Soundtrack sequel psu breakdown

duende · Oct 10, 2020

Hi,

After a PSU capacitor recap on the Seqel 2 psu board the + 17v and the 5V don't work. All the components are ok visually.
The transformer secondary are 24v, and after the diode bridge i mesure 33v and 0V. I don't understand how work the IC L4977A.

I mesure 33v at D1 negative and 0V after the +.

Have you have some info on this circuit ?

Thanks.

duende · Oct 10, 2020

Better view...

Khron · Oct 10, 2020

What exactly do you mean by this? ???

duende said:
I mesure 33v at D1 negative and 0V after the +.

For what it's worth, the datasheet can be found here:
https://www.mouser.fi/datasheet/2/389/cd00000084-1795415.pdf

pucho812 · Oct 10, 2020

are you sure you have the correct polarity on the capacitors?

duende · Oct 13, 2020

Hi,

Yes the polarity is correct... And the -17V is ok .

I mesure 33V at D1/R2 and OV after D1/C2.

I have changed the diode bridge the last week but the problem is the same.

Sometimes when and i turn up the power the 5V led are illuminated during 1 second, maybe a capacitor charge ?

pucho812 · Oct 13, 2020

shotgunning parts is not a good approach. having the -17 volts as o.k. and the other not o.k. could still be related to caps. You also may have a shorted transistors, a shorted resistor or any number of components that have failed. You may even have a failed IC L4977A. hard to say. The Schematic is hard to read. the IC L4977A looks to be a regulator. The pin out was provided. Should be easy to check if that is not working. However if you are not sure about it, I would take it to a repair shop. We doin't want you to hurt yourself to cause further damage.