Heathkit clock speaker?

[ian field]

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 ???

Finding a way to reduce the duty cycle might be a means to get away with a
more commonly available lower impedance speaker.

 

[Franc Zabkar]

Finding a way to reduce the duty cycle might be a means to get away with a
more commonly available lower impedance speaker.

AFAICS, at 25VDC a 40 ohm speaker would draw 600mA when the transistor
switches on, regardless of duty cycle, assuming the transistor has a
high enough gain at whatever base drive the IC provides. An MPS-A20
transistor is only rated for 100mA.

- Franc Zabkar

 

[ian field]

AFAICS, at 25VDC a 40 ohm speaker would draw 600mA when the transistor
switches on, regardless of duty cycle, assuming the transistor has a
high enough gain at whatever base drive the IC provides. An MPS-A20
transistor is only rated for 100mA.

As the speech coil is an inductor it will exhibit a linear rising current
waveform from the initial application of voltage, obviously that will be
shorter for a lower impedance/inductance speech coil so the pulse width must
be made shorter to ensure it terminates before the inductor saturates.

 

[Franc Zabkar]

As the speech coil is an inductor it will exhibit a linear rising current
waveform from the initial application of voltage, obviously that will be
shorter for a lower impedance/inductance speech coil so the pulse width must
be made shorter to ensure it terminates before the inductor saturates.

If my inductance measurements make any sense, then the time constant
of the speaker would be L/R = 0.5mH/40R = 12us, or 80uH/40R = 2us.
That's a very short pulse.

- Franc Zabkar

 

[Franc Zabkar]

As the speech coil is an inductor it will exhibit a linear rising current
waveform from the initial application of voltage, obviously that will be
shorter for a lower impedance/inductance speech coil so the pulse width must
be made shorter to ensure it terminates before the inductor saturates.

I used Bob Parker's ESR meter to measure various 8 ohm speakers. AIUI,
the meter delivers 8us pulses to the device being tested.

FWIW, here are my results:

10W/8R - 35 ohms
2W/8R - 22 ohms
0.5W/8R - 15 ohms
20W/8R/20kHz tweeter - 18 ohms

- Franc Zabkar

 

[Fred McKenzie]

I actually built this thing circa 1980, and still use it. I recently
found pdf's of a manual and circuit diagram for it at

http://www.pestingers.net/Heathkit_manuals_sequential.htm

A quick look at these pdf's didn't tell me any specs for the speaker,
tho.

Joe-

Back in the 70s I built a kit clock using parts and instructions from
Radio Shack. The specified 8 Ohm speaker was driven through a 100 Ohm
series resistor. I found a small 100 Ohm speaker, and figured I might
get more power transfer if it matched the resistor. The result was
definitely louder than the original 8 Ohm speaker.

I didn't try it without the 100 Ohm series resistor, which would be
similar to your configuration. Suppose your square wave was 10 V RMS.
As an approximation, that would produce one watt into a 100 Ohm speaker,
which is probably louder than your neighbors would like!

I still have the old clock and have enhanced it over the years. It now
has a 9 volt battery backup with its 60 Hz signal coming from an
oscillator that divides down a color burst crystal tuned to 3.579540
MHz. The frequency may drift a bit, but it doesn't stop when the power
fails.

Fred