Star Tech Journal

September 1995
... continued from page 17
..,'ROUBLESHOOTING
If fuses below, it is probably a bad
output section or the power sup-
ply. Each output section can be
in-plugged at the fuse board on
the rear wall. This lets you iso-
late the bad one. If it still takes
fuses out, check the power sup-
ply board and the big filter caps
sticking out the front.
Bad output sections usually mean
output transistors. These are un-
der the driver cards, on the back
of the heat sinks. Check them for
shorted. If one is bad, replace
them both -trust me. Check the
related driver card for bad tran-
sistors (the six leg one is actually
two) and three diodes. Though it
is best to use the right parts, I find
ese amps are forgiving of rather
liberal substitutions. Don't forget
the small transistor on the heat
sink.
If it works but sounds terrible,
you either have a bad channel or
a bad power supply. If both chan-
nels sound bad, it is likely a bad
15V supply, but we already fixed
that. Only one channel sound
bad? Blown channels don't al-
ways blow fuses, so if you have a
nasty sounding one, use the same
procedure: check all the transis-
tors, I bet you find a bad one.
Inspect for burnt or open resis-
tors, any you find were caused by
bad semiconductors somewhere.
The preamp circuits on the front
anel rarely cause trouble.
...:,till sound bad? Make sure
speakers are OK. Blown speak-
ers can be reconditioned at about
STAR*TEcH
Tech Library
half the cost of replacement.
Music stores that sell guitar am-
plifiers can refer you to a
reconditioner. Check the exter-
nal speaker wiring or remove it
while testing. Disconnect the
amplifier, then hook some other
source to the speakers to test
them. How is you stylus? It takes
a good "needle" to make good
sound.
STAR* TECH Journal's
Tech Library comes
complete with every
service manual
published.
SAVE OVER
SHOP NOTES
Instead of wasting fuses, keep a
collection of circuit breakers
handy. Solder clip leads to them,
then clip them to the fuse holder
until problems are solved. A well
supplied bench needs a variac
and isolation transformer for re-
pairing monitors. Use it for am-
plifier work as well. Bring up the
power slowly and watch the cur-
rent draw. Don't waste your fuses.
AMP connectors to fit the power
and speaker hookups are easy to
come by. I got mine from the
junk wire bin from stripped out
games. The turntable came from
my stereo and I made a little
adaptor to plug it into the input
jack. The larger size Molex pins
(.09") fit into the input jack nicely.
Make sure to clip the turntable
ground wire to the amp chassis
or you will get hum.
25%
For more information
see page 23.
HOLIDAY VISIT
Remember, after selling a box to
a private home, many customers
will be pleased to have you out
for an annual maintenance, espe-
cially going into the fall party sea-
son. Folks, this is almost free
money! Where else can they buy
styli?
Journal
~
~
STAR*TECH Journal
neither operates, dis-
tributes nor manufac-
tures coin-operated
equipment. It's sole pur-
pose for being is to com-
pile technical and infor-
mational data consid-
ered valuable for dis-
semination to its paid
subscribers. Comments
or suggestions pertain-
ing to this service are
always welcome.
~
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