The driver board keeps burning out transistors when a signal is applied, it had 4 stuffed output transistors when i got it ,i replaced all 8 of these these but it burnt out left side of driver board when signal was applied, i replaced those then i applied power it was ok as before until i applied signal,this time it was right side, 8 transistors burnt out .ihave since found a dicky varistor? double diode? on the regulated power supply & protector board! would that be the cause of my woes? what do you think! i have had some suggestions from ginko,but i think he needs more info (here it is) regards ross
Check for open resitors (All Values)..also make sure that any diode stacks (Varistors) are not open or shorted....mostly make sure you have replaced the BIAS POT! Lastly U could check for open traces and bad solder joints..also look for tiny solder bridges under an eyeloop magnifier on foil side of PCB>especially after replacing multiple components.
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Re: sansui amp au11000
Without seeing the amp nor having the service manual in front of me, it sounds like you have a severe offset problem. There is either an open resistor or transistor in the early stages. With the output transistors removed from the unit, look around at voltage levels to determine where the fault is. I will try to locate the service manual. Can you post the parts that you have replaced?
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When you need to replace the output transistors, do them both at the same time with a matched pair. That way you won't unbalance the power supply and will be assured of the same power out is available on both channels.
most of the time in a sony receiver, when the output transistors are shorted and blowing the fuse, it doesn't take any more than replacing the outputs and fuse then you should be good to go. They don't usually require any resistors or other components to be replaced.
When replacing output transistors on amplifiers it is generally a good practice to replace the output capacitors as well as the biasing resistors.
Just because one transistor is bad does not mean that that is the whole problem.
Look for what caused the problem.
Bad speakers,bad wires,defective traces on circuit board,poor soldering.
Also when mounting the transistor make sure it is insulated and grease it with a dab of silicone heat sink compound.
Whenever I service a board I examine all components for tolerance as well.
A resistor marked at 1% tolerance,example 1k ohm + or - 1% should read no less than 990 ohms and no more than 1010 ohms. Anything else your asking for trouble.
Another point is that some amplifiers use matching pairs.
Pairs complement each other in an output stage and must be replaced in pairs.
I repaired a Sansui Amplifier a while back and it looked as if a power output resistor was burned.
Comparing the two channels I figured the matching value from the other side and replaced all of them and the system has worked flawlessly for years now.
I am a retired electrical engineer and have worked in a few areas of electronics for over 35 years.
Hope this information helps you.
It is possible, but nlikely that the protection circuit is bad. More common are shorted output transistors or defective driver stage transistors. This should not be a difficult repair for anyone familiar with audio amps. Unsolder/remove Q22 and Q23 and measure the voltages at each of the 3 pads per transistor and post them here.
I am working on one right now, it has blown output transistors and several upstream transistors and burned resistors as well. This channel of the amp will need a rebuild.
Use a volt meter and, with black lead on the chassis, check the voltages at the 3 conductor connectors going to the board at the rear speaker terminals. These are the outputs coming from the amp and if there is voltage on any of these (unit at idle, no volume), the protection circuit will keep the output from getting to the speakers. High voltage here means blown amp, most likely.
may i ask you did you put silicone grease in your transistor and apply a good mica insulator in all of your transistor---------look for any burnt resistor or explosive capacitor -----check for scew tie your supply voltage to earth by wrong.