Joba
New Member
Hello I need your help if anyone know to resolve this I will be grateful. My Sansui 8080 wich I love so much has develop a distortion on right channel. Maybe is my fault, I open it because i needed to replace 3 capacitors on tuner power supply board and one of them was bad, after that I wanted to check some other capacitors on main power supply board for esr values, to do so I removed amp driver board because it was on the way, after checking those smaller caps none of them was bad so I left them alone. I decided to reassemble the unit. I put back driver board in socket and I think I saw some sparks (not shore). Unit was power off for a few minutes at the time but maybe main filters did not fully discharge. The unit now fully functioning on low volume but when I raise it a little bit sound is distorted.
I'm struggling to resolve this for a day now and this is what I did so far:
1. Tried to isolate source of distortion so I removed pre-main bridges and realize the problem is in main amp
2. This leads mi to driver board, first checked dc offset about 20mV on both channels...ok, next pull out 10 amp bias fuse for right distorted channel and used my DMM as ammeter, after 10 minutes of warming up I have about 13mA and I can raise it to 16mA at max. Other good channel had about 32mA, Sansui manual suggest 30mA +or- 1mA.
3. Looking at the schematic and measuring voltages on driver board directly, I don't see nothing wrong.
4. Pull out driver board again to check every component in non voltage state with DMM, no short between any transistor or diode, checked every fuse resistor on the board non of them is open or drifted in value too much, other resistors and two big fuses all looks fine.
5. I'm taking the driver board and flipping it for 180 degrees and putting into socket, turn the unit on and problem is still there on the same channel, so distortion did not change channel like I was expecting...now I'm confused is it problem with a board at all?
6. I know that after driver board signal goes to output transistors so I remove them from sockets measured with component tester...no significant difference between gain and voltage drop, put them back but swapped their places on heat sink just in case and nothing.
7. After outputs it only left protection circuit and relay, so I take a shot and desoldered two wires from that board for signal and hooked them directly to speakers bypassing the protection and still the same. At least I know it is not preamplifier or protection board/relay the problem.
Tone controls are clean I use this receiver for years and it worked fine. What else i need to check on driver board? Read about VD1212 diodes are crap, also tried to swap there places on board. For other biasing transistors I don't know replacements. If someone have suggestion how to raise that bias on right that would be outstanding.
Thanks
I'm struggling to resolve this for a day now and this is what I did so far:
1. Tried to isolate source of distortion so I removed pre-main bridges and realize the problem is in main amp
2. This leads mi to driver board, first checked dc offset about 20mV on both channels...ok, next pull out 10 amp bias fuse for right distorted channel and used my DMM as ammeter, after 10 minutes of warming up I have about 13mA and I can raise it to 16mA at max. Other good channel had about 32mA, Sansui manual suggest 30mA +or- 1mA.
3. Looking at the schematic and measuring voltages on driver board directly, I don't see nothing wrong.
4. Pull out driver board again to check every component in non voltage state with DMM, no short between any transistor or diode, checked every fuse resistor on the board non of them is open or drifted in value too much, other resistors and two big fuses all looks fine.
5. I'm taking the driver board and flipping it for 180 degrees and putting into socket, turn the unit on and problem is still there on the same channel, so distortion did not change channel like I was expecting...now I'm confused is it problem with a board at all?
6. I know that after driver board signal goes to output transistors so I remove them from sockets measured with component tester...no significant difference between gain and voltage drop, put them back but swapped their places on heat sink just in case and nothing.
7. After outputs it only left protection circuit and relay, so I take a shot and desoldered two wires from that board for signal and hooked them directly to speakers bypassing the protection and still the same. At least I know it is not preamplifier or protection board/relay the problem.
Tone controls are clean I use this receiver for years and it worked fine. What else i need to check on driver board? Read about VD1212 diodes are crap, also tried to swap there places on board. For other biasing transistors I don't know replacements. If someone have suggestion how to raise that bias on right that would be outstanding.
Thanks
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