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VC3165 Frequency Counter

Discussion in 'Troubleshooting and Repair' started by RB28, Jul 30, 2012.

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  1. RB28


    Jul 30, 2012
    Hi all,

    I have a Frequency Counter VC3165 which has partially failed. It has different modes it operates in controlled by 3 front panel buttons but the buttons no longer work so its stuck on reading frequency in mode 1 (50MHz-2.4GHz). It displays the input signal frequency ok and there is only an Ic2 connection between the main microcontroller and display driver so I figure the main microcontroller in it is working fine and the display driver is also working mostly.

    Please see attached schematic...

    The display driver it uses is a ZLG7289A chip which appears to be a I2C to 8 common cathode 7 segment display driver and button multiplexor. As the frequency display is working fine, but the buttons are not being read, then I figure the problem is with this chip.

    This chip has original markings indicating that it is a PIC16C55 microcontroller that is preprogrammed for a specific application and marketed as a ZLG7289A. I dont' have a programmer for this chip nor the source code to program another.

    This Frequency counter seems to be common and has been marketed under different brand names. (I have checked all solder joints etc on the board by the way and do not think the problem is a physical one!)

    So I am trying to source a replacment ZLG7289A part for it. I have found Chinese datasheets with original manufacturer information on it and I've tried contacting them but they say "sorry not available". Its also not available from the usual suppliers (RS Components/Element14/Digikey).

    So if can anyone help please with sourcing the part I would be very grateful.
    Thanks for reading this... Ray

    Attached Files:

    Last edited: Jul 30, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Sorry, I cannot help you with the chip.
    Before you exchange the chip, provided someone can help you to get one: have you checked the 10k resistor that leads to the pushbuttons and all connected wires? Also the buttons themselves and the resistors and wiring leading from the buttons to the chip (lower edge of the chip symbol in the schematic)?
    Since the display is working o.k., the port pins on the chip seem to do at least the output-part of the job.In between scanning the display, the chip reads from the buttons.
    Do not check for open connections only, check for short circuits, too. If something is stuck or open, this will not work, preventing you from changing the range.

  3. davenn

    davenn Moderator

    Sep 5, 2009
    meaning that sourcing another chip is pointless anyway as you are not going to be able to program it

  4. RB28


    Jul 30, 2012
    Thanks for your comments. Yes I already have gone over those components and checked for bad joints and values. There are 8 resistors going from the top of the switches to the ZLG7289A which are marked as 270 ohms on the schematic, but in real life they are 52 ohm resistors. I assume this was an engineering mod done to brighten the display by the manufacturer. I wondered if this may have pushed the chip to the current limit and that may have caused the problem??

    If I am successful in getting another ZLG7289A, I think I'll change these back to the schematic value.
  5. shrtrnd


    Jan 15, 2010
    Yeah, but did you check the continuity of the push-button switches themselves?
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    You wrote that the display works, so the output pins of the chip seem to be o.k.,
    What obviously doesn't work is the input function. So check the buttons and the 10k resistor.
    Apart from not making contact another issue could be one switch making permanent contact, thus inhibiting selection of other modes.
  7. RB28


    Jul 30, 2012
    Yes... thats one of the first things I checked making sure the 3 switches were operating correctly (ie not stuck on and not open circuit).... Rgds Ray
    Last edited: Jul 31, 2012
  8. shrtrnd


    Jan 15, 2010
    I may be barking up the wrong tree, but I can't help but wonder what Harald told you
    at the beginning of your post. You sound like you have an ohmeter. Did you check
    the continuity of the pushbutton switches themselves.
    Like Harald said, it's not just a matter of if the switches are open. It's a matter of
    if they're making ELECTRICAL contact correcting and breaking it correctly.
    Simply mechanically activating the push-buttons doesn't always make a good check.
    I'd be thoroughly checking to make sure the pushbuttons are ELECTRICALLY (use the
    ohmeter) making and breaking contact correctly, before I tore into the circuit board.
    Good luck
  9. RB28


    Jul 30, 2012
    Hi thanks for your interest. Yes I did use a multimeter to check the switches and they are ok. Because the ZLG7289A chip had a socket, I could easily remove it so I could check the components on the display / button PCB.
    With the ZLG7289A removed I checked all passive components on the display/button pcb and can't find any out of tolerance or faulty. (I removed the capacitors temporarily to test them).
    I measured all resistors and capacitors and they are all within the tolerance of their marked values. The non-electrolytic capacitors are not leaky (dc wise at their rated voltage) and the electrolytic capacitors all have esr values below what is expected for their value/voltage rating. I also checked all leds and segments of all the displays and they all light up ok. I did this by putting current limited source between the respective pins of the empty ZLG7289A socket so it would check the PCB continuity etc all in one go.

    As I said in the original post, I'm pretty confident that the ZLG7289A is a PIC16C55 microcontroller. This has a resistor and capacitor for clock timing on pin 27 OSC1/CLKin as per the PIC16C55 data sheet instead of a crystal. There is no connection to the OSC2/CLKout pin 26. Reading the PIC16C55 datasheet, I would expect that there should be a 5V clock signal available on the OSC2/CLKout pin 26. When I put my scope on pin 26 there is nothing to be measured... only noise at a few millivolt level. So would that indicate that there is a problem? I'm not familiar with this chip so I'll have to read up on it's specs... but now I need to go to work.
    Thanks again for your help.
    Last edited: Aug 1, 2012
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    If it's working at all, then no.

    If there was no clock signal the device would not do anything that required the uC.

    You could try looking at the pin that has the RC network attached. If this is for an external clock then you should see something here.
  11. michael2458


    Aug 27, 2016
    RB28.....would you have that circuit diagram in pdf format
    could you email a copy of it to me please
    [email protected]
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