Carbsandcaps
Super Member
Working on a technics su-v909. The previous owner was charged a $500 “repair” to the protection circuit, there’s some resistors soldered to the board underneath. Both class A and the computer drive bulbs were out. Knowing that they are part of the protection circuit I replaced them. I’m assuming that was the whole problem. Before I replaced the bulbs I did a full recap and all was working fine. Replaced the bulbs and the main power capacitors because they had just come in.
Anyway now I have fluctuating voltages. The negative rail voltages will go to -35v then slowly drop, then pick back up then drop again. The positive rail voltages do the same thing but to about 25v then drop to 16v then pick back up. The AC voltage from the transformer doesn’t change. Seems like something is shutting the dc voltages off. I have 0v on one side of R605 where there is supposed to be 4.7v the resistor tests good but I’m not sure if the 4.7v comes from that resistor or in front of it. Also how much voltage does it take to turn the thyristors on? I have less than 1 volt on the gates when the voltages goes up.
Anyway now I have fluctuating voltages. The negative rail voltages will go to -35v then slowly drop, then pick back up then drop again. The positive rail voltages do the same thing but to about 25v then drop to 16v then pick back up. The AC voltage from the transformer doesn’t change. Seems like something is shutting the dc voltages off. I have 0v on one side of R605 where there is supposed to be 4.7v the resistor tests good but I’m not sure if the 4.7v comes from that resistor or in front of it. Also how much voltage does it take to turn the thyristors on? I have less than 1 volt on the gates when the voltages goes up.