[obcbeatle]

Well...after I sprayed (Deoxit) on all the switch and pot. contacts the
402 worked great for a couple days. But today I turned it on and there
was a fairly loud hiss coming from the left channel and the left
channel is weak (low volume) again. I can gently bump the 402 and the
left channel volume increases, but it is still not equal to the right
channel in volume (but close). I can spray the Tone Defeat and Low
Filter switches again (these seemed to be the offending swithes at the
time), as well as ALL the switch/pots, but I have already done that a
couple times. I don't want to over wash. I suspect I have a failing
component, but I don't feel like it's the Tone Defeat and/or Low Filter
switch anymore since there is hiss in the left channel when I turn the
402 on after it has been off for awhile. I can turn the 402 off and
then immediately back on with no hiss. Does anyone have any ideas? Or
does anyone have a schematic for this amp and a suggesttion where I
might probe first to find an offending component? Also, I have a
multi-meter to test components. I just don't know where to start. Or
if anyone has any other suggestions for me, I'd appreciate a reply. I
work on computers for a living and have repaired my guitar pedals and
tube amps in the past, so I'm familiar with safe practices when dealing
with electricity... Otherwise, I guess I'll try to get this 402
repaired by someone else, or buy a new amp Thanks.

[Peter Larsen]

obcbeatle wrote:

Well...after I sprayed (Deoxit) on all the switch and pot. contacts the
402 worked great for a couple days. But today I turned it on and there
was a fairly loud hiss coming from the left channel and the left
channel is weak (low volume) again. I can gently bump the 402 and the
left channel volume increases, but it is still not equal to the right
channel in volume (but close). I can spray the Tone Defeat and Low
Filter switches again (these seemed to be the offending swithes at the
time), as well as ALL the switch/pots, but I have already done that a
couple times. I don't want to over wash. I suspect I have a failing
component, but I don't feel like it's the Tone Defeat and/or Low Filter
switch anymore since there is hiss in the left channel when I turn the
402 on after it has been off for awhile. I can turn the 402 off and
then immediately back on with no hiss. Does anyone have any ideas?

Oscillation, it may be that some feedback path go through the front
panel switches, Sony loved to build tape recorders like that ....

Or
does anyone have a schematic for this amp and a suggesttion where I
might probe first to find an offending component? Also, I have a
multi-meter to test components. I just don't know where to start.

With a scope.

Or
if anyone has any other suggestions for me, I'd appreciate a reply.

You're in above your head, you need someone on the spot to help you or
to find a trustworthy repairshop nearby.


Peter Larsen



I
work on computers for a living and have repaired my guitar pedals and
tube amps in the past, so I'm familiar with safe practices when dealing
with electricity... Otherwise, I guess I'll try to get this 402
repaired by someone else, or buy a new amp Thanks.

[jakdedert]

Peter Larsen wrote:
obcbeatle wrote:

Well...after I sprayed (Deoxit) on all the switch and pot. contacts the
402 worked great for a couple days. But today I turned it on and there
was a fairly loud hiss coming from the left channel and the left
channel is weak (low volume) again. I can gently bump the 402 and the
left channel volume increases, but it is still not equal to the right
channel in volume (but close). I can spray the Tone Defeat and Low
Filter switches again (these seemed to be the offending swithes at the
time), as well as ALL the switch/pots, but I have already done that a
couple times. I don't want to over wash. I suspect I have a failing
component, but I don't feel like it's the Tone Defeat and/or Low Filter
switch anymore since there is hiss in the left channel when I turn the
402 on after it has been off for awhile. I can turn the 402 off and
then immediately back on with no hiss. Does anyone have any ideas?

Oscillation, it may be that some feedback path go through the front
panel switches, Sony loved to build tape recorders like that ....

Or
does anyone have a schematic for this amp and a suggesttion where I
might probe first to find an offending component? Also, I have a
multi-meter to test components. I just don't know where to start.

With a scope.

Or
if anyone has any other suggestions for me, I'd appreciate a reply.

You're in above your head, you need someone on the spot to help you or
to find a trustworthy repairshop nearby.


I tend to agree with Peter; although it's 'just' possible that every
time you open up the unit you disturb a loose connection, which operates
correctly for a day or two before becoming intermittent again.

Next time you have the top off, try gently tapping various components
with the unit powered up and a source selected. You might find a bad
solder joint.

Beware of high voltages, and always use something nonconductive for your
probe.

jak


Peter Larsen



I
work on computers for a living and have repaired my guitar pedals and
tube amps in the past, so I'm familiar with safe practices when dealing
with electricity... Otherwise, I guess I'll try to get this 402
repaired by someone else, or buy a new amp Thanks.

[obcbeatle]

Peter Larsen wrote:

You're in above your head, you need someone on the spot to help you or
to find a trustworthy repairshop nearby.

Ok. I've been searching the past couple days for an HK service center
near me, but I can't find anything. I can't even find an electronics
repair shop near me in Vermont. At least not one that appears to
repair vintage equipment. Can anyone recommend an electronics service
center in Vermont or western NY? I live about 2 hours south of
Burlington, VT and about 2 hours north/northeast of Albany, NY. Just
thought I'd ask. Thanks.

[obcbeatle]

jakdedert wrote:

I tend to agree with Peter; although it's 'just' possible that every
time you open up the unit you disturb a loose connection, which operates
correctly for a day or two before becoming intermittent again.

Next time you have the top off, try gently tapping various components
with the unit powered up and a source selected. You might find a bad
solder joint.

Using my wood probe I can push down on the PCB (mounted inside 402,
front/top right corner where the Tone Defeat switch and Bass and Treble
pots. are solderd) and the left channel volume goes up and down. So I
think I identified the general area that has a loose connection or bad
solder joint. Unfortunately, I can't find a specific wire/joint on the
PCB that effects the volume when using my probe. The switches/pots.
are mounted on the underside of that PCB, so it's hard to see or get my
probe to every wire/joint. I wish I could figure out how to unmount
that PCB so I can get a better look, because I can solder. All that
said...I'm still uncertain what causes the hiss in the left channel
when the 402 has been off for awhile and I first turn it on. Would a
loose connection or bad solder joint cause the hiss? Seems heat
related too (hiss goes away after warming up). Thanks.

[Peter Larsen]

obcbeatle wrote:

I'm still uncertain what causes the hiss in the left channel

Oscillation comes to mind when it goes away after a turn-off turn-on. I
may be wrong, but it is because of this possiblity I suggest the use of
a scope. I don't know enough to suggest specific causes other than
"something" in a feedback path is involved.


Peter Larsen

[jakdedert]

Peter Larsen wrote:
obcbeatle wrote:

I'm still uncertain what causes the hiss in the left channel

Oscillation comes to mind when it goes away after a turn-off turn-on. I
may be wrong, but it is because of this possiblity I suggest the use of
a scope. I don't know enough to suggest specific causes other than
"something" in a feedback path is involved.


Peter Larsen

If in fact, there's a loose joint, 'shotgunning' the entire area
(reflowing each solder joint, paying most careful attention to the ones
which look questionable) is an option. It really doesn't matter where
in the schematic that the fault occurs. If it's a bad joint, the above
will fix it.

OTOH, if the OP wants to home in on the exact spot, it can be a
nightmare. I've examined until I was blue in the face without finding
the *exact* spot on some boards. Use of a good magnifier (or two...I
use a magnifying light with another glass held between it and the board
at times) is essential. Sometimes a strong light shown up through, from
below the board, helps.

Most helpful is to narrow down the area by applying less and less
pressure to the components until you come up with just a few candidates.
Solder those and try the amp again. If you don't hit it the first
time, go at it again.

Finally, the pressure you are applying 'might' be enough to make a
faulty switch work (possibility that it's not a solder joint, but
internal to a compnent). You would do well to trace down to a single
component....

jak

[obcbeatle]

jakdedert wrote:

If in fact, there's a loose joint, 'shotgunning' the entire area
(reflowing each solder joint, paying most careful attention to the ones
which look questionable) is an option. It really doesn't matter where
in the schematic that the fault occurs. If it's a bad joint, the above
will fix it.

OTOH, if the OP wants to home in on the exact spot, it can be a
nightmare. I've examined until I was blue in the face without finding
the *exact* spot on some boards. Use of a good magnifier (or two...I
use a magnifying light with another glass held between it and the board
at times) is essential. Sometimes a strong light shown up through, from
below the board, helps.

Most helpful is to narrow down the area by applying less and less
pressure to the components until you come up with just a few candidates.
Solder those and try the amp again. If you don't hit it the first
time, go at it again.

Finally, the pressure you are applying 'might' be enough to make a
faulty switch work (possibility that it's not a solder joint, but
internal to a compnent). You would do well to trace down to a single
component....

Well...right now I'm feeling like most of this is beyond what I can do,
particularly w/o the proper tools/knowledge. I sure do appreciate all
the suggestions though. I turned the amp on this evening after being
off since last night and it turned on with no hiss in the left channel
and equal volume on both channels. Go figure Bottomline...I think
this unit needs repair by someone quealified to do it, but I know of no
one in my area. So I guess I'm going to try to but another one. How
much do you think is a reasonable price to pay for a used/refurbished
HK A-402? I haven't seen any on eBay recently. $100-200? Thanks
again for all the assistance.

[Mark D. Zacharias]

Peter Larsen wrote:
obcbeatle wrote:

I'm still uncertain what causes the hiss in the left channel

Oscillation comes to mind when it goes away after a turn-off turn-on.
I may be wrong, but it is because of this possiblity I suggest the
use of a scope. I don't know enough to suggest specific causes other
than "something" in a feedback path is involved.


Peter Larsen

Great idea about the oscillation. I e-mailed the OP directly and sent him a
manual for the 402.

Mark Z.

[obcbeatle]

Mark D. Zacharias wrote:
Great idea about the oscillation. I e-mailed the OP directly and sent him a
manual for the 402.

Many thanks! I will post my results!

[Mark D. Zacharias]

obcbeatle wrote:
jakdedert wrote:

If in fact, there's a loose joint, 'shotgunning' the entire area
(reflowing each solder joint, paying most careful attention to the
ones which look questionable) is an option. It really doesn't
matter where in the schematic that the fault occurs. If it's a bad
joint, the above will fix it.

OTOH, if the OP wants to home in on the exact spot, it can be a
nightmare. I've examined until I was blue in the face without
finding the *exact* spot on some boards. Use of a good magnifier
(or two...I use a magnifying light with another glass held between
it and the board at times) is essential. Sometimes a strong light
shown up through, from below the board, helps.

Most helpful is to narrow down the area by applying less and less
pressure to the components until you come up with just a few
candidates. Solder those and try the amp again. If you don't hit
it the first time, go at it again.

Finally, the pressure you are applying 'might' be enough to make a
faulty switch work (possibility that it's not a solder joint, but
internal to a compnent). You would do well to trace down to a single
component....

Well...right now I'm feeling like most of this is beyond what I can
do, particularly w/o the proper tools/knowledge. I sure do
appreciate all the suggestions though. I turned the amp on this
evening after being off since last night and it turned on with no
hiss in the left channel and equal volume on both channels. Go
figure Bottomline...I think this unit needs repair by someone
quealified to do it, but I know of no one in my area. So I guess I'm
going to try to but another one. How much do you think is a
reasonable price to pay for a used/refurbished HK A-402? I haven't
seen any on eBay recently. $100-200? Thanks again for all the
assistance.

I don't think I'd pay over 100.00 . Even that might be a bit much.

There are lots of cool vintage integrated amps you could pick up for that
money. Kenwood, Pioneer or Onkyo come to mind - I'm not really a big HK fan,
though their older stuff is better IMHO.

Mark Z.