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intermittent failure of Kenwood KR-4070 receiver

H

Halfgaar

Hi,

I've got a very annoying intermittent-failure problem with my 25 year old
Kenwood KR-4070 receiver. I think some 6 years ago, my parents gave it to
me because it had failed. The output would drop, reappear, drop, reappear,
etc. Before I got it, it was sent in for repair first. They couldn't find
anything wrong with it, it just so happend that it was in working order
when in the repair shop. It worked for a while longer at home, but
eventually, no sound came from it at all anymore. That's when they gave it
to me. I used the tuner of it for quite some time, until someday I decided
to test whether the amplifier worked again. It did. After that, it had
worked for years. Then, it didn't work for a few days, and after those
days, it worked fine again for years. It's having the same problem again
now.

When it fails, it sounds just when like you shut it off. Shutting it off
procuces a "plop". When it starts to work again it also sounds like it is
switched on, a similair "plop" is audible. Both speakers and headphone
outputs are affected, and always both the left and right channel, never
either left or right. When giving it a signal, a faint distorted output is
audible when the volume is at maximum.

Because of the symptoms, I am inclined to think that power to the pre-amp
keeps failing. But, I am far from an expert. AC powerinput to the pre-amp
is present all the time.

Is it reasonable to think that it is a mechanical problem (like bad
connections or cold-solderjoints) as suppose to for example dried out
electrolytic capacitors, seeing as how the problem started some 6 years
ago? I already re-heated almost every solderjoint of what seems to be the
pre-amp. It didn't work.

Also, on www.repairfaq.org I read that the signal can be traced by a wet
finger. How exactly do I go about that? When I've reached a point after the
failure-point, should that be audible in the output? Also, is it safe to
touch all parts except those of the power-supply (which has it own PCB),
without shock-hazzard?

TIA

Halfgaar
 
H

Halfgaar

Halfgaar said:
Hi,

I've got a very annoying intermittent-failure problem with my 25 year old
Kenwood KR-4070 receiver. I think some 6 years ago, my parents gave it to
me because it had failed. The output would drop, reappear, drop, reappear,
etc. Before I got it, it was sent in for repair first. They couldn't find
anything wrong with it, it just so happend that it was in working order
when in the repair shop. It worked for a while longer at home, but
eventually, no sound came from it at all anymore. That's when they gave it
to me. I used the tuner of it for quite some time, until someday I decided
to test whether the amplifier worked again. It did. After that, it had
worked for years. Then, it didn't work for a few days, and after those
days, it worked fine again for years. It's having the same problem again
now.

When it fails, it sounds just when like you shut it off. Shutting it off
procuces a "plop". When it starts to work again it also sounds like it is
switched on, a similair "plop" is audible. Both speakers and headphone
outputs are affected, and always both the left and right channel, never
either left or right. When giving it a signal, a faint distorted output is
audible when the volume is at maximum.

Because of the symptoms, I am inclined to think that power to the pre-amp
keeps failing. But, I am far from an expert. AC powerinput to the pre-amp
is present all the time.

Is it reasonable to think that it is a mechanical problem (like bad
connections or cold-solderjoints) as suppose to for example dried out
electrolytic capacitors, seeing as how the problem started some 6 years
ago? I already re-heated almost every solderjoint of what seems to be the
pre-amp. It didn't work.

Also, on www.repairfaq.org I read that the signal can be traced by a wet
finger. How exactly do I go about that? When I've reached a point after
the failure-point, should that be audible in the output? Also, is it safe
to touch all parts except those of the power-supply (which has it own
PCB), without shock-hazzard?

TIA

Halfgaar

I've got some development. By going around the PCB with a wet finger, I can
introduce noise in the left or right channel. I was able to do this when it
was working. As soon as it failed, I tried it again. I could still here the
plops by touching the components, but they sound louder. I don't know if
this means anything, just thought I'd mention it. It also means that the
signal fails before that point.

Halfgaar
 
J

jakdedert

Halfgaar said:
I've got some development. By going around the PCB with a wet finger,
I can introduce noise in the left or right channel. I was able to do
this when it was working. As soon as it failed, I tried it again. I
could still here the plops by touching the components, but they sound
louder. I don't know if this means anything, just thought I'd mention
it. It also means that the signal fails before that point.

Possibly dirty function switch or problem in the balance control...almost
all that's common to the both sides, except for the power supply.

jak
 
M

Mark D. Zacharias

The 4070 is (in)famous for a intermittent or just bad power switch. This
will often be accompanied by dimming or flashing dial lamps, and would often
cause interference to a nearby AM radio during the failure mode, since the
switch was arcing inside.

Mark Z.
 
H

Halfgaar

Mark said:
The 4070 is (in)famous for a intermittent or just bad power switch. This
will often be accompanied by dimming or flashing dial lamps, and would
often cause interference to a nearby AM radio during the failure mode,
since the switch was arcing inside.

Mark Z.

I'm positive it's not the switch (unless the (pre-)amp works of a seperate
power-line), because lighting remains constant, and when I connect the tape
out to another amp, I can still hear the tuner when the amp is in a state
of "disservice".

I've got another possibility. A friend of mine told me about self-repairing
electrolytic capacitors, which are able to seal internal leaks somehow. Do
those things actually exist, and if so, is it likely that my receiver has
such capacitors?

BTW, do you actually have experience with this amplifier? If so, I'd like to
know your opinion/experience with it.
 
A

Asimov

"Halfgaar" bravely wrote to "All" (07 Jun 04 18:59:44)
--- on the heady topic of "Re: intermittent failure of Kenwood KR-4070 receiver"

Ha> From: Halfgaar <[email protected]>

Ha> I'm positive it's not the switch (unless the (pre-)amp works of a
Ha> seperate power-line), because lighting remains constant, and when I
Ha> connect the tape out to another amp, I can still hear the tuner when
Ha> the amp is in a state of "disservice".

Ha> I've got another possibility. A friend of mine told me about
Ha> self-repairing electrolytic capacitors, which are able to seal internal
Ha> leaks somehow. Do those things actually exist, and if so, is it likely
Ha> that my receiver has such capacitors?

BTW it isn't only electrolytic capacitors that self-heal, any
semiconductor can have a bad internal contact which might become
intermittant with temperature or simply by an electrical impulse such
as blasting the volume control. This was it in a bad lot of 2SC828's.

A*s*i*m*o*v

.... Over a hundred billion electrons were used in crafting this tagline.
 
D

David Farber

Subject: intermittent failure of Kenwood KR-4070 receiver
From: Halfgaar [email protected]
Date: 06/06/2004 8:20 AM Pacific Daylight Time
Message-id: <[email protected]>

Hi,

I've got a very annoying intermittent-failure problem with my 25 year old
Kenwood KR-4070 receiver. I think some 6 years ago, my parents gave it to
me because it had failed. The output would drop, reappear, drop, reappear,
etc. Before I got it, it was sent in for repair first. They couldn't find
anything wrong with it, it just so happend that it was in working order
when in the repair shop. It worked for a while longer at home, but
eventually, no sound came from it at all anymore. That's when they gave it
to me. I used the tuner of it for quite some time, until someday I decided
to test whether the amplifier worked again. It did. After that, it had
worked for years. Then, it didn't work for a few days, and after those
days, it worked fine again for years. It's having the same problem again
now.

When it fails, it sounds just when like you shut it off. Shutting it off
procuces a "plop". When it starts to work again it also sounds like it is
switched on, a similair "plop" is audible. Both speakers and headphone
outputs are affected, and always both the left and right channel, never
either left or right. When giving it a signal, a faint distorted output is
audible when the volume is at maximum.

Because of the symptoms, I am inclined to think that power to the pre-amp
keeps failing. But, I am far from an expert. AC powerinput to the pre-amp
is present all the time.

Is it reasonable to think that it is a mechanical problem (like bad
connections or cold-solderjoints) as suppose to for example dried out
electrolytic capacitors, seeing as how the problem started some 6 years
ago? I already re-heated almost every solderjoint of what seems to be the
pre-amp. It didn't work.

Also, on www.repairfaq.org I read that the signal can be traced by a wet
finger. How exactly do I go about that? When I've reached a point after the
failure-point, should that be audible in the output? Also, is it safe to
touch all parts except those of the power-supply (which has it own PCB),
without shock-hazzard?

TIA

Halfgaar
I used to see a lot of these sets come in with a loose metal coupler mounted in
the power/speaker switch. The metal piece would work its way loose, so when you
turned on the power, the speaker switch would not be enabled. Unplug the set,
pull gently on the metal coupler piece that extends slightly behind the
power/speaker switch. It is supposed to be permanently attached. If you can
easily remove the coupler, that could be part or all of your problem. Some
quick drying cement can be used to put it back in place.

Good luck.
David Farber
David Farber's Service Center
L.A., CA
 
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