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Dave , If that fuse is blowing , there is a problem in the power supply or output section. If you have a meter that can test diodes / transistors. please check the output transistors first. Most likely one or more are shorted. If they check ok , you should check the power supply diodes .You can order new ones from a number of online stores by simply putting the part number into google. Hope this helps ?
I assume you checked & tested the fuse? so you have power into the supply but not out. "can" capacitors have a triangular wedge in the top of them. If the top is swollen and or has a smudge discoloration it has been over stressed or is worn out. Some hand held testers have a diode check on them. So you can test the diode The IC chip may be the problem and if it is in a chip plug your in luck you can re & re it easy . If not, that will be a tough job and beyond my know how
It could be the diodes, but whatever is the cause it's before the pre-amp section. Generally a loud hum such as that with no effect by volume control, is mostly caused by the filter capacitor in the power supply. And that hum is the 50HZ mains hum getting into your system!
Since you are able to view the output i'm assuming you are using an oscilloscope and only the positive or negative half of the sine wave is displayed. The output stage will be using a complimentry pair of output transistors, one amplifies the positve half cycle the other the negative. These are driven by a driver stage (phase splitter). One side is not amplifying. First measure the pos' and neg' LT supply rails. If ok check
ouput transistors with analog Ohm meter for base / emitter / collector continuity. If the amp uses an output IC and LT supplies are ok then use scope to view input signal, if input ok but output bad then IC is suspect. Since both channels are affected your problem is most likely a missing LT supply rail.
Fuse blowing is due to excessive current drain and the the first clue is the blown fuse itself. If the fuse blows with a flash a bang and splatters copper across the inside of the glass you have a hard blow caused by a dead short on the power supply, check for power switch flashing over internaly. Short circuit main filter capacitors. Shorted turns in power transformer primary winding. If fuse just separates check for short on output transistors C to E. Try isolating fault by disconnecting LT pos/neg supplies to output stage. Also check for partial shorts on speakers and speaker wires using an analog ohm meter on low ohms setting.
This is a standard amp design which should be a relatively simple repair. The output transistors are probably shorted. Check those first. Second, check the rectifier diodes in the power supply. Usually there are protection circuits that will prevent the fuse from blowing if the outputs are shorted.
Let me know your progress. I will attempt to get the service manual for this unit to get you more details.
I am not familiar with the particulars of your specific model, but with switch mode supplies in gereral, look for defective filter caps on the output side or a defective Zener Diode. If you can post a few photos of the power supply section, I may be able to guide you to the likely parts.
This often indicates either a power supply problem or shorted output transistors. In either case, the unit will require replacement parts. If you have a multi-meter that can check diodes, check the rectifier diodes or bridge rectifier in the power supply first. If those are OK, next check the output transistors. If you don't have access to the meter, expect a parts cost in the $15-$25 range plus the local labor rate for the repair. This should not be a difficult repair for any shop that does audio repair.
Hi, This is not a big issue, but you need to be carefull when you fix this. The main supply is ok. You need to look for that they call a Zener Diode. This will be connect in serie with a resistor so the time you going to spend finding this fault will be a lot. Also check your supply to the front panel and see that the supply is good
Were any fuses
blown? Check this first and
foremost before you do anything else. If there were blown fuses, you
then (if you are familiar with
how), need to check the output stages for shorts. NOT generally to
you have suggested.... but more between the center and outside pins
(collector emitter short we are looking for here) To check the output
transistors properly, you need to unsolder leg and
check each of the output transistors junctions base emitter, and base
collector, and also collector emitter for any leakage. Once you find
the offending channel(normally there is only one), you need to then
carefully check the driver transistors also. Pay attention to low value
resistors about the output stages, small signal and zener diodes and if
they are there, any power diodes. If you are unsure of the condition
of a part, then replace it to be sure :)