Thanks for your answers. I haven’t used this forum before, I think I
inadvertently deleted one post, and also was not sure how to reply
specifically to the author, I might have done this already, sorry for
double posting.
@Dave:
I checked the DC output, it’s below 10mV.
@ William:
The color LED mentioned in the user manual are amber and blue (normal
functioning). The troubleshooting diagram in The Service Manual also
mentions green and red, which is confusing. The test signal was generated
by the receiver, with the signal test function activated, via subwoofer
pre-out.
@ Arfa
I have the manual, from the same source you mentioned.


First, it isn't a forum - it's a usenet newsgroup. If you are accessing it
via the internet, then you will be using a scraper site that acts as a
portal to usenet. Effectively, it's taking usenet content and placing it
onto a website, then taking your replies or original posts, and shunting
them back the other way. It's a 'scruffy' way of accessing newsgroups, and
posts get lost and delayed way too easily. You are much better off accessing
usenet and the groups directly from your own machine. This is easily done
via clients within Windows mail programs - for instance I am using the
client within Windows Live Mail at the moment to post this - but there are
many other free ones. Sometimes, I use XNews for instance. If your ISP
doesn't provide direct access to usenet news servers, there are plenty of
other options. Replying becomes very easy once you have a proper news
client. You just make sure that the message you want to reply to is the
'active' one on your screen, then click "Reply Group". Your reply will then
be posted to the whole group, in chronological order, with the text of the
post you are specifically replying to, included above your reply. If you do
want to reply just to a specific poster for any reason, same procedure, but
click on "Reply".

As to your problem. I am not sure how you would have an amber and blue led
indication. Certainly, there are no dual LEDs in that combination that I
have come across. Red and green duals exist, and if you drive both at once,
you get amber. If you have the unit set for auto switch on on receipt of
signal, do any indicators change when you give it some signal ? If they do,
then that should eliminate a good chunk of circuitry around the front end.
As to trusting that the driving source's test signal is putting out
something valid on the sub channel, I'm not sure that I would. Whenever I'm
repairing subs - which is quite often - I use a proper sine generator set to
80 Hz.

Do you have access to a 'scope ? If you do, and the knowledge to use it,
then this should be an easy problem to at least diagnose, if not actually
fix. The fact that you get a thump both at switch on and switch off, seems
to indicate that the output relay is closing and staying closed, which
pretty much - although not quite definitely - suggests that the output stage
is happy and working. If there were any serious DC offsets or whatever, the
output protect relay would drop back out.

This is not a particularly complicated unit, and having a service manual -
even a block diagram - is, as most of us on here would tell you, a HUGE
plus these days. We have to fix this stuff for a living on a daily basis,
with no info at all, probably at least 50% of the time ...

Do some more tests with a known signal going in, and come back to us with
the results. At a pinch, you could use a small power transformer - say 6v -
with a resistive potential divider of 10:1 across it to provide a crude line
level sine wave signal at 50 or 60 Hz, depending which side of the pond you
are. Note, that's a transformer, on its own. Not a DC power unit of any
kind.

Arfa