Heathkit clock speaker?

On Thu, 18 Sep 2008 13:59:55 -0700, Jeff Liebermann put finger to keyboard and composed:

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

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

greg

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

Try resoldering the flexible wires at the terminal strip. I have seen a few new speakers that size with bad joints. It is a Litz type of wire, and hard to tin if it isn't really clean. Also, I have seen pocket joints where the terminal was hot enough for the solder to wet, but the wire is barely touching the other side.

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I have a few Heathkit gizmos. Stop where you are! Buy a Heath (Brock) candy bar! cuhulin

I like your calcs better than mine. That suggests that Heathkit would have used something like a high impedance earphone "speaker" in the design. That's possible and probably would work quite well.

However, I tore apart a similar Heathkit clock, with the identical p/n speaker, and measured 41.5 ohms DC resistance. Adding the inductance, that I didn't bother measuring, will produce about 50 to 60 ohms impedance. Obviously, this is not the optimum power transfer design, but that's what Heathkit apparently used. The important thing is that a common 4, 8, or 16 ohm speaker will NOT work.

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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.

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

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.

On Fri, 19 Sep 2008 09:41:51 -0700, Jeff Liebermann put finger to keyboard and composed:

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:

Here is some info on the MK5017 clock chip that was used in the GC-1005:

The MK5017's Tone output is shown driving a 2N3904 transistor connected to a 17VDC supply through a transformer-coupled 8 ohm speaker:

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

On Sun, 21 Sep 2008 15:54:26 +0100, "ian field" put finger to keyboard and composed:

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

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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.

On Sun, 21 Sep 2008 21:24:09 +0100, "ian field" put finger to keyboard and composed:

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

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On Sun, 21 Sep 2008 21:24:09 +0100, "ian field" put finger to keyboard and composed:

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

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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