Powering
up an unknown amplifier
If you're
into audio as a hobby and are interested in picking up
equipment (especially amplifiers) untested and of
unknown origin, it really pays to have at least some
knowledge of what's going on inside the case; plugging a
mystery amplifier straight into the mains and hoping for
the best is risky, at the least you might get a fright
if it goes bang and it might suffer expensive damage
too, possibly killing any speakers which you have hooked
up.
I try
really hard to be impartial on this website and to stay
away from giving direct advice, but here's the bare
minimum which I do before I power up an amplifier of
unknown origin:
Look for any signs of repairs, especially bad
ones which look like the person might have abandoned
the attempt; sometimes novices try their hand at
repairs, I see quite a few of those repairs and
sometimes it's confusing trying to work out what
that person was actually trying to repair. There's
no shame here, it really isn't easy to make a neat
repair if you've never done it before, we all
started somewhere; I remember way back when I
started messing around with electronics as a kid, the results weren't
pretty. Still, if there's any signs of recent
repairs they might point to a fault which is still
there, possibly even the reason the equipment was
sold.
Check for any signs of burned components, any
charring on or around components; especially
resistors. Any dubious looking components need to be
tested, if they give a misleading reading and they'd
be difficult to remove, at the very least the same
component on the other channel should be checked and
the readings compared. Also check the inside of the
lid - heat (and smoke) rises, some heat marking over
the area of a component which would normally
generate heat is fine but thick soot or signs
of scorching isn't.
Also check any fuses, especially
on amps where fuses are fitted as protection on the
speaker outputs. Fuses can sometimes play dead
without actually blowing their filament, a bad fuse/fuseholder
connection had caused me headaches more than once in
the past - old panel fuseholders are offenders here
(many models of Quad use these),
sometimes the metal contacts oxidise and cause
intermittent faults.
Test the mains wiring for safety. At the very
least this needs a multimeter, ideally a more
expensive one which can test diode junctions and
capacitors too (I have a Fluke 115 for general
testing, a tool which I couldn't exist without).
With any power switches on the equipment turned off,
start off at the mains plug - if the equipment is
earthed, test for continuity between plug pin and a
bare part of the chassis, flexing the mains lead
while you do it; it should be well under 1
ohm and any fluctuation means a bad connection or
frayed wire which must be repaired before you even
think about applying power; this is the most important safety check and one
you should ALWAYS make; a good earth/ground
connection is more important than ANYTHING else.
Also check resistance between both mains pins and
earth - it should be very high, ideally in the
Megohms. Then test resistance between live and
neutral pins, this should be extremely high (many
Megohms) with the power switch in the off position,
then press the power switch and you should see the
meter read anywhere from a couple of ohms upwards,
depending on the size/power of the transformer - it
definitely shouldn't be a short circuit however. If
the item fails in ANY of these tests, it stands to
reason that you should NOT attempt to apply power
until you're sure that you've rectified the cause.
Test the semiconductors. At the very least
this means the power transistors in the output stage
and the main rectifier diodes, ideally I'd test
every major transistor for short circuits but this
needs a meter with diode test function. This can be
confusing as the discharged reservoir capacitors
will initially show as a short circuit but this
should rapidly rise as they charge up from the
voltage from the meter probes. In some designs you
might need to remove the output transistors to test
them properly; it's definitely worth taking the
trouble in my opinion and could save a lot of
headaches later on. There are exceptions but
transistors often fail short circuit from collector
to emitter; however ANY
component which seems dubious should be removed and
tested separately.
Finally, do a visual check on any wiring
looms, check any connectors are secure, make a final
check of components; especially the electrolytics,
especially any in the power supplies. If any are
bulging or visibly leaking, don't take the chance -
it's better to be safe and replace them. Another
thing to mention here, it might also be an idea to
pull any supply fuses and check that they correspond
to the value printed on the PCB; it's not unheard of
for novices to try changing fuses for a higher value
in the hope that they won't blow.
On an amplifier which looks like it hasn't seen
power for a few years, you need to be careful. Sure, you could
just plug it into the mains, power it straight up and
you might well be lucky and everything will work
perfectly.
 |
|
Technics
SU-V8 after rebuild, final checks before
powering up. Everything nice and clean |
However, things certainly can go wrong and on
something which hasn't seen any power for upwards of 5
years and might have unknown problems it's not a good idea to go in and blindly apply
full power without at least checking everything over and
conducting basic go/no-go tests on some of the main
components.
Testing for shorted junctions on the main output
transistors and the voltage regulators, performing a
health check on the main electrolytic capacitors is the least you should consider
doing.
Why you need to be careful
Finally, an illustration of why you need to be wary
before you apply power to an unknown piece of audio
gear.
 |
|
Fuses replaced with
woodscrews |
The photograph to the left was of a Pioneer
SA-3000 integrated
amp which came to me with a fault where sometimes it
wouldn't power up. On looking inside, I found what
you see in the image to the left - the main fuses
replaced with woodscrews.
The bad connection between them and the fuseholder
caused said fault.
Ironically, the fuses were
probably fine and the actual fault with the amplifier
was a broken emitter resistor which had killed the left
channel, £1.50 of components later and things were
working again (and still are, this is now in one of my
systems, it works perfectly). This is probably one of the worst
bodges I've seen in 15 years of repairing audio,
something like this could easily cause a fire.
There's a reason why you shouldn't do
things like this, fuses are put there for a reason - if
anything does go wrong, they're often the final safety
device which would cut power before anything bad had a
chance to happen. Especially in a power amplifier,
there's a lot of high current available, that current
could create huge amounts of heat. A short circuit power
transistor (quite a common fault) would usually just
blow its supply fuses before any damage was done and the
worst you'd notice might be a pop and a harmless wisp of
smoke.
Take those fuses away and you're then
pulling huge amounts of power from the components
further up the line, if you were lucky the rectifier
diodes would overheat and go open circuit (that said,
most semiconductors will often fail short circuit which
would cause even more damage), or the mains
transformer would overheat and its internal fuse would
blow. At the least, this would cause irreparable damage
to the amplifier - worse, it could possibly cause
something to overheat and catch fire.
Finally
Don't be scared - just be careful,
especially when dealing with safety - fuses and earth
connections are there for a reason.
