Heathkit clock speaker?

[ian field]

"Franc Zabkar" <fzabkar@iinternode.on.net> wrote in message
news:e3add4tqmv9e8h7719l0h1qdf2ovu35qdu@4ax.com...

On Sun, 21 Sep 2008 15:54:26 +0100, "ian field"
gangprobing.alien@ntlworld.com> put finger to keyboard and composed:

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.

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.

 

[Fred McKenzie]

In article
<none-53D617.23594618092008@earthlink.vsrv-sjc.supernews.net>,
Joe <none@given.now> wrote:

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

 

[nobody >]

GregS wrote:

In article <48d295d2$0$23598$9b536df3@news.fv.fi>, "Joe" <natkroPOISTA@dnainternet.net> wrote:
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

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

greg

DC resistance IS NOT impedance.

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

 

[Jeff Liebermann]

On Thu, 18 Sep 2008 22:15:27 +0300, "Joe"
<natkroPOISTA@dnainternet.net> wrote:

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.

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.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[ian field]

"Joe" <natkroPOISTA@dnainternet.net> wrote in message
news:48d2a78b$0$23588$9b536df3@news.fv.fi...

"Dave Plowman (News)" <dave@davenoise.co.uk> wrote in message
news:4fe0df5f75dave@davenoise.co.uk...
In article <48d295d2$0$23598$9b536df3@news.fv.fi>,
Joe <natkroPOISTA@dnainternet.net> wrote:
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?

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.

--
Small asylum seeker wanted as mud flap, must be flexible and willing to
travel

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

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

-Jan

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.

 

[Archon]

Dave Plowman (News) wrote:

In article <_QzAk.15047$O51.7624@newsfe23.ams2>,
ian field <gangprobing.alien@ntlworld.com> wrote:
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,

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

 

[Jeff Liebermann]

On Fri, 19 Sep 2008 06:45:30 +0300, "Joe"
<natkroPOISTA@dnainternet.net> wrote:

"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
news:bge5d4dcdnri9757oi2f503lm3qlcosh9f@4ax.com...
On Thu, 18 Sep 2008 22:15:27 +0300, "Joe"
natkroPOISTA@dnainternet.net> wrote:

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.

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.

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.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Jeff Liebermann]

On Thu, 18 Sep 2008 21:27:37 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Fri, 19 Sep 2008 06:45:30 +0300, "Joe"
natkroPOISTA@dnainternet.net> wrote:


"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
news:bge5d4dcdnri9757oi2f503lm3qlcosh9f@4ax.com...
On Thu, 18 Sep 2008 22:15:27 +0300, "Joe"
natkroPOISTA@dnainternet.net> wrote:

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.

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.

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.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Franc Zabkar]

On Thu, 18 Sep 2008 13:59:55 -0700, Jeff Liebermann <jeffl@cruzio.com>
put finger to keyboard and composed:

On Thu, 18 Sep 2008 22:15:27 +0300, "Joe"
natkroPOISTA@dnainternet.net> wrote:

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.

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.

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
--
Please remove one 'i' from my address when replying by email.

 

[Michael A. Terrell]

Joe wrote:

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?

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.

--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

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

 

[Jeff Liebermann]

On Fri, 19 Sep 2008 17:34:40 +1000, Franc Zabkar
<fzabkar@iinternode.on.net> wrote:

On Thu, 18 Sep 2008 13:59:55 -0700, Jeff Liebermann <jeffl@cruzio.com
put finger to keyboard and composed:

On Thu, 18 Sep 2008 22:15:27 +0300, "Joe"
natkroPOISTA@dnainternet.net> wrote:

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.

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.

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

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.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Franc Zabkar]

On Fri, 19 Sep 2008 09:41:51 -0700, Jeff Liebermann <jeffl@cruzio.com>
put finger to keyboard and composed:

On Fri, 19 Sep 2008 17:34:40 +1000, Franc Zabkar
fzabkar@iinternode.on.net> wrote:

On Thu, 18 Sep 2008 13:59:55 -0700, Jeff Liebermann <jeffl@cruzio.com
put finger to keyboard and composed:

On Thu, 18 Sep 2008 22:15:27 +0300, "Joe"
natkroPOISTA@dnainternet.net> wrote:

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.

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.

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

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.

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
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Sun, 21 Sep 2008 15:54:26 +0100, "ian field"
<gangprobing.alien@ntlworld.com> put finger to keyboard and composed:

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
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Sun, 21 Sep 2008 21:24:09 +0100, "ian field"
<gangprobing.alien@ntlworld.com> put finger to keyboard and composed:

"Franc Zabkar" <fzabkar@iinternode.on.net> wrote in message
news:e3add4tqmv9e8h7719l0h1qdf2ovu35qdu@4ax.com...
On Sun, 21 Sep 2008 15:54:26 +0100, "ian field"
gangprobing.alien@ntlworld.com> put finger to keyboard and composed:

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.

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
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Sun, 21 Sep 2008 21:24:09 +0100, "ian field"
<gangprobing.alien@ntlworld.com> put finger to keyboard and composed:

"Franc Zabkar" <fzabkar@iinternode.on.net> wrote in message
news:e3add4tqmv9e8h7719l0h1qdf2ovu35qdu@4ax.com...
On Sun, 21 Sep 2008 15:54:26 +0100, "ian field"
gangprobing.alien@ntlworld.com> put finger to keyboard and composed:

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.

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
--
Please remove one 'i' from my address when replying by email.

 

[mac-g3]

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.

Joe wrote:

I just managed to get hold of unbuilt Heathkit alarm clock GC-1107.
The electrolytics were dry and the speaker is shot.