Digital piano repair : technics SX PX336 : help needed on PSU

[tepsamps]

It doesn't make sense to wire a zener across a regulator output.

Hi Helmholtz, that's what i was wondering... gonna check the solder side, but my DMM told so...

The icone is a zener. On the other side, for the +, the diode is other direction and seems ok. Maybe it's just a diode...

They are D14 and D15 on the previous picture...

[dmeek]

I've lost track of what the problem is but here is the schematic of the 15V regulators. It doesn't use 3-terminal regulators. Can you post a wider photo of the foil side and try to keep the camera parallel with the board and well lit.

[dmeek]

This is from the Technics SX-PR603 manual. Very similar.



MA4160 is 16V, MA4180 is 18V

[Helmholtz]

Ok, so the output zeners are not normally conducting. Probably serve some protection purpose.

[g1]

I didn't follow the thread but It doesn't make sense to wire a zener across a regulator output.
The rest of the wiring doesn't look right either.

It is unusual, but look at the schematic Jon posted in post #7. It's a different model (332 instead of 336), but I think the circuit should be similar.
(edit: never mind, I missed some posts)

[tepsamps]

This is from the Technics SX-PR603 manual. Very similar.



MA4160 is 16V, MA4180 is 18V

Great man, it helps a lot...
Here is the entire board

[Helmholtz]

It is unusual, but look at the schematic Jon posted in post #7. It's a different model (332 instead of 336), but I think the circuit should be similar.
(edit: never mind, I missed some posts)

Yes but these are 18V zeners, so they are not conducting and not contributing to regulation at normal operation.

[dmeek]

Another thing to check is the "IC Protectors". They look like transistors and are marked N10, N75 etc., they are fast acting, low current fuses. This board has a lot of them.

IC_Protectors_TO92.pdf

[tepsamps]

Another thing to check is the "IC Protectors". They look like transistors and are marked N10, N75 etc., they are fast acting, low current fuses. This board has a lot of them.

[ATTACH]n963491[/ATTACH]

I saw only 6 and they are good. Maybe the digiboard or control panel is shorting somewhere...

[tepsamps]

Hi,
i have 2 questions about this schematic :

why using fuse resistors instead of just fuses?
why using this king of regulators with a lot of component, instead of a 12V regulator and a 3v zener to ground to go to 15 v ?

Thanks

[Enzo]

Because they wanted resistors. By using fusible ones, they wouldn't need a resistor AND a fuse.

Why the discrete regulator? I don't know. The circuit is basic and works. Perhaps a three-terminal regulator like a 7815 wouldn't handle the input voltage. Perhaps it responds better to circuit demands. I just accept it was a design decision.

[tepsamps]

Hello again, i changed Q1, Q2 and D14.
i still have a short between the +15 and the ground. I removed D12, C7 and C11, they are good, and i have 104 ohm between output and ground. Nothing is plugged to the board.

The -15V works...

Any idea?

[dmeek]

Something on the board is shorted, at least to 104 ohms. You will have to follow the +15V trace to wherever it goes. Lift any jumpers or cut the trace to find where the short is. Could be a bad capacitor or chip.

[tepsamps]

OK, i have started the job, but it 's not easy... What would mean +15M and +15H?
It's on the schematic, but i don't know. Have you got the entire schematic? PMUT is labbelled on the board, and the 15V is close.

[dmeek]

Start at D14. You have a short of 104 ohms. Lift jumper J148. Which side of the jumper is the short on? You don't need a schematic to find the problem, just follow and isolate the trace as you go.
The schematic I have is for the SX-PR603, the board is different.