Repair to faulty amp
 

Greetings,

I know there are a few here who are very well qualified with hifi (and
particularly amplifier) electronic design. Can you help?

I have an Alchemist Nexus amplifier which, when turned above a certain
volume, screeches loudly through the speaker output.

I believe that the amp is outputting DC - does that sound right?

I have removed two burnt out resistors - identical on each channel on the
PCB - and I had already replaced two 4 amp output fuses before testing
produced the results described and before I removed the burnt resistors.

Can anyone confirm my translation of the resistor colour codes (gathered
from an identical amp as those in the faulty amp were badly burnt):

1st digit: Brown: 1
2nd digit Black: 0
Multiplier: Black: 0
Quality: Brown: 1

I make this 10 ohms of a high-tolerance 1%/1000 hour anticipated failure?

I want to be certain as, while I am confident that my soldering skills are
up to the task (the components are not surface mount), I have not carried
out such a repair before without the aid of a manufacturer's service manual
before.

I have created a small rollover image at
http://www.alchemisthifi.info/fault/resistor.htm which allows you to compare
the damage on the bad amp with the good amp.

The manufacturer (Alchemist) is no longer in business and there are no
circuit diagrams about.

Beyond the resistor confirmation, any constructive advice you can offer
about my approach would be appreciated.

TIA.

Repair to faulty amp
 

JustMe wrote:

Greetings,

I know there are a few here who are very well qualified with hifi (and
particularly amplifier) electronic design. Can you help?

I have an Alchemist Nexus amplifier which, when turned above a certain
volume, screeches loudly through the speaker output.

I believe that the amp is outputting DC - does that sound right?

I have removed two burnt out resistors - identical on each channel on the
PCB - and I had already replaced two 4 amp output fuses before testing
produced the results described and before I removed the burnt resistors.

Can anyone confirm my translation of the resistor colour codes (gathered
from an identical amp as those in the faulty amp were badly burnt):

1st digit: Brown: 1
2nd digit Black: 0
Multiplier: Black: 0
Quality: Brown: 1

I make this 10 ohms of a high-tolerance 1%/1000 hour anticipated failure?

I want to be certain as, while I am confident that my soldering skills are
up to the task (the components are not surface mount), I have not carried
out such a repair before without the aid of a manufacturer's service
manual before.

I have created a small rollover image at
http://www.alchemisthifi.info/fault/resistor.htm which allows you to
compare the damage on the bad amp with the good amp.

The manufacturer (Alchemist) is no longer in business and there are no
circuit diagrams about.

Beyond the resistor confirmation, any constructive advice you can offer
about my approach would be appreciated.

TIA.

In all probability the burnt out resistor(s) are not the cause of the
problem but an effect of it. Resistors very rarely fail short circuit so
it is likely that whatever this resistor is connected to has actually
failed and taken out the resistor with it.

Ian
--
Ian Bell

Repair to faulty amp
 

Contact Chris Found at He designed a few alchemist
products and would certainly help if he can. He's a friendly chap (lives in
Fulham) Andy

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Repair to faulty amp
 

"Ian Bell" wrote in message
...
JustMe wrote:

Greetings,

I know there are a few here who are very well qualified with hifi (and
particularly amplifier) electronic design. Can you help?

I have an Alchemist Nexus amplifier which, when turned above a certain
volume, screeches loudly through the speaker output.

I believe that the amp is outputting DC - does that sound right?

I have removed two burnt out resistors - identical on each channel on
the
PCB - and I had already replaced two 4 amp output fuses before testing
produced the results described and before I removed the burnt resistors.

Can anyone confirm my translation of the resistor colour codes (gathered
from an identical amp as those in the faulty amp were badly burnt):

1st digit: Brown: 1
2nd digit Black: 0
Multiplier: Black: 0
Quality: Brown: 1

I make this 10 ohms of a high-tolerance 1%/1000 hour anticipated
failure?

I want to be certain as, while I am confident that my soldering skills
are
up to the task (the components are not surface mount), I have not
carried
out such a repair before without the aid of a manufacturer's service
manual before.

I have created a small rollover image at
http://www.alchemisthifi.info/fault/resistor.htm which allows you to
compare the damage on the bad amp with the good amp.

The manufacturer (Alchemist) is no longer in business and there are no
circuit diagrams about.

Beyond the resistor confirmation, any constructive advice you can offer
about my approach would be appreciated.

TIA.

In all probability the burnt out resistor(s) are not the cause of the
problem but an effect of it. Resistors very rarely fail short circuit so
it is likely that whatever this resistor is connected to has actually
failed and taken out the resistor with it.

Ian

Yes, I wondered about that too but, with my limited knowledge, I've not been
able to identify any other faulty components - what else would cause an amp
to go DC in the manner described at the top? The o.p transistors all compare
well with those in the good amp, for instance.

Either way, they need replacing and so I thought I'd start there, even if I
have to replace them again, after.

Have I read the resistor's rating correctly?

Repair to faulty amp
 

"Andy Evans" wrote in message
...
Contact Chris Found at He designed a few alchemist
products and would certainly help if he can. He's a friendly chap (lives
in
Fulham) Andy

I believe the Nexus amp was designed by Glen Gayle - Chris designed their
digital products and the Product8 pre amp too. I could send it to Glen, but
I'd really like to try and sort it at home, if I can.

Repair to faulty amp
 

I believe the Nexus amp was designed by Glen Gayle - Chris designed their
digital products and the Product8 pre amp too. I could send it to Glen, but
I'd really like to try and sort it at home, if I can.

you can email Chris and he'll give you his phone number - he may be able to
tell you what to do over the phone. Andy

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Repair to faulty amp
 

JustMe wrote:

"Ian Bell" wrote in message
...
JustMe wrote:

Greetings,

I know there are a few here who are very well qualified with hifi (and
particularly amplifier) electronic design. Can you help?

I have an Alchemist Nexus amplifier which, when turned above a certain
volume, screeches loudly through the speaker output.

I believe that the amp is outputting DC - does that sound right?

I have removed two burnt out resistors - identical on each channel on
the
PCB - and I had already replaced two 4 amp output fuses before testing
produced the results described and before I removed the burnt
resistors.

Can anyone confirm my translation of the resistor colour codes
(gathered from an identical amp as those in the faulty amp were badly
burnt):

1st digit: Brown: 1
2nd digit Black: 0
Multiplier: Black: 0
Quality: Brown: 1

I make this 10 ohms of a high-tolerance 1%/1000 hour anticipated
failure?

I want to be certain as, while I am confident that my soldering skills
are
up to the task (the components are not surface mount), I have not
carried
out such a repair before without the aid of a manufacturer's service
manual before.

I have created a small rollover image at
http://www.alchemisthifi.info/fault/resistor.htm which allows you to
compare the damage on the bad amp with the good amp.

The manufacturer (Alchemist) is no longer in business and there are no
circuit diagrams about.

Beyond the resistor confirmation, any constructive advice you can offer
about my approach would be appreciated.

TIA.

In all probability the burnt out resistor(s) are not the cause of the
problem but an effect of it. Resistors very rarely fail short circuit so
it is likely that whatever this resistor is connected to has actually
failed and taken out the resistor with it.

Ian

Yes, I wondered about that too but, with my limited knowledge, I've not
been able to identify any other faulty components - what else would cause
an amp to go DC in the manner described at the top? The o.p transistors
all compare well with those in the good amp, for instance.

When you say compare well what do you mean? And when you say the amp outputs
dc, between what points do you measure this, what value do you read and how
does it compare with a good amp?

If the output trannys are OK then it may well be their drive circuit that is
giving the problem but you really need a circuit to get into this levele of
detail.


Either way, they need replacing and so I thought I'd start there, even if
I have to replace them again, after.

Have I read the resistor's rating correctly?

Looks like 10ohms 1% to me.

IAn

--
Ian Bell

Repair to faulty amp
 

JustMe wrote:

Greetings,

I know there are a few here who are very well qualified with hifi (and
particularly amplifier) electronic design. Can you help?

I have an Alchemist Nexus amplifier which, when turned above a certain
volume, screeches loudly through the speaker output.

I believe that the amp is outputting DC - does that sound right?

No.


I have removed two burnt out resistors - identical on each channel on the
PCB - and I had already replaced two 4 amp output fuses before testing
produced the results described and before I removed the burnt resistors.

Can anyone confirm my translation of the resistor colour codes (gathered
from an identical amp as those in the faulty amp were badly burnt):

1st digit: Brown: 1
2nd digit Black: 0
Multiplier: Black: 0
Quality: Brown: 1

I make this 10 ohms of a high-tolerance 1%/1000 hour anticipated failure?

Resistors don't have " 1000 hour anticipated failure " bands !


I want to be certain as, while I am confident that my soldering skills are
up to the task (the components are not surface mount), I have not carried
out such a repair before without the aid of a manufacturer's service manual
before.

I have created a small rollover image at
http://www.alchemisthifi.info/fault/resistor.htm which allows you to compare
the damage on the bad amp with the good amp.

The manufacturer (Alchemist) is no longer in business and there are no
circuit diagrams about.

Beyond the resistor confirmation, any constructive advice you can offer
about my approach would be appreciated.

If you don't know how to troubleshoot - give up or give it to someone who does.


Graham

Repair to faulty amp
 

In article , JustMe
wrote:
Greetings,

I know there are a few here who are very well qualified with hifi (and
particularly amplifier) electronic design. Can you help?

I have an Alchemist Nexus amplifier which, when turned above a certain
volume, screeches loudly through the speaker output.

I believe that the amp is outputting DC - does that sound right?

No. Sounds more like instability causing oscillations for some strange
reason. May be that the PSU is unhappy so the amp is 'motor boating'. (Pure
guess on my part.)

A meter should tell you if the output d.c. is correct, etc. Similarly, you
could use one to check the resistor values.

I have removed two burnt out resistors - identical on each channel on
the PCB - and I had already replaced two 4 amp output fuses before
testing produced the results described and before I removed the burnt
resistors.

Can anyone confirm my translation of the resistor colour codes (gathered
from an identical amp as those in the faulty amp were badly burnt):

1st digit: Brown: 1 2nd digit Black: 0 Multiplier: Black: 0 Quality:
Brown: 1

I make this 10 ohms of a high-tolerance 1%/1000 hour anticipated failure?

I want to be certain as, while I am confident that my soldering skills
are up to the task (the components are not surface mount), I have not
carried out such a repair before without the aid of a manufacturer's
service manual before.

I would tend to recommend testing the amp using a current-limited bench
PSU. Otherwise you may just waste more resistors and/or fuses if the unit
repeats the problem whenever you turn it on. Faulty power amps can tend to
be distructive unless controlled. :-)

You also ideally need a diagram for diagnosing any fault that may not be
obvious.

10 Ohms sounds too high for o/p resistors to me. Sounds more like drivers,
or similar, but I have no idea as I don't know the design. The dead
resistors may mean some transistors have failed. resistors may good
slow-blow fuses. Alas the 'slow' bit means the transistors go short in
microseconds, and then the resistors fry in milliseconds...

I have created a small rollover image at
http://www.alchemisthifi.info/fault/resistor.htm which allows you to
compare the damage on the bad amp with the good amp.

The manufacturer (Alchemist) is no longer in business and there are no
circuit diagrams about.

Take up Andy's advice re contacting the designer. :-)

FWIW I don't have any diagrams for the amp. If I did I'd be happy to try
and advise. (If anyone has a diagram I'd love a copy to add to my
collection. :-) )

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Repair to faulty amp
 

On Tue, 09 Nov 2004 22:05:19 +0000, JustMe wrote:


Can anyone confirm my translation of the resistor colour codes (gathered
from an identical amp as those in the faulty amp were badly burnt):

1st digit: Brown: 1
2nd digit Black: 0
Multiplier: Black: 0
Quality: Brown: 1

I make this 10 ohms of a high-tolerance 1%/1000 hour anticipated failure?


If the amp is pretty old then you are right, apart from the 4th band which
usually means 1% tolerance (silver=10%, gold=5%, red=2%). Some old
resistors had a salmon pink band at the end to signify "high stability" -
but that is a while ago!

There are now resistors with 4 bands, where the 4th band is the
multiplier. In this case your value would be 1k. These are produced so
that you can revel in delightful values like 91k!

How about measuring a known good resistor out of the other amp and
comparing them? A resistor will almost always fail open circuit or
high in value. If the other amp has been working fine, and its resistors
are not burned, then you should be looking for whatever caused the burning
(e.g. a shorted transistor or something like that allowing excessive
current flow for too long).

--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info