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Heathkit clock speaker?

Discussion in 'Electronic Repair' started by Joe, Sep 18, 2008.

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  1. Joe

    Joe Guest

    I just managed to get hold of unbuilt Heathkit alarm clock GC-1107.
    The electrolytics were dry and the speaker is shot.
    Finding replacement caps was easy, but does anyone here know the impedance
    of this 2,5" speaker?
    Is it "normal" 8 ohms or something more exotic?

    -Jan
     
  2. GregS

    GregS Guest

    Can't you measure the dc RESISTANCE ?
    Seems like 3.2 or 4 ohm might be a better match.

    greg
     
  3. Joe

    Joe Guest

    Unfortunately the cone is fine, but the coil reads several megaohms.

    -Jan
     
  4. Joe

    Joe Guest

    To clarify the circuit, the postive side of the speaker receives 17 volts
    thru one diode and a resistor and the negative side is connected via
    transistor to ground and the transistor is driven by 4001 IC.
     
  5. Is it an ordinary electromagnetic speaker? Can't it be some of that piezo
    parts? A 17Vpp is pretty high. Too high for a low impedance 2.5" speaker.
    Try to connect some audio oscillator to it. If you hear nothing at 1kHz/10V
    then the speaker is really gone.

    petrus bitbyter
     
  6. ian field

    ian field Guest

    Sound quality certainly hasn't been a high priority on any radio alarm clock
    I've ever had so maybe you should concentrate on what *IS* important on a
    radio alarm clock - the speaker should be loud enough and the output devices
    should not overheat at maximum volume. If an 8 Ohm works - is loud enough
    and doesn't overheat anything then fine, otherwise 16 Ohm aren't that hard
    to find - 35 Ohm do exist but are harder to find.
     
  7. ian field

    ian field Guest

    A good clue as to whether its a piezo sounder is that some other component
    would provide a DC path for the collector of the driver transistor - maybe
    an inductor or just a resistor if its a real cheapie.
     
  8. Archon

    Archon Guest

    The manual is available here,
    http://www.pestingers.net/Heathkit_manuals_sequential.htm

    I assume the OP has it but for others trying to help in the manual the
    speaker looks like a normal moving coil speaker
     
  9. mac-g3

    mac-g3 Guest

    This is a bit off topic, but just to let you know unbuilt Heathkit kits
    are actually worth quite a bit of money. Unless you really have a
    passion to dig into it, might be a better idea to buy a cheap alarm
    clock and sell that kit on Ebay.
     
  10. Joe

    Joe Guest

    Yes, it is a normal, paper cone speaker.
    And to clarify further, this is not a clock/radio.

    -Jan
     
  11. Joe

    Joe Guest

    Yes, thanks I am aware of this. I have several Heathkits at home.
    It is not a question of needing just a clock, but having a working Heathkit
    from 1978 ;-)

    -Jan
     
  12. Joe

    Joe Guest

    Speaker is a 401-163
    Resistor is 150 ohm

    -Jan
     
  13. Joe

    Joe Guest

    Thanks a million, appreciate your effort very much!

    -Jan
     
  14. Franc Zabkar

    Franc Zabkar Guest

    The GC-1107 supplies the speaker via a rectified 13VAC source and 150
    ohm 1/2W resistor. That's a DC supply of 18V.

    I believe maximum power will be transferred to the speaker if it has a
    resistance equivalent to that of the series resistor, ie 150 ohm. In
    this case, when the transistor is turned on, the current will be
    18/300 = 60mA. Assuming a square wave signal with a duty cycle of 50%,
    the power dissipated in the speaker will then be 9V x 60mA x 0.5 =
    270mW.

    Therefore I'm guessing that the speaker has an impedance/resistance of
    at least 150 ohms and a power rating of at least 0.5W. If the
    speaker's impedance were any less, then the dissipation in the
    resistor would increase.

    If we accept that the speaker should dissipate less than 270mW in both
    clock circuits, then in in the GC-1005 case we have ...

    Power(max) = 0.27 = 25 x 25 x 0.5 / R(min)

    So R(min) = 1157 ohms

    - Franc Zabkar
     
  15. GregS

    GregS Guest

    Rip the cone apart and see if you can find a reading. The wire usually breaks
    away from the coil.

    greg
     
  16. GregS

    GregS Guest

    And sounds nothing like a moving coil speaker. I say,
    throw a resistor across it and measure the AC signal voltage to
    get something we can compute with.

    greg
     
  17. ian field

    ian field Guest

    In this situation I'd probably nick a replacement speaker from an old pocket
    radio along with the O/P transformer, in which case the resistor could
    probably be omitted without overstressing the driver transistor or supply..

    Actually, radios of that vintage are probably worth a bit, but many people
    will have such transformers in the junk box, Maplin among others still stock
    the Eagle Electronics LT700 transformer that would do the job.
     
  18. GregS

    GregS Guest


    I remember ordering some 100 ohm paper cone speakers from Mouser many years ago
    for my Johnson walki-talkies. I know they still have some 100 ohm speakers.

    greg
     
  19. ian field

    ian field Guest

    The highest I've seen were 150 Ohm as used in the Philips EE kits, they were
    driven class A by a single transistor - an AC128 in the EE20, the EE1003
    used a BC148 - those horrible "lockfit" transistors which were quite large
    for only 220mW, one of the two BC148s in the kit had a pressed steel 'heat
    fin' that didn't fit at all snugly to the transistor and stayed pretty much
    cold as the transistor got hotter and hotter.
     
  20. Franc Zabkar

    Franc Zabkar Guest

    Neither of the clock circuits makes any sense to me.

    Your clock has a 25VDC supply which, at a 50% duty cycle, would cause
    a 41.5 ohm speaker to dissipate 7.5W.

    In the OP's clock circuit, a 41.5 ohm speaker would cause the 150 ohm
    1/2W resistor to dissipate ...

    (18/191.5 x 0.5) x (18 x 150/191.5) = 0.66W

    I can only assume that the speaker's impedance at the operating
    frequency of the alarm is *much* higher than one would expect. For
    example, at 1kHz an impedance of 100 ohms would require an inductance
    of 16mH. I measured the inductance of an 8 ohm 1W 3" speaker on my
    DMM's 2mH scale as 0.08mH and about 0.5mH on the 2mH and 20mH scales.
    I could hear a high pitched tone on the 2mH range (1kHz ?) and a low
    pitch on the 20mH range (100Hz ?).

    This site appears to be dedicated to saving and restoring old Heathkit
    clocks:
    http://www.decodesystems.com/heathkit-clocks.html

    Here is some info on the MK5017 clock chip that was used in the
    GC-1005:
    http://www.decodesystems.com/mk5017.html

    The MK5017's Tone output is shown driving a 2N3904 transistor
    connected to a 17VDC supply through a transformer-coupled 8 ohm
    speaker:
    http://www.decodesystems.com/mk5017-2.gif

    The transformer is spec'ed as "2K/8R".

    I'm really clutching at straws now, but is it possible that the
    Heathkit speaker has a built-in 2K/8R transformer ??? Does it have the
    usual permanent magnet? Would it make sense to have a stationery 2K
    winding and an 8R moving coil on a soft iron former ???

    - Franc Zabkar
     
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