Amp problem, Pulsing Woofers with low volume

[Mike Wilson]

Hello there
Love the site! Anyhoo, I need help with my amp. I have a old Kenwood amp. I
think it was manufactured around 1988, 120W, 4 or 8 Ohm rated. A few years
ago (when I knew little about electronics) I had the amp serviced, one cap
and 2 voltage regulators where replaced. The one cap that was replaced is a
large, 42v 6800microferret cap (3 leads bottom of cap). The amp has been
running fine for years. Recently, the woofers start pulsing when the volume
is turn down low and the bass is about half way. If the volume is turned up
to about half way, the problem stops. This pulsing will happen with or
without playing any music. The pulsing only happens in the bass (woofers).
The pulsing does go away when one speaker is removed.

When testing the amp, no speakers attached, I used a volt meter set to 200
ma to see if there is any voltage leaking to the speakers, yep sure enough,
there is about 40ma leaking. Next I tried the two large caps that are near
the power supply. One is a 63v 6800 microferret and the other is a 42v 6800
microferret. with the amp turned on, both caps charge up to their respective
voltages. When the amp is turned off, the voltage in both caps are gone in
about 2 seconds and only a slight charge is left over, around 140 ma
Shouldn't the caps hold a charge, is this normal? Next I decided to take one
of the suspected caps off of the board. Charged it up with a 12v power
source, when I tested it, the reading dropped from 12v to less than a volt
in a matter of a second, is this normal for a 42v 6800 microferret cap?

Please help as I cannot seem to figure out why the amp is doing this!!!!!!
: (
Thanks in advance
Mike

 

[Jerry G.]

In many of these amps, there are more caps that are bad from age. It may
not be necessarily the larger caps. Using an ESR meter would be the best
way to locate the bad ones. The hardway to find the defective caps is to sit
down with a scope and DVM and go through the amp stage by stage. Sometimes
one stage may effect another, so this can be difficult to find. It may be a
cap in a decoupling or feedback operation circuit that is the fault.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


Hello there
Love the site! Anyhoo, I need help with my amp. I have a old Kenwood amp. I
think it was manufactured around 1988, 120W, 4 or 8 Ohm rated. A few years
ago (when I knew little about electronics) I had the amp serviced, one cap
and 2 voltage regulators where replaced. The one cap that was replaced is a
large, 42v 6800microferret cap (3 leads bottom of cap). The amp has been
running fine for years. Recently, the woofers start pulsing when the volume
is turn down low and the bass is about half way. If the volume is turned up
to about half way, the problem stops. This pulsing will happen with or
without playing any music. The pulsing only happens in the bass (woofers).
The pulsing does go away when one speaker is removed.

When testing the amp, no speakers attached, I used a volt meter set to 200
ma to see if there is any voltage leaking to the speakers, yep sure enough,
there is about 40ma leaking. Next I tried the two large caps that are near
the power supply. One is a 63v 6800 microferret and the other is a 42v 6800
microferret. with the amp turned on, both caps charge up to their respective
voltages. When the amp is turned off, the voltage in both caps are gone in
about 2 seconds and only a slight charge is left over, around 140 ma
Shouldn't the caps hold a charge, is this normal? Next I decided to take one
of the suspected caps off of the board. Charged it up with a 12v power
source, when I tested it, the reading dropped from 12v to less than a volt
in a matter of a second, is this normal for a 42v 6800 microferret cap?

Please help as I cannot seem to figure out why the amp is doing this!!!!!!
: (
Thanks in advance
Mike

 

[Mike Wilson]

Thanks Jerry
I still would like to know if a cap (as described before) would loose it's
charge fast?

 

[Tweetldee]

Thanks Jerry
I still would like to know if a cap (as described before) would loose it's
charge fast?

be amp. is

No, a capacitor that large should hold on to its charge for quite a while
with only a voltmeter connected. Replace both of the large capacitors.You
mentioned that the capacitors near the power supply have different voltage
ratings.. has either or both ever been replaced? Measure the voltage
across each of them. If the voltage is close to the capacitor's rating,
replace it with a higher voltage rated capacitor. I'm particularly
concerned about the 42V capacitor. Doesn't seem right that the power
supply capacitors should have different voltage ratings.
Cheers!!!!
--
Tweetldee
Tweetldee at att dot net (Just subsitute the appropriate characters in the
address)

Never take a laxative and a sleeping pill at the same time!!

 

[Asimov]

"Mike Wilson" bravely wrote to "All" (01 Nov 03 13:20:18)
--- on the heady topic of "Re: Amp problem, Pulsing Woofers with low volume"

MW> From: "Mike Wilson" <[email protected]>

MW> Thanks Jerry
MW> I still would like to know if a cap (as described before) would loose
MW> it's charge fast?

Regardless the cap should keep its charge for a considerable time when
left open circuit and unloaded. That it loses the charge in about 1
second indicates a couple of possible problems. The first obvious is it
might have a very large leakage.

The other is that its capacitance value is greatly reduced. If we assume
a constant current discharge using I=CV/t we get around 100mA of leakage
when it should be say a few 10's of microamps. OTOH the alternative of
too little capacitance would, assuming 10uA leakage, estimate to about
1uF instead of your 6800uF. This can't be so because that would result
in a lot of audible hum.

However, the pulsation might come from an instability in the preamp gain
stage. You mentioned that the problem seems worse with the volume set
low. Perhaps this indicates the input coupling capacitor at the volume
wiper contact may have lost some value. I know from my experience in
studying preamp stage design that some common configurations in the past
could present instability if either the input coupling capacitor was a
too small value, or the emitter bypass.

In my opinion the supply caps are very leaky, and the instability comes
from a preamp cap being dry. A scope would be an ideal instrument to
confirm the problem.


.... Is reactance then illusory? No, it just appears that way...

 

[Leonard Caillouet]

Perhaps it is all those little micro ferrets running around in there. The
correct term for the unit of capacitance is the Farrad.

Look for more bad caps in an old amp. I would also get the right voltage
rating cap. A 42v cap installed where the original was 63V is a big
mistake. It is likely operating at its limit in an amp of that power rating
and is likely failing.

Leonard Caillouet

 

[Mike Wilson]

Thanks Tweetledee, that good avice, I'll try tthat first, replace the two
large caps and see where I am from there.

 

[Mike Wilson]

Thanks Jerry!!!
Again, Thanks a bunch, you confirmed what I thought all along...the person
that servised this amp, replace a 63v cap with a 42v cap. I'll replace the
two large caps first and see where I'm at from there. I can trouble shoot
computer hardware and software like it's no one's bussiness (computer tech
here), but I'm kinda new at actually replacing componets. The "input
coupling capacitor at the volume > wiper contact" that you mentioned, where
might I find this?
Thanks is advance.

 

[Mike Wilson]

I was wondering if anybody knows where I can find a online maual for Kenwood
Amps. I tried their website, found a model that was close to mine, but it
doesn't have any technical data, like, is that 42v cap the correct one?
Thanks in advance
Mike

 

[Mike Wilson]

Micro Ferret, it my little joke...seems like you are the only one that
picked up on it ; )
Yep I do know a little bit about electronics Ohms laws ETC, not like the ppl
that respond to my inquiries though.
So Leonard do you think that the 42v cap is correct, I mean, do you think
that the person that serviced that amp replaced a 63v with a 42v cap? One
other piece of info I forgot to mention is: the amp seems to shut off
abruptly with a bit of a thump when I turn it off. Before I had this
problem, when shutting off the amp, the music would fade, giving me more
cause to think that I have a bad cap or a power supply problem. But again
the amp works, jut have that annoying pulsing hum when it is on low volume.
Anyother suggestions?
Thanks in advance.
Mike

 

[Leonard Caillouet]

Your message said that there were two caps, one 42v and one 63v and that the
42v was the one that was changed. What does that suggest? If that is not
what you meant, please be more clear. Also, if you would post the model
number someone may be able to help you more specifically. I have a stack of
Kenwood manuals about 4 feet tall and might have it, but like we tell people
asking for help all of the time, we are not psychics....

Leonard Caillouet

 

[Mike Wilson]

Sorry if I didn't expain this as well as I could, computer tech here, kinda
new at this thing. : )
Anyhoo upon futher investigation I realized that it's the 63v cap that had
been replaced as the 42v
had the orginal manufacture's glue on the bottom of the cap. The 42v is
definaltely bad as it won't even hold
a 12 volt charge. The 63v will hold a charge, I must also mention, I tested
these caps with them off
of the board. The model number of the amp is Kenwood KA-3500
Any further assistence would be much appreciated Leonard!
Thanks in Advance
Mike Wilson

"Never pet a burning dog"

 

[Leonard G. Caillouet]

Sorry, Mike. All of the manuals that I have are either older or newer than
this one.

Call East Coast Transistor for Kenwood manuals.
800-645-3516

Leonard Caillouet

 

[Mike Wilson]

Thanks for the info