dr*audio
Fish fingers and custard!
When repairing AU-11000 and AU-9900 amps the following work should be done:
I have found the pre - driver transistors in these and other Sansui amps to be failure prone, at least when they get this old. They are the ones that have a tab on the top, but they are not TO-220 case. Motorola used to call this case "Uni-Watt" because they had a 1 watt power rating. I have found that they go intermittent and sometimes fail outright, causing possible damage to the rest of the power amp. I recommend replacing them with KSC3503ESTU and KSA1381ESTU, available from Mouser Electronics.
There is a screw behind the front panel that provides a ground connection for the entire unit to the case. It gets loose and the ground is lost. I solder a braid from the input board (right hand side on the bottom) to the chassis.
Watch for loose or worn out connectors between boards. These cause intermittent problems.
When adjusting the bias, don't use the method in the manual. Instead, do the following:
The bottom panel and feet must be in place for this to work properly.
1. From the top, with a hemostat, you attach alligator clip jumper wires to the emitters of 2 output transistors; one NPN, one PNP on each channel. You can access the emitters on the transistor sockets on the heatsink.
2. Drape the wires over the back of the amp, out of the way, and make sure they can't short to each other or the chassis. Connect your meter to one set of wires for one channel. I use 2 meters so I can do both channels at once.
3. Place the top cover onto the unit with no screws. If you are careful you will not crimp or damage the testing wires.
4. remove the cover over the driver board.
5. Turn the amp on and allow it to warm up for a few minutes, then adjust the idle current for 20mA across the 2 emitter resistors. I don't remember the values of the resistors, but you multiply 20mA X the total resistance to get the voltage reading you want.
6. Take a piece of electrical tape and stick it to the top of the driver cover, to use as a handle. Place the cover over the drive board and attach with one screw.
7. Monitor the voltage and adjust after 10 minutes. Always replace the cover over the driver board.
8. After it has been stable for 30 minutes you are done.
I have found the pre - driver transistors in these and other Sansui amps to be failure prone, at least when they get this old. They are the ones that have a tab on the top, but they are not TO-220 case. Motorola used to call this case "Uni-Watt" because they had a 1 watt power rating. I have found that they go intermittent and sometimes fail outright, causing possible damage to the rest of the power amp. I recommend replacing them with KSC3503ESTU and KSA1381ESTU, available from Mouser Electronics.
There is a screw behind the front panel that provides a ground connection for the entire unit to the case. It gets loose and the ground is lost. I solder a braid from the input board (right hand side on the bottom) to the chassis.
Watch for loose or worn out connectors between boards. These cause intermittent problems.
When adjusting the bias, don't use the method in the manual. Instead, do the following:
The bottom panel and feet must be in place for this to work properly.
1. From the top, with a hemostat, you attach alligator clip jumper wires to the emitters of 2 output transistors; one NPN, one PNP on each channel. You can access the emitters on the transistor sockets on the heatsink.
2. Drape the wires over the back of the amp, out of the way, and make sure they can't short to each other or the chassis. Connect your meter to one set of wires for one channel. I use 2 meters so I can do both channels at once.
3. Place the top cover onto the unit with no screws. If you are careful you will not crimp or damage the testing wires.
4. remove the cover over the driver board.
5. Turn the amp on and allow it to warm up for a few minutes, then adjust the idle current for 20mA across the 2 emitter resistors. I don't remember the values of the resistors, but you multiply 20mA X the total resistance to get the voltage reading you want.
6. Take a piece of electrical tape and stick it to the top of the driver cover, to use as a handle. Place the cover over the drive board and attach with one screw.
7. Monitor the voltage and adjust after 10 minutes. Always replace the cover over the driver board.
8. After it has been stable for 30 minutes you are done.
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