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Sansui 5000a questions

Carbsandcaps

Carbsandcaps

Super Member
Just finished rebuilding the driver boards on a 5000a. They are the 1040 boards and I did the updated biasing setup and replaced all the transistors. One channel has some distorted sound and a fluctuating 2-3vdc on the output. It makes a scratching sound over the music. I’ve replaced the output capacitors and thought they are supposed to take care of all the dc voltage on the outputs. It doesn’t seem to be coming from feedback circuit because there’s no voltage on the other side of the feedback resistor.

The other channel works fine until you turn the volume up and the protection light comes on and it shuts down the circuit. I have the bias pots turned all the way counter clockwise which according to the instructions should be lowest bias setting. When adjusting the bias pot and measuring amperage by removing the protection fuse and putting an ameter across the fuse terminals I get no change. It stays very low around .002 amps. There’s also another potentiometer on the driver boards that is nowhere in the service manual that I’d like to know about. Any ideas would be great. IMG_3733.jpegIMG_3734.jpeg
 
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One of the pots is idle current, the other is DC balance. The second one is for centering the voltage on the + terminal of the output capacitor at around half of the supply voltage. If you have the bias turned all the way down, it's no surprise the sound is distorted. Set it as per the manual, and set the DC balance to get the correct voltage, and then try listening again and see if it improves. You should have about 0.023A or so. First of all, what did you use to replace the bias diodes? If they are wrong, it may be why it won't get enough current.

As for the fluctuations, there should be no DC at all on the output since there's a blocking capacitor. Make sure this measurement is taken with a load attached to the amplifier or else it is meaningless. If there is really DC on the output, check to make sure C006 is not installed backwards.
 
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i didn't see this thread till now. amazing job and looks like a lot of work . when i bought mine(5000x) i showed it to a tech when i went to pick up my vintage rca receiver and kenwood kr-720 and told me to check inside when i get home cause he said he had atleast 100 or so come in due to...
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I bought a kit that includes all this for biasing. I double checked everything in the kit it seems ok but the bias pots don’t seem to be affecting the bias current. I’ll have to try again tomorrow
 
@Carbsandcaps always worthwhile checking the multimeter meter fuse that is usually present internally to protect current measurement circuitry (may even be two fuses depending on meter type).

Cheers
John
 
I don't think DC Balance is set correctly. Do you have this under a Dim Bulb ?
What is the voltage on driver board Resistor R827?
 
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