Heathkit clock speaker?

J

Joe

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

greg
 
"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.
Click to expand...
Unfortunately the cone is fine, but the coil reads several megaohms.

-Jan
 
"Joe" <natkroPOISTA@dnainternet.net> wrote in message
news:48d295d2$0$23598$9b536df3@news.fv.fi...
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



To clarify the circuit, the postive side of the speaker receives 17 volts
Click to expand...
thru one diode and a resistor and the negative side is connected via
transistor to ground and the transistor is driven by 4001 IC.
 
"Joe" <natkroPOISTA@dnainternet.net> wrote in message
news:48d295d2$0$23598$9b536df3@news.fv.fi...
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
Click to expand...
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.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster
it goes.
 
"Joe" <natkroPOISTA@dnainternet.net> schreef in bericht
news:48d2a8de$0$23608$9b536df3@news.fv.fi...
"Joe" <natkroPOISTA@dnainternet.net> wrote in message
news:48d295d2$0$23598$9b536df3@news.fv.fi...
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



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.
Click to expand...
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
 
"petrus bitbyter" <pieterkraltlaatditweg@enditookhccnet.nl> wrote in message
news:48d2b61e$0$5041$e4fe514c@dreader26.news.xs4all.nl...
"Joe" <natkroPOISTA@dnainternet.net> schreef in bericht
news:48d2a8de$0$23608$9b536df3@news.fv.fi...

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



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.


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

--
*Why is it called tourist season if we can't shoot at them?

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.
 
"Archon" <Chipbee40_SpamNo@yahoo.com> wrote in message
news:zoBAk.185691$vn1.23307@fe03.news.easynews.com...
Dave Plowman (News) wrote:
In article <_QzAk.15047$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
Click to expand...
Yes, it is a normal, paper cone speaker.
And to clarify further, this is not a clock/radio.

-Jan
 
"mac-g3" <mac-g3@hotmail.com> wrote in message
news:48d3068a$0$12373$c3e8da3@news.astraweb.com...
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.
Click to expand...
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
 
"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.


--
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
Click to expand...
Speaker is a 401-163
Resistor is 150 ohm

-Jan
 
"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
news:bva6d497n710je3nupp0etnavom075u1uj@4ax.com...
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
Click to expand...
Thanks a million, appreciate your effort very much!

-Jan
 
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
Click to expand...
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
 
In article <48d2a78b$0$23588$9b536df3@news.fv.fi>, "Joe" <natkroPOISTA@dnainternet.net> wrote:
"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.
Click to expand...
Rip the cone apart and see if you can find a reading. The wire usually breaks
away from the coil.

greg
 
In article <4fe0f7a77edave@davenoise.co.uk>, "Dave Plowman (News)" <dave@davenoise.co.uk> 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.
Click to expand...
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
 
"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
news:l0l7d492rcl8vdt5e2fgfs4uqhcmk6a26n@4ax.com...
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.
Click to expand...
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.
 
In article <d4RAk.14839$Ax1.13283@newsfe12.ams2>, "ian field" <gangprobing.alien@ntlworld.com> wrote:
"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
news:l0l7d492rcl8vdt5e2fgfs4uqhcmk6a26n@4ax.com...
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.

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.
Click to expand...

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
 
"GregS" <zekfrivo@zekfrivolous.com> wrote in message
news:gb11af$h13$1@usenet01.srv.cis.pitt.edu...
In article <d4RAk.14839$Ax1.13283@newsfe12.ams2>, "ian field"
gangprobing.alien@ntlworld.com> wrote:

"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
news:l0l7d492rcl8vdt5e2fgfs4uqhcmk6a26n@4ax.com...
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.

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
Click to expand...
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.
 
"Franc Zabkar" <fzabkar@iinternode.on.net> wrote in message
news:p0a8d4pi7n71559o3s5kf0b96544e07n13@4ax.com...
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 ???
Click to expand...
Finding a way to reduce the duty cycle might be a means to get away with a
more commonly available lower impedance speaker.
 
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