I am trying to repair a Quantum Audio pre-amp that has no output. There is no power to the opamps. There is a small transformer on the board that is getting voltage on it's primary windings and auxiliary windings but there is no voltage bon the secondary windings which feeds some diodes and capacitors which then power the opamps VCC circuit. The remote turn on circuit appears to be working properly as well as the 3 transistors prior to the transformer. What can the problem be? The buck stops at the transformer but I see no sign of a problem or damage...
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Q3 has 13v on all 3 legs
Q2 and Q1 both have 13v on legs 2 and 3 and 0v on leg 1 which is connected to the diode at D14. I tried changing the 4148 diode and there was no difference.
Q2 and Q1 both have 13v on legs 2 and 3 and 0v on leg 1 which is connected to the diode at D14. I tried changing the 4148 diode and there was no difference.
Some of these self oscillating supplies require a pulse to start. If the large transistor is shorted, the supply may not start.
Did you check the other two transistors to determine if they were shorted?
Did you check the other two transistors to determine if they were shorted?
Are any traces broken on the power connector?
What's the part number on Q3?
Was the voltage precisely 13.00v on all 3 terminals?
What's the part number on Q3?
Was the voltage precisely 13.00v on all 3 terminals?
Desolder one leg of each of the 4 rectifier diodes and check them. Check carefully to confirm that none are leaky.
With them out of the circuit, do you see any oscillation on the power transformer?
Do as you did here and apply remote after you connect B+ and ground.
With them out of the circuit, do you see any oscillation on the power transformer?
Do as you did here and apply remote after you connect B+ and ground.
Desoldered all four diodes and they all read good. 620 or so on diode check one way, OL when reversed.
There is no oscillation on the transformer with the diodes lifted, just straight lines on the scope.
There is no oscillation on the transformer with the diodes lifted, just straight lines on the scope.
When checking for leakage, you have to use ohms. They may be OK but those diodes failed regularly on other crossovers that used them.
See if this will work, try grounding one end of a 100 ohm resistor and tapping the other end to the base leg of one of the driver transistors (Q1, Q2). Have your scope on one of the primary windings and monitor what happens. Does tapping the base cause the supply to oscillate?
See if this will work, try grounding one end of a 100 ohm resistor and tapping the other end to the base leg of one of the driver transistors (Q1, Q2). Have your scope on one of the primary windings and monitor what happens. Does tapping the base cause the supply to oscillate?
Tested the four diodes on ohms and got 0 ohms in both directions for all.
I tapped the base of Q1 and Q2 and got some readings on the scope but only on the 2 outer windings of the secondary ones. There were no reactions visible on the 2 inner windings where they enter the board. There was barely any visible response to a single tap of the 100 ohm resistor connected to ground, so I tapped in quick succession to achieve these screenshots. These were with the diodes resoldered back in.
I tapped the base of Q1 and Q2 and got some readings on the scope but only on the 2 outer windings of the secondary ones. There were no reactions visible on the 2 inner windings where they enter the board. There was barely any visible response to a single tap of the 100 ohm resistor connected to ground, so I tapped in quick succession to achieve these screenshots. These were with the diodes resoldered back in.
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Make sure that the 2 small windings are not broken.
0 ohms is a direct short and would indicate that all 4 diodes are shorted.
The solder connections on many of the components between the transformer and the closest end of the board look dodgy. Desolder them completely, stand the legs up straight and resolder them.
0 ohms is a direct short and would indicate that all 4 diodes are shorted.
The solder connections on many of the components between the transformer and the closest end of the board look dodgy. Desolder them completely, stand the legs up straight and resolder them.
I removed the transformer and checked all the windings. No breaks, continuity along each lead, and no evident cross-conducting.
I retested the diodes with the meter on auto-ranging. They read 1.6Mohms in one direction, and around 60Mohms in the other.
I removed and resoldered all of the components in the power supply area. I removed and tested the capacitors in the area and they are within tolerance.
Can't get this power supply to start, and don't know what to do next. The only action I see with this is when I touch the board /transformer with my fingers, then I see voltage, as much as +7v on the secondary side and the opamps VCC pins...
I retested the diodes with the meter on auto-ranging. They read 1.6Mohms in one direction, and around 60Mohms in the other.
I removed and resoldered all of the components in the power supply area. I removed and tested the capacitors in the area and they are within tolerance.
Can't get this power supply to start, and don't know what to do next. The only action I see with this is when I touch the board /transformer with my fingers, then I see voltage, as much as +7v on the secondary side and the opamps VCC pins...
You may be able to tell something about the condition of the supply by injecting a signal from another power supply. I don't know how badly you want to repair this piece.
When you checked Q1 and Q2, did you check for open junctions and leakage, in addition to checking for shorted junctions?
Ok, so Q1 and Q2 were defective. I don't have KSP06 so I used MPSA06 as the specs were identical. The power supply powers up but not sure if the output voltage is correct. The supply voltage to the opamps is +29.5v when measured across the + and - legs of the ICs. I am just accustomed to 5v and 15v supplies so this may be correct and normal but I just need to be told that nothing is amiss. I haven't tested for sound or anything either...