First, let me admit to "rank" amateur status on troubleshooting and
repair. I am working on the subject reciever for a friend. The
problem is that the main fuse blows within one or two seconds of
switching on the power. I think I once read here that one of the first
things to check is the output transistors, which I have done. I am
confident enough to say that they are ok, neither shorted or open.
Since I don't have a variac at my presen t location I left the
transistors disconnected and used a fuse as my variac. No change, It
blew immediately. I would appreciate any suggestions on a procedure.
I do have the schematic. Is there any publication to which I might be
directed which outlines "the basic standard procedure" for analyzing,
troubleshooting, etc. for solid state stereo amps and receivers.
Thanks in advance.

Bud Pember

In article .com, wrote:
First, let me admit to "rank" amateur status on troubleshooting and
repair. I am working on the subject reciever for a friend. The
problem is that the main fuse blows within one or two seconds of
switching on the power. I think I once read here that one of the first
things to check is the output transistors, which I have done. I am
confident enough to say that they are ok, neither shorted or open.
Since I don't have a variac at my presen t location I left the
transistors disconnected and used a fuse as my variac. No change, It
blew immediately. I would appreciate any suggestions on a procedure.
I do have the schematic. Is there any publication to which I might be
directed which outlines "the basic standard procedure" for analyzing,
troubleshooting, etc. for solid state stereo amps and receivers.
Thanks in advance.

Bud Pember

A variac is handy for checking equipment, and even handier
if you also have a series lightbulb device. Different sized lamps
will provide different current ranges. I even have a box with switches to
turn on and off different sized lamps. A variac with a volt and current meter is also nice,
as well as an isolated output tap from the variac.

Most often, checking things will reveal problems as you have done.
There is a bruit test which works sometimes, or at least alows
you to make some voltage checks, using that variac, and, or the lamp box.
Shorted paths will heat up, sometimes giving a clue of a shorted part, for inctance.

I,m sure the repair FAQ has some info......................

http://zekfrivolous.com/faq/

greg

wrote:
First, let me admit to "rank" amateur status on troubleshooting and
repair. I am working on the subject reciever for a friend. The
problem is that the main fuse blows within one or two seconds of
switching on the power. I think I once read here that one of the first
things to check is the output transistors, which I have done. I am
confident enough to say that they are ok, neither shorted or open.
Since I don't have a variac at my presen t location I left the
transistors disconnected and used a fuse as my variac. No change, It
blew immediately. I would appreciate any suggestions on a procedure.
I do have the schematic. Is there any publication to which I might be
directed which outlines "the basic standard procedure" for analyzing,
troubleshooting, etc. for solid state stereo amps and receivers.

Start near the beginning. Disconnect the secondary windings of
the transformer from the rectifier block or diodes. If the fuse blows,
you probably have a bad power transformer. If it doesn't, connect the
secondary windings back up but now disconnect the connections
from the filter caps to the amp itself. If the fuse blows, you have a
shorted rectifier or filter cap. If it doesn't, you're back to trouble-
shooting the amp itself.

Disconnecting the output transistors by themselves may or may
not be a bad idea, becuase it can soradically change the operating
of the drivers so that they themselves may fail.

Basically,, when you have such a frank catastrophic failure as you
describe, you have to start as early in the chain as possible and
work from there, not from the last point and work back..

wrote:
wrote:
First, let me admit to "rank" amateur status on troubleshooting and
repair. I am working on the subject reciever for a friend. The
problem is that the main fuse blows within one or two seconds of
switching on the power. I think I once read here that one of the first
things to check is the output transistors, which I have done. I am
confident enough to say that they are ok, neither shorted or open.
Since I don't have a variac at my presen t location I left the
transistors disconnected and used a fuse as my variac. No change, It
blew immediately. I would appreciate any suggestions on a procedure.
I do have the schematic. Is there any publication to which I might be
directed which outlines "the basic standard procedure" for analyzing,
troubleshooting, etc. for solid state stereo amps and receivers.

Start near the beginning. Disconnect the secondary windings of
the transformer from the rectifier block or diodes. If the fuse blows,
you probably have a bad power transformer. If it doesn't, connect the
secondary windings back up but now disconnect the connections
from the filter caps to the amp itself. If the fuse blows, you have a
shorted rectifier or filter cap. If it doesn't, you're back to trouble-
shooting the amp itself.

Disconnecting the output transistors by themselves may or may
not be a bad idea, becuase it can soradically change the operating
of the drivers so that they themselves may fail.

Basically,, when you have such a frank catastrophic failure as you
describe, you have to start as early in the chain as possible and
work from there, not from the last point and work back..


Completely agree with the above, except there is a lot of testing which
could be done *in-circuit* without disconnecting everything first.
OTOH, since the outputs are usually the first thing to go, that was not
an unreasonable starting point in this case.

jak

wrote in message
oups.com...
First, let me admit to "rank" amateur status on troubleshooting and
repair. I am working on the subject reciever for a friend. The
problem is that the main fuse blows within one or two seconds of
switching on the power. I think I once read here that one of the first
things to check is the output transistors, which I have done. I am
confident enough to say that they are ok, neither shorted or open.
Since I don't have a variac at my presen t location I left the
transistors disconnected and used a fuse as my variac. No change, It
blew immediately. I would appreciate any suggestions on a procedure.
I do have the schematic. Is there any publication to which I might be
directed which outlines "the basic standard procedure" for analyzing,
troubleshooting, etc. for solid state stereo amps and receivers.
Thanks in advance.

Bud Pember


Disconnect the transformer secondaries from the Power Supply. If the fuse
still blows, the mains transformer is faulty, or there could be some small
suppressor caps connected to either side of the transformer that have gone
short circuit.

If you have ruled out the output transistors, the fuse blowing probably
means a short circuit.. Don't keep plugging in more fuses as you may cause
further damage. A short can easily be found in circuit using a multimeter.
Check the rectifier diodes or Bridge Rectifier with the secondaries still
disconnected. Check for shorts between each power supply rail and ground.

Make sure that if any transistors need to have their cases electrically
insulated from the heatsink that they are. Could someone have forgotten to
put these insulators back? (Check for shorts between the transistor casing
or metal tabs and the metal of the heatsink).



Gareth.

I just want to thank everyone for their help. Because of the very
specific suggestions, the problem was traced to a shorted rectifier
block, and the problem is fixed. Thanks again!

Bud Pember