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.
The likely cause of this is a dead short in the power supply. I don't receal the specifics of this unit, but there should be a set of four(4) diodes or a single bridge rectifier in the main power supply area. Check this/these for a direct short using a diode checker. Other than that, the output transistors being shorted can cause hte same symptom. Without seeing the actual unit, I can not tell for sure. If you know how, check the output transistors as well. Report your results here.
Connect your voltmeter`s probes on the "emitter" of the two output transistor, or in other words the measurement is made between the emitters of the output transistors.
Turn on the amp but do not let the output get hot.
If only one side was repaired then you could use the good side and measure its biais as reference. Or
Adjust the biais at between .007 to .014 volt, so it is fair to say that at .010 volt you would be in the range.
repeat the same for the other side if needed.
Hope this help and good luck
The fact that the channel works in mono mode indicates that the output amplifier section is working correctly. The problem is in the front end. This can be as simple as a bad control, or the stereo separation circuit. Without seeing the unit and testing some components inside, this will be a difficult problem to solve remotely. The expensive parts are usually in the output amp section, so this repair should not be expensive from a parts perspective. I would budget around $20 for parts and have a repair shop take a look. I work on units of this vintage all the time, and usually the problem is a failing transistor or cap in the front end. Finding it is the problem not replacing it. Look for a shop with experience with this vintage.
this could be the pre amp transistor or the power amp that needs replacing
the pre amp transistor is cheap but the power amp is not
if you did not crank it , then it is probably just the transistor that needs replacing
It sounds like there is a transistor that is breaking down because of heat. With the unit placed away from the amp, there is less heat. This unit needs a quick overhaul. If a tech can reproduce the problem, the use of a "freeze" spray will make finding the problem transistor easy. Figure on $5-$10 in parts plus the local labor rate for the total repair.
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.
You have a shorted output channel. You probably see burnt resistors and very heat stressed areas of the board. That is a sign of the outputs being shorted. Unless you have experience in repaired amplifiers you will not be able to repair this yourself. If you change the burnt resistors, they will most likely burn up again. The output transistors for one of the channels are also shorted. They must be replaced along with every other bad component. If you miss just one of the bad components you will most likely burn up all the good parts you just put in. Resistors must be checked to make sure they are within the rated tolerance, capacitors must be checked for being shorted or being dry, diodes must be checked for being open or shorted, output transistors and driver transistors must be checked for being open or shorted, and the list goes on. It is a very difficult job for anybody not experienced in repairing electronics.
Take it to a reapir center that will work on those type of amplifiers, not all repair centers will.
I am very sorry to give you such bad news, but it is the problem you are having.
If you need any additonal help feel free to ask, if you decide you are going to try and fix it yourself I can tell you what to look for and how to check them, but without the proper equipment you will not be able to do this.
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