DIYbanter - help with amp repair (cont.)

I think I answered you before, but I can help you out a little more.
Without a schematic, it is really hard to know what happened. but here
goes.

1. Can you read the color values for the burned resistor? If so, replace
it with its exact match. BTW this device could also be a picofuse, as they
can sometimes look like a 1/4 W resistor. They usually have a green
body with colored bands. Most 1/4 W resistors have light brown or flesh
colored bodies with colored bands.

2. Can you read the number on the transistor? If so, you really need to
replace them with their exact match.

If you can't identify the devices explicitly, look around on the PCB and try
to find those same two devices elsewhere on the PCB. Typically, amplifiers
will have a great deal of symmetry.

3. What usually "blows" are "hang-offs", the devices that are physically
pressed against the heatsink by some form of clamping clip or bar.

Try turning on your amplifier and "feeling" around for any *really* hot
devices. Bad devices will generally be too hot to touch. Do this only
for a short time, and then let everything cool off. Look for any
devices that might be overheating or giving off smoke. At any sign of
trouble,
TURN THE AMPLIFIER OFF. Probe the suspect area for any bad devices.

Power supplies typically have MOSFETS for the "hang-offs". With the B+
disconnected, measure the Gate to Source and Gate to Drain
resistances. None should be zero or very low readings. Should be very
high, sometimes in the MegOhm range. Measuring the Drain to Source, you
might read a low resistance, but swap your meter probes and measure
again. Drain to Source sometimes has an intrinsic diode that will give a
shorted
reading in one direction. If you measure high resistance in at least
one direction, then the device is probably fine.

The "hang-off" rectifying diodes need to be measured in both directions
as well. If you can, use your diode check function on the meter. Some
power diodes will have three leads as well. These may be common
cathode or common cathode. The should show their forward voltage drop in
one
direction ( .6 for most diodes, but can be .3 if shottky) and open or
"OL" in the other direction. If they exhibit this behavior, they will
probably be
goof.

Output devices should also be checked. Most of the time these are BJT's
but might be MOSFET's. If they are BJT's check their Collector to Emitter
resistance. Should be very high, maybe over 50K Ohms. If they read
zero ohms, they are bad. They sometimes blow open as well. This will
appear
as "OL" or some really high Meg Ohm reading. You might want to check
Base to emitter resistance as well.

If they are MOSFET, follow the above directions for checking the PS
MOSFET's.

4. If you think all of the "hang-offs" appear good, but the amplifier still
doesn't turn on, you will need to isolate the problem. Could be in the
power
supply, could be in the output section. You will need an oscilloscope
to do any further probing.

Is the power supply operating? Check to see that there is a square
wave signal being driven into the transformer ( the big wire wound donut ).
If it isn't, try carefully desoldering the rectifier diodes and look
again. I usually cut them out and then use a pair of tweezers to remove the
remaining leads from the PCB. Too much heat applied to the PCB can
cause trace lifting. If you want to try and desolder them, use a *lot* of
solder and move your solder iron tip back and forth across leads and
then gently "rock" the device out. BTW, be careful about crimped leads.
If the leads are crimped or "staked" on the back side of the PCB, it is
often better to cut the devices out to keep from lifting a trace.

5. Anything past these cursory methods will require a schematic to
effectively troubleshoot your amplifier. If it isn't the power devices, the
process
of finding the problem becomes much more difficult.

This aint much, but I hope it might help,

John Andreen

"Johan Wagener" wrote in message
...
Sorry that I had to repost this again. I took a closer look at what
happened
and I noticed that there is a neutral wire also going out from the
switcher
PSU. this is what touched the metal body of my amp. So basically the amp
was
connected to the PSU and also the ground was connected to the Neutral of
the
mains.

I have not yet checked the diode but this is where I will start. The burnt
resistor is a 1/4W one so its not a power resistor. There is also a
transistor close to the burnt resistor that seems to have taken some heat.
All smoothing caps seem fine.

I just received news that there is no warranty on the amp so I can now
start
working on it myself.
Any other Ideas?