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