Jack said:
My IA drops out every once in awhile. I hear what sounds like a relay
clicking when this happens. I have the SM and it looks like it might
be the "Speaker Relay which is controlled be an IC and some components
that is called the "Speaker Protection Circuit." Has anyone had any
experience with this and how this SPC works ?
Thank You
Jack
If it's a simple protect circuit, it only monitors for DC offset at the
outputs. There is usually a resistor or diode network from each output, to
some collective point that the chip monitors for average zero voltage. When
the unit is initially powered, this chip also provides the turn-on delay,
via a simple R-C timing network on one of its pins, to allow for the various
supply rails to establish and stabilize, before the speakers are connected
by the relay. This serves the dual purpose of preventing false triggers of
the protect circuit whilst the rails are settling, and also of preventing
major speaker thumps if the main rails don't come up exactly together.
When they get a bit more elaborate, these protect circuits can also check
for output stage overloads and total shorts, and the most sophiticated
protect circuits also monitor AC supplies to the various rectifier circuits,
as well as the DC outputs from them.
If you hear a relay click, and the unit appears to otherwise stay powered,
then almost certainly, it is the speaker relay you are hearing. The first
thing is to listen whether the relay ever comes *straight* back on. If it
does, then it could be something as simple as a bad joint on one of its coil
pins - it's fairly common. If there is always a delay, then it's probably
that the chip is sensing a real problem. It will always put the delay back
in after an 'event'. The fact that the problem is intermittent, and there
seems to be otherwise no signs of trouble with the performance or audio,
might well suggest a simple bad joint, so the next move would be to run the
amp with the lid off, and have a bit of a tap around with the butt end of a
screwdriver. If this makes the problem occur, you can refine the location of
the area, by switching to something lighter like a plastic Biro pen for the
tapping.
Other possible causes, depending on the sophistication of that particular
protect circuit, are wire whiskers at the output terminals, and an
intermittently defective speaker. It is unlikely (although possible of
course) that there is any genuine component fault in the output stages being
monitored. Most outputs, discrete or hybrid module, either work or don't.
One slight rider on that though. If the amp uses an STK series hybrid, these
can get intermittent connections where the pins connect to the ceramic
substrate. This gives all the appearances of a bad joint that you just can't
find, no matter how hard you look.
Finally I have, in the past, had the timing cap mentioned above, go faulty
or low value or poor ESR, which has resulted in intermittent erroneous
operation of the protect circuit on various makes and models. Usually, there
are other symptoms as well, such as the relay being reluctant or slow to
kick in at initial power up. Hope this helps.
Arfa
