"Arfa Daily" wrote in message
...

"Jamie" t wrote in
message ...
wrote:

One of the two main fuses blows instantly as soon as power is applied
to receiver. I checked the main power supply caps 2@10,000uf and they
were ok. I also checked the bridge rectifier and it was ok. When I
remove the two power leads to the transformer it does not blow fuse.
The main windings of the transformer are 1.0 ohm. The output
transistors are each about 800 ohms at the speaker terminal (prior to
relay).

What else to check?????


Thanks!!


To save your self some fuses and hassles, find
your self a high wattage incandescent lamp rated
for the line voltage your using (120v 200W) for
example. Use that in place of the fuse until you can
removed the solid short.
WHen you say you removed the main windings? I can
only assume you're referring to the primary side of the
xformer?. If so, how about disconnecting the secondary
sides?
You may want to perform a short test to ground to see if you have
a shorted xformer. The preferred method is to use a megga meter.

Usual causes are arc's from lightning storms or over heating of the
enamel if you do find a short to ground.
--
If disconnecting the secondary from the bridge removes the short,
then try disconnecting the output supply that goes from the bridge and
caps into the amp.

Also, you may have a time delayed on or soft start circuit that may not
be soft starting.

P.S.
Because a CAP or bridge test ok at DMM voltage levels does not mean
they are good.


--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5


If you can lay hands on a variac, use that to power the unit. This kind of
fault then becomes a breeze to locate (the basic cause of). If it turns
out to be related to the output stages - and I'm not quite sure of what
you are measuring to come up with your "800 ohms" figure - then how simple
or not the *actual* problem is, is a whole new ball game with DC coupled
amps like this ...

Arfa


I'm going to assume the typical.

1. An amp channel is blown.
2. The associated emitter resistor is not blown yet since the fuse blows.
3. You can measure from the center leg of each emitter resistor to the
collector tabs of the associated output transistors.
4. When you find a channel where the emitter is shorted to one or both
collectors, you've found your blown channel.
5. There are often other parts bad as well, though the Yamaha amp channels
are pretty easy. Often there is a resistor between the driver transistors
which is burned - usually 220 ohms, and of course the drivers are suspect,
the bias transistor could be bad OR HAVE BAD SOLDER CONNECTIONS which caused
the failure in the first place, and there could be another resistor bad
which feeds the B+ or B- voltage to a driver - usually 47 ohms.


Mark Z.