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

[tepsamps]

Hello,
i'm posting here even if it's not a guitar amp. Sorry...

So i have a digital piano to repair, but it seems to be a power supply issue.
And i've got no special repairing skills...
The transformer gives 36-0-36 VAC, and there are 2 sets of 4 diodes. The diodes are quite big.
When testing the diodes with a multimeter, there are all dead, because there are conductive in reverse direction. And they seem to be the 4 in parallel, not in a bridge configuration.
And the filtering caps seem to have leaked on the board, but again i'm not a repair man, it can be glue from the manufacturer.
So i think i'm gonna remove the PCB to see the hidden side.
But i can read +50 VDC and -50 VDC but the reading is the same at each diode cathode or anode of a set.




My first question : what is this red/brown stuff on the board near the big caps?

Thanks for the help

[xtian]

My first question : what is this red/brown stuff on the board near the big caps?

It's glue.

[The Dude]

What's wrong with the keyboard? What are your symptoms? On many keyboards, the connector that exits the supply is labeled with voltages near the connector itself. If so, check voltages at the connector and see if they match the label. If no label, typically you'll see +&-15V, 5V, and maybe 3.3V. See if you can find those voltages or something close on the connector.

[Jon Snell]

The power supply is a transformer, bridge rectifier and two regulators.
Check there is +15volts on the collector of Q6 and there is -15volts on the collector of Q4.
If there is not, go to the emitters of each and check for voltage on those, still no voltage on both, check the bridge rectifier outputs, if yes at that point, check R3, R4 & R5. If blown, find out why and replace ONLY with the same type and rating!
Before the bridge is a 20/0/20 volt transformer a mains fuse and that is it.

[tepsamps]

Thank you for the support...
The issue is that when i power on the piano, only the indicator led glows. All the rest is dark and there is no sound. The digi board has no voltage from the PSU.
Fuse is OK.
I'm gonna follow Jon Snell advice, thank you guys.
Keep you informed...

After check : R3 R4 R5 are OK, 10K, 3K and 10K and voltage across when in use.
There is no voltage at the input of the regulators, but i can read -0.4 v at the output of Q8, dedicated for the -15 v.

But there is a mess with the big diodes, i don't get it.
After switch off, all the 8 diodes are faulty according to my DMM. After a 1 hour break, D11 D9 D7 and D5 (for the -50 v) are OK but D10 D8 D6 and D4 (for the +50 v) are still bad. And if i remove the 36-0-36 vac plug from the tranny they are good. And if i plug the tranny back, there are bad again.

Jon Snell, have you got the schematic? Or your advices are just from the pics?

[tepsamps]

Hey,
i removed the pcb and the diodes are OK. It's a dual full wave rectifier with 2 // diodes instead of 1. It gives + and - 50 VDC.
I took pictures with a light to see the circuit.


The inputs of the 2 regulators are connected to a transistor, Q6 and Q5. I don't know why and i don't know the pinout. The center leg of Q6 is connected to the center leg of the regulator. Before Q6 i'm lost.
I found R21 faulty with my DMM, i really don't know if my DMM is sometimes bad, but i don't have the same readings always. I'm sure i have seen it to 0 ohm, after infinite, it seems it's OK at 600k. And why i have a negative reading of R112? Negative for a resistor... I don't get it....

Anyway, after the outputs of the regulators, the +- 15vdc goes to an external outlet for the digi board. Many jumpers,

I hope that with the pics you will be able to have an clever idea...

Thank you again!!!

[Jon Snell]

You cannot accurately measure a component whilst in circuit.
The most you can do is verify a short or not on a diode and then sometimes not.
Here is the schematics.
Check the regulators for the correct voltages first. To find the pin outs, look up the data sheet for each component.

If you are getting lost, take it to a repair shop.

[tepsamps]

As i said before, i get no voltage at the input and output of the regulators, and the schematic you posted does not match my circuit.

On the voltage path, there is R112 R21 and R114 R22 that i can't check correctly, because i have never the same readings, it's very strange, with negative resistance...or several megohm, but not always ;(

The repair shop in the area, it's me,

Thank you by the way...

[tepsamps]

So i removed R21 and R22. One is bad, open, one is 3.8 M with my DMM, but the colors are not obvious to check out. They are old, and the resistor has green paint on it.
Could some one help me? One side is connected to the B+ or B- and the other side is connected to the center leg of each regulator.
The issue is : is the 3.8M already nearly open?

It should be orange white green for 3.9M, but it's not... Inside the circuit, i tested it at 5.6K, but it does not match either. It seems to have 2 gold rings.
What do you see?
Many thanks

[dmeek]

They are probably less than 10 ohms. Can you post photos of the area they came from, both sides of the pcb.

Take a photo of the foil side. What are the part numbers of the regulators and transistors Q4,Q5,Q6?

[tepsamps]

I made a schem from the pcb. I don't have the part number of the reguator, it's just black. The resistor is 6.8R and acts as a fuse.

I come back for the photos...

I can't see the transistor for the + side, B1015 maybe?

Well i don't really understand why a transistor, to split input voltage for the regulator (less than 50 V)?

[dmeek]

The maximum input voltage for many 3-terminal regulators is 35V. The transistor/zener pre-regulator drops the voltage to less than 35V.

[tepsamps]

Here there are, it is what you needed?



There is a zener to ground at the output, i forgot it...

[tepsamps]

New schematic and i think i found the guilty. The zener is conductive in both directions. The output is at 1400 ohm from the ground...

[Helmholtz]

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.