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!!

On Oct 9, 7:27 pm, 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!!

Disconnect all loads from the electrolytics, so you just have the
transformer, diodes and capacitors in the circuit and see what
happens. The diodes may test ok but break down under higher voltage
than your tester applies.

H. R. (Bob) Hofmann

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

"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

"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.

On Oct 10, 4:15 am, "Mark D. Zacharias"
wrote:
"Arfa Daily" wrote in message

...







"Jamie" t wrote in
...
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.- Hide quoted text -

- Show quoted text -

I do have a variac, I am not sure how it really helps finds the fault,
though. I have not determined yet whether the short is in the power
supply board or the output transistors.
Thanks

Ok, one step closer. I replaced the two 10,000uf 71v power supply
caps with two 12,000uf 80v caps I had and now the fuse does NOT blow
when I push the power button. But, I hear a relay click and the unit
does still not power up.

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 ...



Use a large incandescent lightbulb wired in series with the unit for
testing. It's cheaper than a variac and in this case is a better solution.
200-300W should do it.

On Oct 10, 10:32 pm, "James Sweet" wrote:
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 ...

Use a large incandescent lightbulb wired in series with the unit for
testing. It's cheaper than a variac and in this case is a better solution.
200-300W should do it.

Ok, I dug a bit deeper and found that the two center channel power
transistors were shorted. I replaced them with the rear channel
devices to see if it would work.
The fuse no longer blows but the standby relay still kicks it out.
The difference is that the display comes on momentarily and no fuse
blowing.
So either I have more issues on the center channel or the protection
circuitry knows that the rear channel transistors are missing.

wrote in message
ups.com...
On Oct 10, 10:32 pm, "James Sweet" wrote:
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 ...

Use a large incandescent lightbulb wired in series with the unit for
testing. It's cheaper than a variac and in this case is a better
solution.
200-300W should do it.

Ok, I dug a bit deeper and found that the two center channel power
transistors were shorted. I replaced them with the rear channel
devices to see if it would work.
The fuse no longer blows but the standby relay still kicks it out.
The difference is that the display comes on momentarily and no fuse
blowing.
So either I have more issues on the center channel or the protection
circuitry knows that the rear channel transistors are missing.



There's a good chance the latter is the case.