[Kele]

I fear my Adcom GTP-600 surround preamp ( http://oi38.tinypic.com/sfuxap.jp=
g
) has a problem (mine is cleaner than the one pictured). It=92s about
15 years old, I like it allot and can=92t afford to replace it with same
level of performance.

Here's what happened and what I know... there was a single semi-loud
pop sound; I was not in the room at the time but heard it. Upon
arriving, noticed the overall volume level had diminished. After
fiddling, I found that just the two surround modes: Adcom's Cinema and
Dolby Surround are bad. The other modes: Stadium, Concert Hall, Club,
five channel, and 2ch stereo seem ok.

When switched to either of the Dolby modes, as I turn up the Master
volume (to about 9:00 position) the main speaker's woofers extended
outward about an inch and stays there=85 and that is with no source
playing. I can move the volume higher and the woofer extends
further. When extended, the woofers move in and out slightly and
slowly. It=92s ugly.

For now, everything other than Dolby modes sound good and appear
normal. I don=92t go to Dolby modes at all and continue to use preamp
daily=85 it=92s rarely turned off.

Wondering if the Dolby section is like a chip that can be swapped out
=96 If I could get the part, a DIY job? I=92ve only found scary
electronic repair options locally, but could get some soldering done
if necessary. Adcom says contact Auth Service Center in Jersey.
Jersey=92s response to my email was simply, =93call us=94. I=92m not in th=
e
continental US, Jersey is far. I haven=92t cracked the un-vented case
ever; have no idea of internal. Before I do, I=92m trying to get
advice.

Bummed,

Kele

[Audio Empire]

On Wed, 1 Dec 2010 10:53:06 -0800, Kele wrote
(in article ):

I fear my Adcom GTP-600 surround preamp ( http://oi38.tinypic.com/sfuxap.jp=
g
has a problem (mine is cleaner than the one pictured). It=92s about
15 years old, I like it allot and can=92t afford to replace it with same
level of performance.

Here's what happened and what I know... there was a single semi-loud
pop sound; I was not in the room at the time but heard it. Upon
arriving, noticed the overall volume level had diminished. After
fiddling, I found that just the two surround modes: Adcom's Cinema and
Dolby Surround are bad. The other modes: Stadium, Concert Hall, Club,
five channel, and 2ch stereo seem ok.

When switched to either of the Dolby modes, as I turn up the Master
volume (to about 9:00 position) the main speaker's woofers extended
outward about an inch and stays there=85 and that is with no source
playing. I can move the volume higher and the woofer extends
further. When extended, the woofers move in and out slightly and
slowly. It=92s ugly.

For now, everything other than Dolby modes sound good and appear
normal. I don=92t go to Dolby modes at all and continue to use preamp
daily=85 it=92s rarely turned off.

Wondering if the Dolby section is like a chip that can be swapped out
=96 If I could get the part, a DIY job? I=92ve only found scary
electronic repair options locally, but could get some soldering done
if necessary. Adcom says contact Auth Service Center in Jersey.
Jersey=92s response to my email was simply, =93call us=94. I=92m not in th=
e
continental US, Jersey is far. I haven=92t cracked the un-vented case
ever; have no idea of internal. Before I do, I=92m trying to get
advice.

Bummed,

Kele


One thing seems likely. You defiantly have a power supply problem. The
extended woofer cone indicates DC in the audio. The preamp is so busy
amplifying a slight DC offset that it's using up all the power supply to do
so . Obviously, your preamp and power amp are DC coupled. AS to the surround
functions not working, I suspect that whatever has put DC on your audio path
has also stopped your surround decoder from working properly.

Don't use it this way, Amplifiers aren't designed to amplify DC as it causes
1/2 of the output circuit to be driven "on" all the time. I'll bet if you
feel the amps heat sinks for the main channels, you'll find that they're very
hot (careful, now don't burn yourself). If you keep using the amp like that
you could destroy the output transistors. They aren't made to have their
duty-cycle exceeded in that manner.

[Kele]

I felt the Amp; I leave it on all the time normally. The pre-amp was
also on; nothing playing volume full off. Amp kinda warm (ADCOM 8200
250 watt x2). Turned off preamp; will check amp sink temp in a couple
hours to see if different with preamp off. Probably should disconnect
the RC jacks too? Ok, they're unplugged at the amp. The two pieces
are on the same AC circuit but different outlets. Ok, the preamp is
unpluged from the AC; there is now nothing physically connecting the
preamp and amp.

You are saying the Dolby circuit not functioning is a symptom of the
power supply having a problem. Check this out one more time... The
speaker extension (moving and holding in the outward position) ONLY
occurs when the Dolby circuit is selected. When Dolby is not
selected, the speakers don't extend. When Dolby on, the sound that
does come through the speakers isn't good, but when Dolby is off, the
sound is normal with all other DSP options, five channel, and two
channel. I was thinking that it's only the Dolby circuit because
everything seems fine with Dolby off.

Oh, maybe you're saying it's not fine, the Dolby circuit is just more
easily suseptable to stray DC and that's why it appears to be a
problem when Dolby is on. I was hoping it is only the Dolby chip
because everything seems ok with Dolby not selected... as if the audio
path does not go through Dolby when not selected, like the way some
EQs are out of the signal path when off.

Please consider this one more time... I really hope I can still play
stereo (with Dolby off). You know, without the preamp (the hub), I'm
SOL. Do you still recommend leaving it off? IF this isn't a DIY fix,
do you think a 15 year old preamp (this caliber) is worth the $400 to
make a trip to Jersey and back?

Thanks though, but now I'm worried. - Kele

[Audio Empire]

On Wed, 1 Dec 2010 17:30:16 -0800, Kele wrote
(in article ):

I felt the Amp; I leave it on all the time normally. The pre-amp was
also on; nothing playing volume full off. Amp kinda warm (ADCOM 8200
250 watt x2). Turned off preamp; will check amp sink temp in a couple
hours to see if different with preamp off. Probably should disconnect
the RC jacks too? Ok, they're unplugged at the amp. The two pieces
are on the same AC circuit but different outlets. Ok, the preamp is
unpluged from the AC; there is now nothing physically connecting the
preamp and amp.

You are saying the Dolby circuit not functioning is a symptom of the
power supply having a problem.

NO, I am saying that the DC offset that has the speaker cone moving out in
response to the volume control setting is the symptom of a problem with the
power supply. Somehow, you've got DC in your audio, which you should not
have. Based on that, I'm postulating that the power supply problem is LIKELY
the problem with your Dolby decode circuit as well.


Check this out one more time... The
speaker extension (moving and holding in the outward position) ONLY
occurs when the Dolby circuit is selected. When Dolby is not
selected, the speakers don't extend. When Dolby on, the sound that
does come through the speakers isn't good, but when Dolby is off, the
sound is normal with all other DSP options, five channel, and two
channel. I was thinking that it's only the Dolby circuit because
everything seems fine with Dolby off.

Then the DC is getting into the audio via the Dolby circuit. Perhaps an
op-amp has failed, of perhaps even the Dolby decoder chip itself. At any
rate, you need to have the pre-amp looked at by a pro, unless you know how to
use a voltmeter or an oscilloscope to do troubleshooting yourself. At any
rate, it's doubtful that even if you can find the problem, given today's
surface-mount components, that you could fix it yourself.

[Kele]

PS: Audio Empire, I re-read your response and don't fully understand
DC offset. Does that mean there is some stray AC on my signal path.
It seems you are saying that my preamp's power supply is overtaxed
because of stray voltage. It's power supply isn't necessarily the
culprit then. Maybe the preamp isn't actually faulty? And I don't
understand DC coupled unless that means the stray AC is flowing to the
amp or maybe from the amp... hmmm. May be I should try swapping out
the APC (not PS-Audio) power strip(s)? I'll put in my Irrational But
Efficious CD to remove the stray electrons from my system (ha-ha).
But really, is this an AC issue? I just checked the AC outlets and
they appear "ok" using a polarity & ground tester (the plug-in type
with the lights). I checked some outlets on the power strips too.
What's left... RC patch cables? Anything can do, I'll try.

It's been over an hour with preamp and amp isolated from one another;
the amp wasn't any cooler. Something elso too... The amps distortion
& thermal protection lights don't illum even with the Dolby on &
speaker cone extension previously described.

[Kele]

Thank you Audio Empire. Your were so quick in responding, part 2 of
mine came after your last comment. I will head your warning and
advice. I'll try to find a Pro service person(s) to look at the pre-
amp; know of any?

[Audio Empire]

On Thu, 2 Dec 2010 11:18:00 -0800, Kele wrote
(in article ):

Thank you Audio Empire. Your were so quick in responding, part 2 of
mine came after your last comment. I will head your warning and
advice. I'll try to find a Pro service person(s) to look at the pre-
amp; know of any?


Unfortunately, not really. One of the downsides of today's electronics
revolution is that more often than not, products are so specialized that only
the maker can fix them when they break. The days of the repair shop down the
street who can fix your TV or your amplifier are mostly gone.

You are going to have to find an authorized factory service center for your
product. Did you say it was an Adcom? If so, I'd start he

http://www.yamasinc.com/adcom_service.htm

Don't know these folks, have no financial interest, etc. Just a suggestion of
a starting place.

[Audio Empire]

On Thu, 2 Dec 2010 03:35:12 -0800, Kele wrote
(in article ):

PS: Audio Empire, I re-read your response and don't fully understand
DC offset. Does that mean there is some stray AC on my signal path.
It seems you are saying that my preamp's power supply is overtaxed
because of stray voltage. It's power supply isn't necessarily the
culprit then. Maybe the preamp isn't actually faulty? And I don't
understand DC coupled unless that means the stray AC is flowing to the
amp or maybe from the amp... hmmm. May be I should try swapping out
the APC (not PS-Audio) power strip(s)? I'll put in my Irrational But
Efficious CD to remove the stray electrons from my system (ha-ha).
But really, is this an AC issue? I just checked the AC outlets and
they appear "ok" using a polarity & ground tester (the plug-in type
with the lights). I checked some outlets on the power strips too.
What's left... RC patch cables? Anything can do, I'll try.

It's been over an hour with preamp and amp isolated from one another;
the amp wasn't any cooler. Something elso too... The amps distortion
& thermal protection lights don't illum even with the Dolby on &
speaker cone extension previously described.


I'm saying that there is a DC voltage in the signal path. This shouldn't
happen. There are two ways to get an AC signal (such as audio) though an
amplifying device. One is to AC couple the signal (where there are capacitors
in the signal path which will allow the AC (the audio) through, but none of
the DC which is necessary to operate the active circuitry). The second way
is to DC couple the signal through. In DC coupling there are no capacitors in
the signal path (desirable where practical) and the DC is kept out of the
signal by designing the circuit so that the plus and minus rails on the DC
power supply are the same (example: If you put +15 volts on one side of a
resistor and -15 volts on the other, and tap the center of that resistor, the
voltage at that tap will be zero because (-15) + (+15) = 0. The same is true
if you put equal + and - voltages across a transistor in an amp. When the
transistor is conducting (I.E. "on"), all else being equal, the DC voltage at
the output of that transistor should be zero and the only thing there will be
the AC signal (the audio). Now it's a bit more complex than that, but this
will do for the purpose of this discussion. Now. Let's say, that something
has gone wrong with the power supply, and instead of there being + and - 15
volts across that transistor, there is now +20 and -15 volts. That means that
there will be 5 volts of DC along with the AC (audio) signal at the output of
that transistor ((-15) + (+20)= +5). That +5 volts is called the "DC
offset".

If your power amp has a sensitivity of 2 volts AC to give full output and if
the power amp is DC coupled, then that offset will be enough to drive that
power amp into clipping (exceeding it's possible output) because it is trying
to amplify that DC voltage (which shouldn't be there ). That means that
one-half of your amplifier's push-pull output transistors are turned hard-on
continuously and are conducting at their full capacity to amplify that +5
volts (the amp doesn't know the difference between audio and a DC voltage
that shouldn't be there). The problem here is that amplifier output
transistors weren't designed to be full-on all the time. They are supposed to
switch to the negative going output transistors every half cycle. To keep
them on all the time, is "exceeding their duty cycle" IOW, operating them
outside of their worse-case design parameters. Some amp designs can handle
this kind of abuse with no damage and others will simply fail if the fault is
ignored long enough.

In your case, your speaker cone moves out proportionally to your volume
control setting. That indicates that the DC is getting into your audio signal
BEFORE the volume control and that the volume control is altering the
amplitude of that offset DC voltage. You're lucky here because that means the
entire DC offset is not being applied to your power amp (keeping it from
failing) and also keeping it from pumping so much DC current into your
speaker system that it destroys the speaker(s) as well. That kind of DC
signal exceeds the speaker's design duty cycle as well. You could burn-up the
voice coil by pumping DC through the speaker.

[Kele]

Thank you again, Audio Empire... The Adcom Service Center you
provided a link to isn't even listed on Adcom's own site. Yama's
looks like the real deal and half as far from where I live as the
place in Jersey that Adcom would have me contact. BTW, I=92ve been
sending queries to all seemingly high-end stereo dealers here with no
recommendation for a repair person locally. I hear you that it=92s
become specialized [and I'll add: and/or they don=92t have a good after
sale program and/or there actually aren=92t any here]. I know you
laughed when I said =93high-end=94 referring to fixing my Adcom =96 that=92=
s
ok.

I read and will do so again your description of DC voltage in the
signal path. I'm still processing. You've been very generous with
your replies, much appreciated. I'll be so bold and ask you one more
thing because of my current situation of not being able to proceed
with sending the defective pre-amp anywhere at this time. Since
reading you, I turned everything off, but would like to continue using
the stereo for now with the Dolby off (bypassed). Here's my
question... If there is no source playing and I turn the Master
volume knob way up and the speaker base cones don't move or emit sound
at all, can I continue to use the stereo until I save more pennies?
This is how it appears with Dolby off - no abnormal speaker movement.

I can fill in with everything you've already written to me about this,
so I'll understand a Yes or No response.

No =3D DC is still transferring and frying my circuits; you're gambling
with the life of other components. Yes =3D There is a power supply
problem, but it only manifests itself when using the Dolby chip
(basically the Dolby chip or it=92s power circuit is creating the DC off-
set); it=92s ok to use cautiously with Dolby off until qualified repairs
can be performed.

When I do get it fixed, I=92ll post here my experience.






On Dec 2, 12:21=A0pm, Audio Empire wrote:
On Thu, 2 Dec 2010 11:18:00 -0800, Kele wrote
(in article ):

Thank you Audio Empire. =A0Your were so quick in responding, part 2 of
mine came after your last comment. =A0I will head your warning and
advice. =A0I'll try to find a Pro service person(s) to look at the pre-
amp; know of any?

Unfortunately, not really. One of the downsides of today's electronics
revolution is that more often than not, products are so specialized that =
only
the maker can fix them when they break. The days of the repair shop down =
the
street who can fix your TV or your amplifier are mostly gone.

You are going to have to find an authorized factory service center for yo=
ur
product. Did you say it was an Adcom? If so, I'd start he

http://www.yamasinc.com/adcom_service.htm

Don't know these folks, have no financial interest, etc. Just a suggestio=
n of
a starting place. =A0

[Audio Empire]

On Fri, 3 Dec 2010 03:54:26 -0800, Kele wrote
(in article ):

Thank you again, Audio Empire... The Adcom Service Center you
provided a link to isn't even listed on Adcom's own site. Yama's
looks like the real deal and half as far from where I live as the
place in Jersey that Adcom would have me contact. BTW, I=92ve been
sending queries to all seemingly high-end stereo dealers here with no
recommendation for a repair person locally. I hear you that it=92s
become specialized [and I'll add: and/or they don=92t have a good after
sale program and/or there actually aren=92t any here]. I know you
laughed when I said =93high-end=94 referring to fixing my Adcom =96 that=92=
s
ok.

I read and will do so again your description of DC voltage in the
signal path. I'm still processing. You've been very generous with
your replies, much appreciated. I'll be so bold and ask you one more
thing because of my current situation of not being able to proceed
with sending the defective pre-amp anywhere at this time. Since
reading you, I turned everything off, but would like to continue using
the stereo for now with the Dolby off (bypassed). Here's my
question... If there is no source playing and I turn the Master
volume knob way up and the speaker base cones don't move or emit sound
at all, can I continue to use the stereo until I save more pennies?
This is how it appears with Dolby off - no abnormal speaker movement.

I can fill in with everything you've already written to me about this,
so I'll understand a Yes or No response.

No =3D DC is still transferring and frying my circuits; you're gambling
with the life of other components. Yes =3D There is a power supply
problem, but it only manifests itself when using the Dolby chip
(basically the Dolby chip or it=92s power circuit is creating the DC off-
set); it=92s ok to use cautiously with Dolby off until qualified repairs
can be performed.

When I do get it fixed, I=92ll post here my experience.

IF there is no speaker cone excursion with the Dolby decode stuff turned off,
there is no reason why you cannot continue to use the preamp as an analog
preamp for tuner, phono (if any), CD player (analog inputs) etc. However, you
want to be sure. If you don't have one already go buy a cheap Volt/Ohm meter
(sometimes called a multimeter). You can get them from places like Harbor
Freight,, even Radio Shack, often for $20 or less:

http://www.harborfreight.com/7-funct...ter-92020.html

http://www.radioshack.com/product/in...ductId=2999093

Merely set the meter to DC volts, 20 Volt scale, and with the preamp and amp
turned ON, and nothing playing, but the volume turned all the up, put the
lest leads across the speaker terminals of both (all) channels, one at a
time. There should be NO DC (or at least almost none) measurable. If it's
more than a couple of volts, probably shouldn't use it until the preamp is
fixed.

BTW, one of these meters is handy to have around the house, so this likely
wouldn't be a one-use buy. You'll use it to check the AC mains voltage, check
continuity on connections, test for blown fuses, etc. they have 1001 uses.