I have a mid-80's HK integrated amp (PM-650). It's been very reliable and
sounds good, but there are a few things that have been bugging me:

1. The output of L & R channels are not equal... the left channel is louder
for any given source. It's in the pre-amp section, because I can make the
right speaker louder by flipping to "reverse stereo" so it's not the power
amp. I've aligned/calibrated the amp using the adjustments in the service
manual.

2. The balance knob is scratchy.

3. Every once in awhile one or both speakers cuts out. I have to toggle
the subsonic filter switch a few times to get it back on.

Anyway, last night I decided to give the amp a throrough cleaning, and I
replaced all of the electrolytic caps upstream of the reverse stereo switch.
I liberally sprayed all of the knobs (bass/treble/balance/tape monitor
select/volume) with MG contact cleaner and operated the knobs vigorously for
a minute or two. I then let the amp sit for an hour. The contact cleaner
evaporates almost instantaneously so I figured an hour was a reasonable time
to wait for the switches to dry out. When I powered up the amp everything
looked normal i.e. it lit up, but when I switched the speakers on (volume
all the way down), WHOA! CRACKLY STATIC, LOUD! None of the preamp controls
have any effect on the volume of static except my old nemesis, the subsonic
filter which creates a hum in the right channel when switched on. Switched
it back off, waited for four more hours. Double-checked all the solder
joints on the caps I had replaced for solder bridges, bad joints, correct
orientation on polar caps, etc.

When I fired it up this time, much much less static, but a) definitely some
residual low-level static on the right channel and b) left channel keeps
cutting out... need to crank up the volume and it "pops" back on. Sounds
good when it's on, and both channels very clear at high volume. Could be
worse I suppose.

My question is: how long does it take for this stuff to dry out? Could I
have just moved the dirt around inside the pots and made it worse? Should I
clean everything again?

I seem to have transformed a infrequent mild annoyance into a show-stopper.

Any replies greatly appreciated.

Dave

"Dave" wrote in message
news:n6KZh.19757$_G.10243@edtnps89...
I have a mid-80's HK integrated amp (PM-650). It's been very reliable and
sounds good, but there are a few things that have been bugging me:

1. The output of L & R channels are not equal... the left channel is louder
for any given source. It's in the pre-amp section, because I can make the
right speaker louder by flipping to "reverse stereo" so it's not the power
amp. I've aligned/calibrated the amp using the adjustments in the service
manual.

2. The balance knob is scratchy.

3. Every once in awhile one or both speakers cuts out. I have to toggle the
subsonic filter switch a few times to get it back on.

Anyway, last night I decided to give the amp a throrough cleaning, and I
replaced all of the electrolytic caps upstream of the reverse stereo switch. I
liberally sprayed all of the knobs (bass/treble/balance/tape monitor
select/volume) with MG contact cleaner and operated the knobs vigorously for a
minute or two. I then let the amp sit for an hour. The contact cleaner
evaporates almost instantaneously so I figured an hour was a reasonable time
to wait for the switches to dry out. When I powered up the amp everything
looked normal i.e. it lit up, but when I switched the speakers on (volume all
the way down), WHOA! CRACKLY STATIC, LOUD! None of the preamp controls have
any effect on the volume of static except my old nemesis, the subsonic filter
which creates a hum in the right channel when switched on. Switched it back
off, waited for four more hours. Double-checked all the solder joints on the
caps I had replaced for solder bridges, bad joints, correct orientation on
polar caps, etc.

When I fired it up this time, much much less static, but a) definitely some
residual low-level static on the right channel and b) left channel keeps
cutting out... need to crank up the volume and it "pops" back on. Sounds good
when it's on, and both channels very clear at high volume. Could be worse I
suppose.

My question is: how long does it take for this stuff to dry out? Could I
have just moved the dirt around inside the pots and made it worse? Should I
clean everything again?

I seem to have transformed a infrequent mild annoyance into a show-stopper.

Any replies greatly appreciated.

Dave


Since, by your own observations, the subsonic filter switch could be the
problem, or something in the immediate vicinity of the switch. What kind of
switch is it (slide switch, toggle switch, rotary switch)? If you can easily
disassemble it to get at the contacts, you might find that they are very
tarnished or even corroded; caused by airborne contaminants.
Also sounds like the volume pots are still dirty. Use a cleaner that lubricates
as well as cleans. A tip... when you spray the cleaner into the pot, turn the
set so that the residual cleaner can drain back out. That allows the cleaner to
float out any contaminants as it drains.
I have also found transistors to cause that exact problem. I surmise that the
wire bond from the external leads to the silicon inside breaks and becomes
extremely intermittent, and can be very difficult to find if you aren't wary of
this kind of fault. Lightly tap on each semiconductor in the area and see if
the audio is affected.
Actually, just about all types of components can become intermittent
(microphonic) in this manner, so using an insulated tool such as a plastic or
nylon tuning wand, tap components and circuit board to see if that affects the
audio.

Good luck...

--
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 to the end, the faster it goes.

"DaveM" wrote in message
. ..

Since, by your own observations, the subsonic filter switch could be the
problem, or something in the immediate vicinity of the switch. What kind
of switch is it (slide switch, toggle switch, rotary switch)? If you can
easily disassemble it to get at the contacts, you might find that they are
very tarnished or even corroded; caused by airborne contaminants.

It's a plastic push-button PCB-mounted switch, not easy to disassemble at
all. I'd worry about breaking the plastic parts which are, I'm sure, quite
irreplaceable. Not particularly accessible either as there is a display
controller board above the tone control board where the switch is located.

Also sounds like the volume pots are still dirty. Use a cleaner that
lubricates as well as cleans. A tip... when you spray the cleaner into
the pot, turn the set so that the residual cleaner can drain back out.
That allows the cleaner to float out any contaminants as it drains.

I was using zero residue cleaner, I'll pick up some lubricating contact
cleaner and give it another go. There appears to be a hole in the back of
each pot where you can see the shaft turn, plus a small (1-1.5mm) hole on
one side. Maybe a blast of compressed air after cleaning wouldn't hurt
either...

I have also found transistors to cause that exact problem. I surmise that
the wire bond from the external leads to the silicon inside breaks and
becomes extremely intermittent, and can be very difficult to find if you
aren't wary of this kind of fault. Lightly tap on each semiconductor in
the area and see if the audio is affected.
Actually, just about all types of components can become intermittent
(microphonic) in this manner, so using an insulated tool such as a plastic
or nylon tuning wand, tap components and circuit board to see if that
affects the audio.

This amp is totally discrete and there are no soldered wire connections (all
wound) , so I think (hope) the fact that BOTH channels are messed up
indicates a simple dirty pot problem... I did check voltages at maybe a
dozen transistors, both left and right channel from PS through the pre-amp
and amp and they were all really close to expected at idle. I don't know if
+/- 1V is significant on a 43VDC rail or not... anyway there are quite a
few transistors but nothing unmanageable and as there are no proprietary,
obscure, or out of production IC's in this dog, whatever I did is completely
fixable. I just hope that it doesn't take me 30 or 40 hours to figure it
out.

I may try just bridging the "open" pair of contacts on the subsonic filter
switch with a jumper wire and see if that helps.

Thanks for your help

Dave


Good luck...

--
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 to the end, the faster it
goes.

On 1 May, 18:49, "Dave" wrote:
"DaveM" wrote in message
. ..

Since, by your own observations, the subsonic filter switch could be the
problem, or something in the immediate vicinity of the switch. What kind
of switch is it (slide switch, toggle switch, rotary switch)? If you can
easily disassemble it to get at the contacts, you might find that they are
very tarnished or even corroded; caused by airborne contaminants.

It's a plastic push-button PCB-mounted switch, not easy to disassemble at
all. I'd worry about breaking the plastic parts which are, I'm sure, quite
irreplaceable. Not particularly accessible either as there is a display
controller board above the tone control board where the switch is located.

Also sounds like the volume pots are still dirty. Use a cleaner that
lubricates as well as cleans. A tip... when you spray the cleaner into
the pot, turn the set so that the residual cleaner can drain back out.
That allows the cleaner to float out any contaminants as it drains.

I was using zero residue cleaner, I'll pick up some lubricating contact
cleaner and give it another go. There appears to be a hole in the back of
each pot where you can see the shaft turn, plus a small (1-1.5mm) hole on
one side. Maybe a blast of compressed air after cleaning wouldn't hurt
either...

I have also found transistors to cause that exact problem. I surmise that
the wire bond from the external leads to the silicon inside breaks and
becomes extremely intermittent, and can be very difficult to find if you
aren't wary of this kind of fault. Lightly tap on each semiconductor in
the area and see if the audio is affected.
Actually, just about all types of components can become intermittent
(microphonic) in this manner, so using an insulated tool such as a plastic
or nylon tuning wand, tap components and circuit board to see if that
affects the audio.

This amp is totally discrete and there are no soldered wire connections (all
wound) , so I think (hope) the fact that BOTH channels are messed up
indicates a simple dirty pot problem... I did check voltages at maybe a
dozen transistors, both left and right channel from PS through the pre-amp
and amp and they were all really close to expected at idle. I don't know if
+/- 1V is significant on a 43VDC rail or not... anyway there are quite a
few transistors but nothing unmanageable and as there are no proprietary,
obscure, or out of production IC's in this dog, whatever I did is completely
fixable. I just hope that it doesn't take me 30 or 40 hours to figure it
out.

I may try just bridging the "open" pair of contacts on the subsonic filter
switch with a jumper wire and see if that helps.

Thanks for your help

Dave



Good luck...



The still bad switch probably needs disassembling & reassembling, or
else replacing the individual switch or switch block. Rows of
pushbutton things on PCBs were a standard design around for many years
in the 70s & 80s, so finding a replacement from some scrap item is
likely.

The pot is either still dirty, probably not, or else is worn out or
cracked, or possibly the rivet at the end of the track is loose.

Replacing the pot is most likely the solution, you've already cleaned
it. Is its an oddity, rivets can be squashed, and worn out tracks can
be painted, tho this would unbalance the volume L-R tracking,

Re the off balance, there are reasons amps have balance controls. If
yours has none you can fit a potential divider in the channel with the
higher output, and set it up and replace the covers. If youre lcky and
the amp has external connections between pre and power sections, you
can put what you want between those 2, such as a new volume & balance
control.


NT

wrote in message
ups.com...
On 1 May, 18:49, "Dave" wrote:

The still bad switch probably needs disassembling & reassembling, or
else replacing the individual switch or switch block. Rows of
pushbutton things on PCBs were a standard design around for many years
in the 70s & 80s, so finding a replacement from some scrap item is
likely.


I re-cleaned the volume pot with some lubricating cleaner and dried it out
with a hair dryer (on low heat setting). The crackling in the right channel
has now all but disappeared.

The left channel, however, will now work for about 10-30 seconds when the
amp is first powered up, then distorts rapidly over about 1-3 seconds and
cuts out completely after that.

The amp is set up as follows:

Input jacks -- input selector PCB -- volume knob/loudness PCB -- tone
control PCB -- power amp PCB.

I'm going to take out the input caps off the tone control and route the
signal directly from the volume knob to the power amp. That will at least
tell me if the problem is with the preamp or power amp. I'm pretty sure
it's going to be in the power amp, as the second-to-last component in the
pre-amp (tone control board) is the stereo/reverse stereo/mono switch. If
the problem was in the preamp prior to this point, changing this switch
should move the problem from one speaker to the other. Which it does not.
The only downstream component from the stereo/reverse stereo/mono switch is
the balance pot.

When I replaced the caps on the power amp, I cleaned the board with spray
cleaner. It was filthy, like 30 years of dust and grime. In retrospect, I
probably shouldn't have bothered, maybe the cold temperature of the spray
cracked off an internal lead in one of the transisotrs or something.

The last component in the amp is "speaker protector". This is a white
rectangular box about 1-1/4"L x 3/16"W x 1/2"H (approx, I'm not looking at
it right now.). I don't know how this works, but could it possibly be my
problem? Or should I be thinking transistors? I guess if I've got the
whole amp apart it wouldn't be such a big deal to measure bias currents on
both channels at idle... there are 14 transistors per channel including the
outputs.


Re the off balance, there are reasons amps have balance controls. If
yours has none you can fit a potential divider in the channel with the
higher output, and set it up and replace the covers. If youre lcky and
the amp has external connections between pre and power sections, you
can put what you want between those 2, such as a new volume & balance
control.

My amp is integrated, but fortunately is very simple to split as the
connection between pre-amp and amp is a three-conductor wire. I've been
meaning to do this for awhile, I've just purchased a sub with line inputs
and outputs... will have to cannibalize some piece of junk for a block of
RCA jacks.

All of my signal path connections are wire-wound, not soldered. Is there
some tool to do this if I wanted to stay true to design and wind the
connections on my pre-out/main-in connections? Is there any benefit to
doing it this way? I'm guessing there probably is as it looks like a fair
bit more work than just plonking a blob of solder on a ribbon cable lead.

Thanks for the help.

Dave



NT

Dave wrote:
wrote in message
ups.com...
On 1 May, 18:49, "Dave" wrote:

The still bad switch probably needs disassembling & reassembling, or
else replacing the individual switch or switch block. Rows of
pushbutton things on PCBs were a standard design around for many
years in the 70s & 80s, so finding a replacement from some scrap
item is likely.


I re-cleaned the volume pot with some lubricating cleaner and dried
it out with a hair dryer (on low heat setting). The crackling in the
right channel has now all but disappeared.

The left channel, however, will now work for about 10-30 seconds when
the amp is first powered up, then distorts rapidly over about 1-3
seconds and cuts out completely after that.

The amp is set up as follows:

Input jacks -- input selector PCB -- volume knob/loudness PCB --
tone control PCB -- power amp PCB.

I'm going to take out the input caps off the tone control and route
the signal directly from the volume knob to the power amp. That will
at least tell me if the problem is with the preamp or power amp. I'm
pretty sure it's going to be in the power amp, as the second-to-last
component in the pre-amp (tone control board) is the stereo/reverse
stereo/mono switch. If the problem was in the preamp prior to this
point, changing this switch should move the problem from one speaker
to the other. Which it does not. The only downstream component from
the stereo/reverse stereo/mono switch is the balance pot.

When I replaced the caps on the power amp, I cleaned the board with
spray cleaner. It was filthy, like 30 years of dust and grime. In
retrospect, I probably shouldn't have bothered, maybe the cold
temperature of the spray cracked off an internal lead in one of the
transisotrs or something.
The last component in the amp is "speaker protector". This is a white
rectangular box about 1-1/4"L x 3/16"W x 1/2"H (approx, I'm not
looking at it right now.). I don't know how this works, but could it
possibly be my problem? Or should I be thinking transistors? I
guess if I've got the whole amp apart it wouldn't be such a big deal
to measure bias currents on both channels at idle... there are 14
transistors per channel including the outputs.


Re the off balance, there are reasons amps have balance controls. If
yours has none you can fit a potential divider in the channel with
the higher output, and set it up and replace the covers. If youre
lcky and the amp has external connections between pre and power
sections, you can put what you want between those 2, such as a new
volume & balance control.

My amp is integrated, but fortunately is very simple to split as the
connection between pre-amp and amp is a three-conductor wire. I've
been meaning to do this for awhile, I've just purchased a sub with
line inputs and outputs... will have to cannibalize some piece of
junk for a block of RCA jacks.

All of my signal path connections are wire-wound, not soldered. Is
there some tool to do this if I wanted to stay true to design and
wind the connections on my pre-out/main-in connections? Is there any
benefit to doing it this way? I'm guessing there probably is as it
looks like a fair bit more work than just plonking a blob of solder
on a ribbon cable lead.
Thanks for the help.

Dave



NT

The posts I've seen in this thread do seem to suggest a problem with
coupling caps. Also, I've seen bad (open circuit or intermittent)
polystyrene caps on HK's in the past.

If you're not already using them, a sine/square generator and an
oscilloscope would be most useful here. In addition to rapidly narrowing
down to the affected stage, a square wave might show that you have a
frequency response problem in the affected stage. If so, the source of the
problem may well be more obvious.

Remember that with coupling caps, you can often get a clue with DC voltages.
There may be DC, or a higher DC than the other channel, on the output side
of a bad cap, or the following transistor stage may show a low collector
voltage.

If indeed the static was not controlled by the volume or balance, then your
problem is narrowed down a bit, assuming there is only the one problem
involved here

I have the service manual pdf if you need it.


Mark Z.

On Tue, 01 May 2007 16:42:59 GMT, "Dave"
wrote:

I seem to have transformed a infrequent mild annoyance into a show-stopper.

Had the exact same problem once in a mixing panel (unbalanced audio,
one channel fading away and coming back, incidental cracle and pop
sounds - cleaning did not help)

turned out to be a leaky electrolyte. Found it by measuring biasses on
every transistor in sight. Tapping the cap did not reveal anything
though.

--
- René

On May 1, 12:42 pm, "Dave" wrote:
I have a mid-80's HK integrated amp (PM-650). It's been very reliable

snip

Any replies greatly appreciated.

In no particular order:

a) cleaning pots and switches is only the first step. You need to
provide some (slight) amount of lubrication afterwards. 100% volatile
cleaners can cause serious friction after the skunge is removed
rebuilding additional skunge almost instantly. The idea is to exercise
the pot only as long as the cleaner is wet, then stop, then re-
saturate to rinse out the residue. *AFTER* that, a wee spritz of
lubricating cleaner makes all go smoothly.

b) switches are even 'more so' as wear can happen in any of several
ways including breaking of tabs or contact springs, and additional
friction can cause failure. So, same process. Saturate, exercise for a
few cycles whilst saturated, rinse, lubricate.

c) Look for broken traces on the boards, look (still) for bad caps. Do
you test the new caps *before* you install them? I have found about
0.5% of new electrolytic caps are bad or sufficiently out of spec to
be problematic. "Badness" typically manifests as intermittents... and
your delay may be related. Or, one you did not replace for whatever
reason...

d) you may believe that the power-amp section is clean, but one thing
I have experienced that is quite peculiar but as I have seen it
twice, it cannot be all that rare... an input transistor on the power-
amp side has become sufficiently different from its other-channel mate
that similar-volume signals to it are quite different in output
volume. RARE, and the only way you will catch this is to remove the
questionable transistors and put them either on a scope or tester that
can determine their actual response.

e) lastly and I have seen this only once and it was A BEAR to find. A
perfectly fine looking 1/2 watt resistor in the signal path had
drifted to 3X its nominal value. How did I find it? Well, every other #
$%^&*( part checked out, so I started measuring resitance "cold"
across both channels and comparing the results component by component.
When I found a difference, I started lifting legs about that point.
And there it was.

Good luck with it.

Peter Wieck
Wyncote, PA

wrote in message
ups.com...
On May 1, 12:42 pm, "Dave" wrote:

a) cleaning pots and switches is only the first step. You need to
provide some (slight) amount of lubrication afterwards. 100% volatile
cleaners can cause serious friction after the skunge is removed
rebuilding additional skunge almost instantly. The idea is to exercise
the pot only as long as the cleaner is wet, then stop, then re-
saturate to rinse out the residue. *AFTER* that, a wee spritz of
lubricating cleaner makes all go smoothly.


I re-cleaned with a cleaner with lubricant last night.

b) switches are even 'more so' as wear can happen in any of several
ways including breaking of tabs or contact springs, and additional
friction can cause failure. So, same process. Saturate, exercise for a
few cycles whilst saturated, rinse, lubricate.

Did not re-do switches. I will though.



c) Look for broken traces on the boards, look (still) for bad caps. Do
you test the new caps *before* you install them? I have found about
0.5% of new electrolytic caps are bad or sufficiently out of spec to
be problematic. "Badness" typically manifests as intermittents... and
your delay may be related. Or, one you did not replace for whatever
reason...


I don't have an ESR meter. Is testing plain ol' capacitance any help? I'd
like to think it's a capacitor given that that's all I changed out.

d) you may believe that the power-amp section is clean, but one thing
I have experienced that is quite peculiar but as I have seen it
twice, it cannot be all that rare... an input transistor on the power-
amp side has become sufficiently different from its other-channel mate
that similar-volume signals to it are quite different in output
volume. RARE, and the only way you will catch this is to remove the
questionable transistors and put them either on a scope or tester that
can determine their actual response.


This was an overnight change, so unless the change was mechanically induced
(definitely not ruling that out) I wouldn't be inclined to look there
justyet.

The more I think about it the less likely a pre-amp problem seems... the
signal goes through primary amplification (a few transisitors) then through
tone controls, subsonic filter, high cut filter, balance control -- power
amp. I've got a switch to bypass the tone control, and switches to bypass
the filters, plus a switch to reverse the channels between pre-amp and amp.
None of these affect the amp output. If I've got a bad cap in the amp
stages and am getting a DC offset at my outputs, that MAY be engaging the
speaker protection. Or, I suppose, worst case scenario I've baked one of my
giant expensive output transistors. I am assuming the speaker protection
(being two leaded) is a polyswitch. The distorted sound for a second or so
prior to speaker cut-out might match a polyswitch tripping, I don't know as
I've only ever had amps with relay protection (or none) in the past. Do you
know offhand if polyswitches tend to fail? I suppose probably as much as
any other component exposed to variable voltage and current.

I'm going to bypass the preamp section. If I can confirm that the problem
is in the amp, I could a) check DC voltages at all transistors until I find
one that's off or b) start swapping electrolytic capacitors from channel to
channel. there are only four per channel in the amp section.

e) lastly and I have seen this only once and it was A BEAR to find. A
perfectly fine looking 1/2 watt resistor in the signal path had
drifted to 3X its nominal value. How did I find it? Well, every other #
$%^&*( part checked out, so I started measuring resitance "cold"
across both channels and comparing the results component by component.
When I found a difference, I started lifting legs about that point.
And there it was.


Yes I've had several of those; they can try your patience.

Good luck with it.


Thanks, I may just need it.

Peter Wieck
Wyncote, PA

On May 2, 6:39 pm, "Dave" wrote:
wrote in message

ups.com...

On May 1, 12:42 pm, "Dave" wrote:

a) cleaning pots and switches is only the first step. You need to
provide some (slight) amount of lubrication afterwards. 100% volatile
cleaners can cause serious friction after the skunge is removed
rebuilding additional skunge almost instantly. The idea is to exercise
the pot only as long as the cleaner is wet, then stop, then re-
saturate to rinse out the residue. *AFTER* that, a wee spritz of
lubricating cleaner makes all go smoothly.

I re-cleaned with a cleaner with lubricant last night.

b) switches are even 'more so' as wear can happen in any of several
ways including breaking of tabs or contact springs, and additional
friction can cause failure. So, same process. Saturate, exercise for a
few cycles whilst saturated, rinse, lubricate.

Did not re-do switches. I will though.



c) Look for broken traces on the boards, look (still) for bad caps. Do
you test the new caps *before* you install them? I have found about
0.5% of new electrolytic caps are bad or sufficiently out of spec to
be problematic. "Badness" typically manifests as intermittents... and
your delay may be related. Or, one you did not replace for whatever
reason...

I don't have an ESR meter. Is testing plain ol' capacitance any help? I'd
like to think it's a capacitor given that that's all I changed out.

d) you may believe that the power-amp section is clean, but one thing
I have experienced that is quite peculiar but as I have seen it
twice, it cannot be all that rare... an input transistor on the power-
amp side has become sufficiently different from its other-channel mate
that similar-volume signals to it are quite different in output
volume. RARE, and the only way you will catch this is to remove the
questionable transistors and put them either on a scope or tester that
can determine their actual response.

This was an overnight change, so unless the change was mechanically induced
(definitely not ruling that out) I wouldn't be inclined to look there
justyet.

The more I think about it the less likely a pre-amp problem seems... the
signal goes through primary amplification (a few transisitors) then through
tone controls, subsonic filter, high cut filter, balance control -- power
amp. I've got a switch to bypass the tone control, and switches to bypass
the filters, plus a switch to reverse the channels between pre-amp and amp.
None of these affect the amp output. If I've got a bad cap in the amp
stages and am getting a DC offset at my outputs, that MAY be engaging the
speaker protection. Or, I suppose, worst case scenario I've baked one of my
giant expensive output transistors. I am assuming the speaker protection
(being two leaded) is a polyswitch. The distorted sound for a second or so
prior to speaker cut-out might match a polyswitch tripping, I don't know as
I've only ever had amps with relay protection (or none) in the past. Do you
know offhand if polyswitches tend to fail? I suppose probably as much as
any other component exposed to variable voltage and current.

I'm going to bypass the preamp section. If I can confirm that the problem
is in the amp, I could a) check DC voltages at all transistors until I find
one that's off or b) start swapping electrolytic capacitors from channel to
channel. there are only four per channel in the amp section.

e) lastly and I have seen this only once and it was A BEAR to find. A
perfectly fine looking 1/2 watt resistor in the signal path had
drifted to 3X its nominal value. How did I find it? Well, every other #
$%^&*( part checked out, so I started measuring resitance "cold"
across both channels and comparing the results component by component.
When I found a difference, I started lifting legs about that point.
And there it was.

Yes I've had several of those; they can try your patience.

Good luck with it.

Thanks, I may just need it.

Peter Wieck
Wyncote, PA

There used to be a spray on the market - can't remember who made it at
the time, might have been GC - it was called "Blue Stuff". I worked
in a stereo service department thru college, and that spray was the
best we could find for cleaning stubbornly noisy pots and switches.
When you sprayed it on it formed a mildly abrasive blue paste, which
would actually scrub the tarnish and dirt off of switch contacts. I
never saw it fail, even with the noisiest, most tarnished switches and
controls. I think it has diatomaceous earth in it.

Anyway, I think TechSpray carries it - might be worth a try
(www.techspray.com)

On May 3, 11:12 pm, "Mr. Land" wrote:
On May 2, 6:39 pm, "Dave" wrote:



wrote in message

oups.com...

On May 1, 12:42 pm, "Dave" wrote:

a)cleaningpots and switches is only the first step. You need to
provide some (slight) amount of lubrication afterwards. 100% volatile
cleaners can cause serious friction after the skunge is removed
rebuilding additional skunge almost instantly. The idea is to exercise
the pot only as long as the cleaner is wet, then stop, thenre-
saturate to rinse out the residue. *AFTER* that, a wee spritz of
lubricating cleaner makes all go smoothly.

Ire-cleaned with a cleaner with lubricant last night.

b) switches are even 'more so' as wear can happen in any of several
ways including breaking of tabs or contact springs, and additional
friction can cause failure. So, same process. Saturate, exercise for a
few cycles whilst saturated, rinse, lubricate.

Did notre-do switches. I will though.

c) Look for broken traces on the boards, look (still) for bad caps. Do
you test the new caps *before* you install them? I have found about
0.5% of new electrolytic caps are bad or sufficiently out of spec to
be problematic. "Badness" typically manifests as intermittents... and
your delay may be related. Or, one you did not replace for whatever
reason...

I don't have an ESR meter. Is testing plain ol' capacitance any help? I'd
like to think it's a capacitor given that that's all I changed out.

d) you may believe that the power-ampsection is clean, but one thing
I have experienced that is quite peculiar but as I have seen it
twice, it cannot be all that rare... an input transistor on the power-
ampside has become sufficiently different from its other-channel mate
that similar-volume signals to it are quite different in output
volume. RARE, and the only way you will catch this is to remove the
questionable transistors and put them either on a scope or tester that
can determine their actual response.

This was an overnight change, so unless the change was mechanically induced
(definitely not ruling that out) I wouldn't be inclined to look there
justyet.

The more I think about it the less likely a pre-ampproblem seems... the
signal goes through primary amplification (a few transisitors) then through
tone controls, subsonic filter, high cut filter, balance control -- power
amp. I've got a switch to bypass the tone control, and switches to bypass
the filters, plus a switch to reverse the channels between pre-ampandamp.
None of these affect theampoutput. If I've got a bad cap in theamp
stages and am getting a DC offset at my outputs, that MAY be engaging the
speaker protection. Or, I suppose, worst case scenario I've baked one of my
giant expensive output transistors. I am assuming the speaker protection
(being two leaded) is a polyswitch. The distorted sound for a second or so
prior to speaker cut-out might match a polyswitch tripping, I don't know as
I've only ever had amps with relay protection (or none) in the past. Do you
know offhand if polyswitches tend to fail? I suppose probably as much as
any other component exposed to variable voltage and current.

I'm going to bypass the preamp section. If I can confirm that the problem
is in theamp, I could a) check DC voltages at all transistors until I find
one that's off or b) start swapping electrolytic capacitors from channel to
channel. there are only four per channel in theampsection.

e) lastly and I have seen this only once and it was A BEAR to find. A
perfectly fine looking 1/2 watt resistor in the signal path had
drifted to 3X its nominal value. How did I find it? Well, every other #
$%^&*( part checked out, so I started measuring resitance "cold"
across both channels and comparing the results component by component.
When I found a difference, I started lifting legs about that point.
And there it was.

Yes I've had several of those; they can try your patience.

Good luck with it.

Thanks, I may just need it.

Peter Wieck
Wyncote, PA

There used to be a spray on the market - can't remember who made it at
the time, might have been GC - it was called "Blue Stuff". I worked
in astereoservice department thru college, and that spray was the
best we could find forcleaningstubbornly noisy pots and switches.
When you sprayed it on it formed a mildly abrasive blue paste, which
would actually scrub the tarnish and dirt off of switch contacts. I
never saw it fail, even with the noisiest, most tarnished switches and
controls. I think it has diatomaceous earth in it.

Anyway, I think TechSpray carries it - might be worth a try
(www.techspray.com)

I just investigated this stuff, in fact I think I ordered some and
have not yet received it. Its been discontinued I think. Yes, I
ordered it two months ago, and it said they had the quanity i ordered.
Glad you brought it up. A recording studio asked me about this stuff,
and they said they depended on it. I know of no other cleaner other
than Cramolin Contaclean, that actually has something in it to
dissolve oxides.

greg