Heathkit clock speaker?

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

greg

Presumably it's the cone which has gone? If so the coil should still be ok so measure the DC resistance. That will be near enough the impedance for a guide. Early small transformer less (output) small transistor amps sometimes used 35 ohm speakers, but they got round this when silicon transistors arrived.

DC resistance IS NOT impedance.

To the OP: it may not be totally correct, but 8 ohms is a valid choice.

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

-Jan

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.

The speaker is used as the collector load for am MPSA20 transistor, and supplied by an 18V source. The only "audio" from the speaker is a square wave from the clock chip.

I haven't looked up the max collector current for the MPSA20, but you need to consult the datasheet for that, and calc the impedance for the speaker from that. I'd suggest something like a 100-ohm speaker for this clock.

"Joe" schreef in bericht news:48d2a8de$0$23608$ snipped-for-privacy@news.fv.fi...

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

What value resitor? What's the part number on the speaker?

I just happen to have the manual for a similar Heathkit GC-1005 "Electronic Clock" handy. No specs on the speaker (401-163). The circuit is similar in that the speaker has one lead going to the collector of an MPS-A20 and the other to 18.5VAC (not DC) through a diode and 1200 uf to ground for a DC voltage of 25.7VDC. No resistor in series.

So, let's do the math. My guess(tm) is that 1 watt will be sufficiently loud to wake the dead. Yours has about a 15VDC swing. Assuming a 50% duty cycle and a pure sine wave (yeah sure): Power = E^2 / R 1 watt = 15^2 / R R = 225 ohms. So, it's probably a high impedance speaker of some sorts. I have the digital clock and an LRC meter and could probably measure the impedance. If you can't get the info any other way, bug me and I'll rip it apart.

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.

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.

Isn't it easier just to look at it? A piezo sounder looks nothing like a moving coil speaker.

The manual is available here,

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

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

-Jan

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

Speaker is a 401-163 Resistor is 150 ohm

-Jan

Ok the speakers are the same. 225 - 150 = 75 ohms which would be my guess for the proper speaker impedance. I don't know what value to suggest based on these calcs. I guess I have to meaure it. That's going to be rough because the speaker is UNDER the PCB. Give me an hour or so.

Done. The speaker measures 41.5 ohms DC resistance. I'm too lazy to measure the inductance, but my guess(tm) is that added inductance will raise the impedance to somewhere between 50 and 60 ohms. Standard impedances (from the Digikey search page) are 40, 45, 48, and 60 ohms. I'm at a loss as to where you would find such a speaker, but at least you have a better clue as to what to search for. For a temporary kludge, the impedance of many headsets are in this range. Sony headsets tend to be about 45 ohms.

Incidentally, I plugged in my GC-1005. It made a loud noise and only the NE-2 lamp lit. Looks like yet another repair project. Sigh.

Thanks a million, appreciate your effort very much!

-Jan