earthsled
Well-known member
Hi folks!
I'm hoping someone can help me sort out this power supply. I have an old CD player I'm repairing for a friend (yes, feel free to laugh). The CD player in question is a California Audio Labs model Delta. Anyway, the unit has 90s era fluorescent display and I expect the driver IC is bad (the display appears to be dead). The display driver IC is a Technics / Panasonic MN15283PEY-1. I've ordered a replacement IC, but I haven't received it yet.
From what I understand, if the driver IC were to fail it would often damage the -33V section of the power supply (see attached schematic). Indeed, I can see that someone has attempted a repair to the components in the -33V rail.
When I look closer, I can see that D306 is not a 6V2 zener like the schematic shows. In it's place is a 17V zener. I assume this change was to adjust the output of this supply from -33V to something closer to -28V.
I'm unable to find a data sheet for the display driver IC, but I have found another service manual that uses it. In this other service manual, I can see the supply voltage for the driver IC is -28.3V. This is why I assume someone was trying to adjust the voltage from the original value.
So here are my questions:
1. I'm struggling to understand why D306 is attached to an AC voltage instead of ground. What is the function for the zener in this position?
2. I gather that the circuit is using a voltage doubler (or tripler ?). In any case, I would assume D305 is being used for regulation. If the original repair was intended to lower the voltage, wouldn't it be a better idea to change D305 to a 28V zener?
3. The voltage on this rail seems to be rising extremely slow - over the span of 60 seconds or more. Is this typical for the way the circuit was designed, or should I suspect other faulty components in the circuit?
Thanks for reading!
I'm hoping someone can help me sort out this power supply. I have an old CD player I'm repairing for a friend (yes, feel free to laugh). The CD player in question is a California Audio Labs model Delta. Anyway, the unit has 90s era fluorescent display and I expect the driver IC is bad (the display appears to be dead). The display driver IC is a Technics / Panasonic MN15283PEY-1. I've ordered a replacement IC, but I haven't received it yet.
From what I understand, if the driver IC were to fail it would often damage the -33V section of the power supply (see attached schematic). Indeed, I can see that someone has attempted a repair to the components in the -33V rail.
When I look closer, I can see that D306 is not a 6V2 zener like the schematic shows. In it's place is a 17V zener. I assume this change was to adjust the output of this supply from -33V to something closer to -28V.
I'm unable to find a data sheet for the display driver IC, but I have found another service manual that uses it. In this other service manual, I can see the supply voltage for the driver IC is -28.3V. This is why I assume someone was trying to adjust the voltage from the original value.
So here are my questions:
1. I'm struggling to understand why D306 is attached to an AC voltage instead of ground. What is the function for the zener in this position?
2. I gather that the circuit is using a voltage doubler (or tripler ?). In any case, I would assume D305 is being used for regulation. If the original repair was intended to lower the voltage, wouldn't it be a better idea to change D305 to a 28V zener?
3. The voltage on this rail seems to be rising extremely slow - over the span of 60 seconds or more. Is this typical for the way the circuit was designed, or should I suspect other faulty components in the circuit?
Thanks for reading!