I
ian field
- Jan 1, 1970
- 0
Franc Zabkar said: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.