3105 modem chip.
    TCM3105 is a single-chip asynchronous frequency-shift-keying (FSK) voice-band modem that uses silicon-gate CMOS technology to implement a switched-capacitor architecture.
    Source for replacement TCM3105 modem chips

    Data Sheet (Texas Instruments discontinued TCM3105 production in 1998).

    TAPR : DCD Assembly Manual (1991)

    2019 paper on TCM3105
    Last edited: May 16, 2019

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