Pulsing Woofers Continued...

M

Mike Wilson

Guest
Hi all
I decided to repost the problem that I'm having with my amp as it hasn't
been fixed yet.
I have a Kenwood KA-3500 amp, the woofers would pulse when the volume was
set to low (1 or 2)
If the volume is turned up to say 4, the problem goes away. Took the amp
apart, the two large power supply capacitors (42V 6800uF) were defective.
Replaced the two caps, turned the amp on and blew the fuse. I double checked
to make sure the caps were installed correctly, double checked to make sure
no bits of solder or wire fell onto the power board. Replaced the fuse,
turned her back on, the pulsing speaker problem is gone; however I have a
new problem. When the amp is turned on, the speakers extend to their full
length of excursion and stay there. There is a slight hum from the speakers,
then it fades until you can barely hear it. The heat sink gets quite hot,
very uncomfortable to the touch. So, of course I turned her off right away.
Am I right to think I blew one or more of the output transistors mounted on
the heat sink? Would it be a good test to unsolder, the transistors, one at
a time to find out which one is causing the problem? One thing I did try,
is test the output voltage of each transistor. I don't know how to reference
each transistor, so I'll say from left to right when looking at the amp from
the front. #1 transistor is 46.6v #2 transistor is 23.3v #3
transistor is .47v #4 transistor is 0.0v (no milliamps) Can you give me
a idea as to what would be cause this?

Thanks in advance : )
Mike
 
In article <vs5dh71364no19@corp.supernews.com>,
wilsonjamesmichael@yahoo.ca says...
Hi all
I decided to repost the problem that I'm having with my amp as it hasn't
been fixed yet.
I have a Kenwood KA-3500 amp, the woofers would pulse when the volume was
set to low (1 or 2)
If the volume is turned up to say 4, the problem goes away. Took the amp
apart, the two large power supply capacitors (42V 6800uF) were defective.
Replaced the two caps, turned the amp on and blew the fuse. I double checked
to make sure the caps were installed correctly, double checked to make sure
no bits of solder or wire fell onto the power board. Replaced the fuse,
turned her back on, the pulsing speaker problem is gone; however I have a
new problem. When the amp is turned on, the speakers extend to their full
length of excursion and stay there. There is a slight hum from the speakers,
then it fades until you can barely hear it. The heat sink gets quite hot,
very uncomfortable to the touch. So, of course I turned her off right away.
Am I right to think I blew one or more of the output transistors mounted on
the heat sink? Would it be a good test to unsolder, the transistors, one at
a time to find out which one is causing the problem? One thing I did try,
is test the output voltage of each transistor. I don't know how to reference
each transistor, so I'll say from left to right when looking at the amp from
the front. #1 transistor is 46.6v #2 transistor is 23.3v #3
transistor is .47v #4 transistor is 0.0v (no milliamps) Can you give me
a idea as to what would be cause this?
If the speakers move all the way out and stay there then you have DC
appearing on the output. This can burn your speakers out quite nicely.
Don't leave it like that for very long. This can be caused by a blown
transistor on the output stage (usually mounted on the heatsink). Is this
on both channels?
 
Yes Mainlander this problem happens on both channels. There is more heat on
the right side of the heat sink where there is less votage. Is there a test
that I can do to find out which output transistors are blown?
Thanks
Mike
"Mainlander" <*@*.*> wrote in message
news:MPG.1a2e868c5fc55b2898a1eb@news.paradise.net.nz...
In article <vs5dh71364no19@corp.supernews.com>,
wilsonjamesmichael@yahoo.ca says...
Hi all
I decided to repost the problem that I'm having with my amp as it hasn't
been fixed yet.
I have a Kenwood KA-3500 amp, the woofers would pulse when the volume
was
set to low (1 or 2)
If the volume is turned up to say 4, the problem goes away. Took the amp
apart, the two large power supply capacitors (42V 6800uF) were
defective.
Replaced the two caps, turned the amp on and blew the fuse. I double
checked
to make sure the caps were installed correctly, double checked to make
sure
no bits of solder or wire fell onto the power board. Replaced the fuse,
turned her back on, the pulsing speaker problem is gone; however I have
a
new problem. When the amp is turned on, the speakers extend to their
full
length of excursion and stay there. There is a slight hum from the
speakers,
then it fades until you can barely hear it. The heat sink gets quite
hot,
very uncomfortable to the touch. So, of course I turned her off right
away.
Am I right to think I blew one or more of the output transistors mounted
on
the heat sink? Would it be a good test to unsolder, the transistors, one
at
a time to find out which one is causing the problem? One thing I did
try,
is test the output voltage of each transistor. I don't know how to
reference
each transistor, so I'll say from left to right when looking at the amp
from
the front. #1 transistor is 46.6v #2 transistor is 23.3v #3
transistor is .47v #4 transistor is 0.0v (no milliamps) Can you
give me
a idea as to what would be cause this?

If the speakers move all the way out and stay there then you have DC
appearing on the output. This can burn your speakers out quite nicely.
Don't leave it like that for very long. This can be caused by a blown
transistor on the output stage (usually mounted on the heatsink). Is this
on both channels?
 
On Tue, 25 Nov 2003 23:12:05 -0600, "Mike Wilson"
<wilsonjamesmichael@yahoo.ca> put finger to keyboard and composed:

Yes Mainlander this problem happens on both channels. There is more heat on
the right side of the heat sink where there is less votage. Is there a test
that I can do to find out which output transistors are blown?
http://www.repairfaq.org/sam/semitest.htm#sttvd
http://www.repairfaq.org/sam/semitest.htm#stttm


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
 
Step One is Disconnect the Spreakers !!!

Do not connect the speakers unless you measure less than 0.05 volts DC
across the speaker terminals with no speaker connected. You can usually
faultfind the amp quite nicely without the speakers. Without the speakers,
you can turn on the power and use a multimeter on volts range to see what is
happening.

One trick is to replace the + and - rail fuses with 100 or 200 ohm 10 Watt
resistors. Under no-signal conditions there should only be a volt or less
dropped across the resistors. Once the amp is fixed, you can connect the
speakers and play at a low level. When all sounds right and there is no DC
across the speaker terminals with no speaker connected, you can remove the
resistors, connect the speakers and turn on.

Roger

If the speakers move all the way out and stay there then you have DC
appearing on the output. This can burn your speakers out quite nicely.
Don't leave it like that for very long. This can be caused by a blown
transistor on the output stage (usually mounted on the heatsink). Is this
on both channels?
 
In article <vs8dl95s95a960@corp.supernews.com>,
wilsonjamesmichael@yahoo.ca says...
Yes Mainlander this problem happens on both channels. There is more heat on
the right side of the heat sink where there is less votage. Is there a test
that I can do to find out which output transistors are blown?
Basically the power supply will be split rails, one side will be X volts
above ground (positive) and the other side X volts below ground
(negative). Each channel of the amp is a fairly symmetric design in the
output stages with half of the output transistors on the positive side
and the other half on the negative side. The transistors on the positive
side produce the positive half of the AC waveform at the output while
those on the negative side produce the negative half of the waveform. If
you have a schematic diagram this will aid greatly in determining which
transistors are which.

The transistors on the positive side are one of the two types and those
on the negative side are the opposite type (the types being PNP and NPN
except that I can't remember which is which. I have a feeling NPN is on
the positive side but don't quote me on that) - the type numbers on them
will obviously be different. You should be able to look up the type
numbers somewhere. This should aid you in tracking down where to start.
Measuring the output DC voltage, whether it is + or - respect to ground
is the starting point. Then find out which side of the amp is supplied
with that polarity of voltage.

It seems strange that both channels have the same problem - unless some
sort of power spike on one half of the power supply blew the transistors
on that side of the amp in both channels.
 
Thanks Mainlander
My good friend in GA gave me a link to test the output transistors (on
heatsink) I used the diode test on my digital multimeter
I'll have to refer to the transistors from left to right facing the front of
the amp.

#1 NEC Transistor B617 R76B (PNP, Pos) 647 Ohms
#2 NEC Transistor D587 R76B (NPN, Neg) 651 Ohms
#3 SK Transistor C258 2DYL (NPN, Neg) 636 Ohms
#4 No-Name Transistor B617 R76B (PNP, Pos) 704 Ohms
All tests were done with leads on base and emt, when I tried base and
collector, I got a slightly lower reading.

Set my multimeter to 200 Ohms scale and tried to test between base and emt,
the multimeter number flashes once and goes away, reversed the leads for
PNP, same thing the number flashes, is this normal, for testing transistors?
One thing I noticed is that the Ohms reading is a lot lower then using the
diode test, for instance, #4 transistor is 179 Ohms

Do you think that #4 transistor is the bad one? I'm not sure if I should
scrap the amp or not.

<*@*.*> wrote in message
news:MPG.1a3079ed54e7657b98a213@news.paradise.net.nz...

In article <vs8dl95s95a960@corp.supernews.com>,
wilsonjamesmichael@yahoo.ca says...
Yes Mainlander this problem happens on both channels. There is more heat
on
the right side of the heat sink where there is less votage. Is there a
test
that I can do to find out which output transistors are blown?

Basically the power supply will be split rails, one side will be X volts
above ground (positive) and the other side X volts below ground
(negative). Each channel of the amp is a fairly symmetric design in the
output stages with half of the output transistors on the positive side
and the other half on the negative side. The transistors on the positive
side produce the positive half of the AC waveform at the output while
those on the negative side produce the negative half of the waveform. If
you have a schematic diagram this will aid greatly in determining which
transistors are which.

The transistors on the positive side are one of the two types and those
on the negative side are the opposite type (the types being PNP and NPN
except that I can't remember which is which. I have a feeling NPN is on
the positive side but don't quote me on that) - the type numbers on them
will obviously be different. You should be able to look up the type
numbers somewhere. This should aid you in tracking down where to start.
Measuring the output DC voltage, whether it is + or - respect to ground
is the starting point. Then find out which side of the amp is supplied
with that polarity of voltage.

It seems strange that both channels have the same problem - unless some
sort of power spike on one half of the power supply blew the transistors
on that side of the amp in both channels.
 
In article <vscj2r3u7i8320@corp.supernews.com>,
wilsonjamesmichael@yahoo.ca says...
Thanks Mainlander
My good friend in GA gave me a link to test the output transistors (on
heatsink) I used the diode test on my digital multimeter
I'll have to refer to the transistors from left to right facing the front of
the amp.

#1 NEC Transistor B617 R76B (PNP, Pos) 647 Ohms
#2 NEC Transistor D587 R76B (NPN, Neg) 651 Ohms
#3 SK Transistor C258 2DYL (NPN, Neg) 636 Ohms
#4 No-Name Transistor B617 R76B (PNP, Pos) 704 Ohms
All tests were done with leads on base and emt, when I tried base and
collector, I got a slightly lower reading.

Set my multimeter to 200 Ohms scale and tried to test between base and emt,
the multimeter number flashes once and goes away, reversed the leads for
PNP, same thing the number flashes, is this normal, for testing transistors?
One thing I noticed is that the Ohms reading is a lot lower then using the
diode test, for instance, #4 transistor is 179 Ohms

Do you think that #4 transistor is the bad one? I'm not sure if I should
scrap the amp or not.

Sorry I don't know enough about transistors to be able to help with these
results.
 
In article <vscj2r3u7i8320@corp.supernews.com>,
wilsonjamesmichael@yahoo.ca says...
Thanks Mainlander
My good friend in GA gave me a link to test the output transistors (on
heatsink) I used the diode test on my digital multimeter
I'll have to refer to the transistors from left to right facing the front of
the amp.

#1 NEC Transistor B617 R76B (PNP, Pos) 647 Ohms
#2 NEC Transistor D587 R76B (NPN, Neg) 651 Ohms
#3 SK Transistor C258 2DYL (NPN, Neg) 636 Ohms
#4 No-Name Transistor B617 R76B (PNP, Pos) 704 Ohms
All tests were done with leads on base and emt, when I tried base and
collector, I got a slightly lower reading.

Set my multimeter to 200 Ohms scale and tried to test between base and emt,
the multimeter number flashes once and goes away, reversed the leads for
PNP, same thing the number flashes, is this normal, for testing transistors?
One thing I noticed is that the Ohms reading is a lot lower then using the
diode test, for instance, #4 transistor is 179 Ohms

Do you think that #4 transistor is the bad one? I'm not sure if I should
scrap the amp or not.

The web pages the other guy referred to are for the measurement of the
voltage drops, not the resistance. The semiconductor junctions have a
certain characteristic voltage drop across them, usually around 0.6V per
junction. It seems to me that would be more valuable than a resistance
measurement as that will show you whether there has been a breakdown of
one of the junctions. I don't know how to interpret the resistance
readings.

To give an example, in a diagram I have here of an amp, the NPN
transistor on the +ve side has the +ve supply rail connected to its
collector with the emitter connected to the output. The base is connected
to the rest of the amp circuitry. With no signal coming in there should
not be anything at the output, DC or otherwise, or very low, because the
transistor should be turned off. But you can see that if there is a
breakdown across the transistor then the current could flow from the
supply rail straight to the output. And then I would expect the
transistor would get very hot because of all the current flow through it.
 
Some questions:
How old is the amplifier
Did you wire in the capacitors with the correct polarity. BTW how did you
determine that the main 6800uF caps were defective. If the amp is less than
20 years old, they should be okay. I have several large 100W rms in a big
audio installation; I built these over 20 years ago - the ETI480 ( a really
robust amp) and the Caps are fine despite their age. Sounds as if the caps
are wired in back to front.
Is the Diode bridge defective.
Motorboating speakers (pulsing) can be caused by an output transistor going
into very high gain. I had this problem in an electronic organ some years
ago.
Make a note of the connections, and disconnect the output transistors, check
for emitter/collector short circuit.
Sounds like you have several problems with this amplifier.
This site would be a good place to start,
http://sound.westhost.com/site-map.htm, lots of info here

2SB617 PNP; is obsolete. replacement TIC42;2SB688(R)
2SD587 NPN ; also obsolete. = 2SD718,
- complimentary power output transistors. Not readily available, might
have to do a search on the net.
This amp must be an old one? circa 1977. wow!
Disconnect the Speakers, otherwise you can kiss them goodbye!
Solder 100 ohm 1/2 watt resistors across the plus and minus dc fuses. If
they vaporise or smoke when you turn on the amp you have got a short in the
output transistors.
Have a look at this url http://www.nbizz.com/soundsclassic/listings/219.html

Might be worth upgrading to a new tuner/amp, you can get a good one for
under $500 AUD. I bought a Panasonic and a Pioneer for under $500 both
surround sound amplifiers.
Best of Luck Mike.


"Mike Wilson" <wilsonjamesmichael@yahoo.ca> wrote in message
news:vs5dh71364no19@corp.supernews.com...
Hi all
I decided to repost the problem that I'm having with my amp as it hasn't
been fixed yet.
I have a Kenwood KA-3500 amp, the woofers would pulse when the volume was
set to low (1 or 2)
If the volume is turned up to say 4, the problem goes away. Took the amp
apart, the two large power supply capacitors (42V 6800uF) were defective.
Replaced the two caps, turned the amp on and blew the fuse. I double
checked
to make sure the caps were installed correctly, double checked to make
sure
no bits of solder or wire fell onto the power board. Replaced the fuse,
turned her back on, the pulsing speaker problem is gone; however I have a
new problem. When the amp is turned on, the speakers extend to their full
length of excursion and stay there. There is a slight hum from the
speakers,
then it fades until you can barely hear it. The heat sink gets quite hot,
very uncomfortable to the touch. So, of course I turned her off right
away.
Am I right to think I blew one or more of the output transistors mounted
on
the heat sink? Would it be a good test to unsolder, the transistors, one
at
a time to find out which one is causing the problem? One thing I did try,
is test the output voltage of each transistor. I don't know how to
reference
each transistor, so I'll say from left to right when looking at the amp
from
the front. #1 transistor is 46.6v #2 transistor is 23.3v #3
transistor is .47v #4 transistor is 0.0v (no milliamps) Can you give
me
a idea as to what would be cause this?

Thanks in advance : )
Mike
 
The amp was built around 1972, the caps don't hold a 12 volt charge from a
12 volt power adapter, which they should at least charge up to. Thanks for
the info, but I gave up on the amp. I bought another amp. I will however
check out the website and see if I can fix the amp in my spare time. Thanks
"mrocus" <silent@example.com> wrote in message
news:3fd34f32@news.comindico.com.au...
Some questions:
How old is the amplifier
Did you wire in the capacitors with the correct polarity. BTW how did you
determine that the main 6800uF caps were defective. If the amp is less
than
20 years old, they should be okay. I have several large 100W rms in a big
audio installation; I built these over 20 years ago - the ETI480 ( a
really
robust amp) and the Caps are fine despite their age. Sounds as if the
caps
are wired in back to front.
Is the Diode bridge defective.
Motorboating speakers (pulsing) can be caused by an output transistor
going
into very high gain. I had this problem in an electronic organ some years
ago.
Make a note of the connections, and disconnect the output transistors,
check
for emitter/collector short circuit.
Sounds like you have several problems with this amplifier.
This site would be a good place to start,
http://sound.westhost.com/site-map.htm, lots of info here

2SB617 PNP; is obsolete. replacement TIC42;2SB688(R)
2SD587 NPN ; also obsolete. = 2SD718,
- complimentary power output transistors. Not readily available, might
have to do a search on the net.
This amp must be an old one? circa 1977. wow!
Disconnect the Speakers, otherwise you can kiss them goodbye!
Solder 100 ohm 1/2 watt resistors across the plus and minus dc fuses. If
they vaporise or smoke when you turn on the amp you have got a short in
the
output transistors.
Have a look at this url
http://www.nbizz.com/soundsclassic/listings/219.html

Might be worth upgrading to a new tuner/amp, you can get a good one for
under $500 AUD. I bought a Panasonic and a Pioneer for under $500 both
surround sound amplifiers.
Best of Luck Mike.


"Mike Wilson" <wilsonjamesmichael@yahoo.ca> wrote in message
news:vs5dh71364no19@corp.supernews.com...
Hi all
I decided to repost the problem that I'm having with my amp as it hasn't
been fixed yet.
I have a Kenwood KA-3500 amp, the woofers would pulse when the volume
was
set to low (1 or 2)
If the volume is turned up to say 4, the problem goes away. Took the amp
apart, the two large power supply capacitors (42V 6800uF) were
defective.
Replaced the two caps, turned the amp on and blew the fuse. I double
checked
to make sure the caps were installed correctly, double checked to make
sure
no bits of solder or wire fell onto the power board. Replaced the fuse,
turned her back on, the pulsing speaker problem is gone; however I have
a
new problem. When the amp is turned on, the speakers extend to their
full
length of excursion and stay there. There is a slight hum from the
speakers,
then it fades until you can barely hear it. The heat sink gets quite
hot,
very uncomfortable to the touch. So, of course I turned her off right
away.
Am I right to think I blew one or more of the output transistors mounted
on
the heat sink? Would it be a good test to unsolder, the transistors, one
at
a time to find out which one is causing the problem? One thing I did
try,
is test the output voltage of each transistor. I don't know how to
reference
each transistor, so I'll say from left to right when looking at the amp
from
the front. #1 transistor is 46.6v #2 transistor is 23.3v #3
transistor is .47v #4 transistor is 0.0v (no milliamps) Can you
give
me
a idea as to what would be cause this?

Thanks in advance : )
Mike
 

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