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