pcb board recover help

[baristini]

Hello to everyone.

I have a pcb board that stoped to work. I can find the problem...

Can anyone help me the to find the fault?

In attach i have the schematic of the board.

When conecting the board to power (9.5volt) i get power (9.5V) before D1 diode, but 0V after this diode.

Already replaced this diode and same result.
Disconected TR8 TR9 and LA4700 ( pins 14 and 5 ) and still have the same result....
TR8 and TR9 seem good. Replaced also LA4700 amp.

if someone have a clue or test to perform please let me know...

Thanks all!

 

[baristini]

Well, in fact i was using a wrong PS.

Now, with the right PS i found that TR9 has no power in it. TR8 has 9V in pins 1 and 3, but no power ( 0v ) in pin 2 (colector ).

My doubt is, does TR8 needs TR9 feeding power to work?


Or maybe TR8 is faulty?


Curiously, in one of many measures, TR9 showed 9v on pin 2 and 3 and voltage regulators IC19 and IC23
did work ( 4.98v ).


PS. When i bypassed D1 i did injected +9V, not -9V.

Thanks everyone.

 

[rjenkinsgb]

My doubt is, does TR8 needs TR9 feeding power to work?

TR8 is the on/off power control.

It should be turned on by either TR13 via the on/off switch, or by TR9.

Grounding either side of D4 should cause TR8 to turn on.

Without the turn on signal, both the emitter and base of TR8 should read at near positive power voltage.

 

[baristini]

TR8 is the on/off power control.

It should be turned on by either TR13 via the on/off switch, or by TR9.

Grounding either side of D4 should cause TR8 to turn on.

Without the turn on signal, both the emitter and base of TR8 should read at near positive power voltage.

Thks!!!

 

[baristini]

So, as i understand in standby mode ( without it power button ), TR9 should be on when conecting the board to power supply. Right?

Or not?

 

[rjenkinsgb]

TR9 should be on when conecting the board to power supply. Right?

The signal to TR9 comes from some other logic, so no idea!

In any normal use where the power button should turn it on & off, TR9 should be off.

It could be used to do an ordered shutdown? eg. when the switch is turned off, the CPU can keep power on briefly to stop any processes or MIDI notes playing etc?
(There is a signal from TR13 collector which could be used by the CPU system to sense the power switch state).

 

[baristini]

Well after few tests on this board, in 2 times the board came to live, but disconecting power and conecting again it didnt start again.

When it turned on, i measure TR9 and found that it has 5v on its base, so voltage drops on TR9 colector near to 0volt
making TR8 turns on and feeding 9v to the board by its colector!!

So i think maybe TR8 is faulty or even the processor or maybe conections... i dont know!

What do you think?

THX!

 

[rjenkinsgb]

TR8 is working correctly, from what you describe; remember it is a PNP transistor with its emitter to incoming power.

TR13 is the one directly controlled by the power on/off switch - TR9 should not matter, for the keyboard to turn on.

The power switch is fed by the positive power from after D1, and when the switch is on, that should be applied to the digital transistor TR13 base, causing its collector to go low and turn on TR8 via D4.

 

[Nigel Goodwin]

Well after few tests on this board, in 2 times the board came to live, but disconecting power and conecting again it didnt start again.

When it turned on, i measure TR9 and found that it has 5v on its base, so voltage drops on TR9 colector near to 0volt
making TR8 turns on and feeding 9v to the board by its colector!!

So i think maybe TR8 is faulty or even the processor or maybe conections... i dont know!

What do you think?

THX!

I think that it's VERY, VERY, VERY rare for a processor to be at fault - they get changed a lot, but it almost always never cures the problem. Assuming the processor isn't doing what it's supposed to, then it's usually external influences, such as the reset or clock circuits, or something holding a pin at the wrong polarity, and the processor is waiting for the pin to be correct.

The circuitry around TR8 is absolutely simple, measure the voltage on TR9 collector - if that is low, then TR8 is switched ON, if it's high, then TR8 is switched OFF (this would be the normal powered off condition).

Either TR9 or TR13 (base high or collector low) can turn on TR8 - if the collector of TR9 is high, then check the bases of TR9 and TR13.

 

[baristini]

Thanks a lot for your help!!

It's already working.

In standby state TR8 is off.

When you press the on/off button then the TR9 turns high, causing the TR9 to go hight too, powering the entire PCB.
Thank you so much for your help. Have a great weekend!