+3 Power Supply

[krypdtocken]

Hi

My +3 power supply has 10,9v on the 5v output.
T2 was shorted, replaced but no change.
The coils all show the correct AC voltage.

Any ideas?

[1024MAK]

It’s normally T1, T2 and sometimes IC1 (the uA723N).

With T2 not fitted, T1 should be off, then the +5V should be at or near 0V. if it’s not, T1 is faulty.

Before changing IC1, keep in mind that it compares the reference voltage on pin 5 with the feedback voltage on pin 4. If either of these is too far away from the other, it will drive T2, and hence T1 hard on.

Hence a problem with the circuitry connected to pin 4 or pin 5 could also be the cause of problems.

There is a schematic here. Keep in mind that not all units may be exactly the same.

Mark

[krypdtocken]

Thank you for the “pointers”, pin 4 showed 7.5v as I remember it.
I will dig a bit deeper tonight.

Kristoffer

[1024MAK]

Pin 4 is feeding back a proportion of the actual voltage from the +5V line, if the +5V line is higher than it should be, the voltage on pin 4 should be higher than normal. Similarly, if the actual voltage from the +5V line is lower than it should be, the voltage on pin 4 will be lower than normal.

Mark

[krypdtocken]

Hi

I removed T1 and measured it out of circuit and it’s OK.
Measured pin 4 and 5 as follows ,4, 7.5v and pin 5.7

I’m leaning towards a faulty IC1, pin 10 show 12.2v( not sure if that is a fault though)

Kristoffer

[krypdtocken]

Conclusion, T2 & C1 failed.
I now have 5.00 v !