power supply problem?

[algorithm]

I was curious to know if losing voltage lines on the PSU after putting a load on it is always going to be due to the board that PSU is connected with?

I have a PSU that has everything correct until I plug it into the corresponding board.
The ribbon cable goes into a "motherboard" with lots of SMD capacitors, etc.
When plugged in, 3 of the four 5v lines is gone. the only remaining 5v line is the 1st one of the 4, under "MUTE-C". Mute-C read 12v.

It is sporadically switching through things on its own, like the buttons are being pressed when they are not. Leds fade out & in.


Here are the service notes. Any help is appreciated.

http://manuals.fdiskc.com/flat/Roland%20JP-8000%20Service%20Notes.pdf

[mij59]

Hi,

You could check the connectors an solder joints, e.g. CN203 etc.

[algorithm]

Hi,

You could check the connectors an solder joints, e.g. CN203 etc.

Hi,

Thanks!
I will take a look at where the ribbon is soldered to the PSU board.

It is confusing to me how the PSU output is perfect until I plug the PSU into the rest of the machine.
I wondered if it was normal for those 5v lines to not appear, the schematic shows those four 5v lines all together.
That is confusing to me as well, if those points were bridged, I would not get respective 5v readings for each wire prior to connecting the PSU to motherboard..
Unless maybe the motherboard connection is at fault?

Thanks again!

[tautech]

Hi,

You could check the connectors an solder joints, e.g. CN203 etc.

Hi,

Thanks!
I will take a look at where the ribbon is soldered to the PSU board.

It is confusing to me how the PSU output is perfect until I plug the PSU into the rest of the machine.
I wondered if it was normal for those 5v lines to not appear, the schematic shows those four 5v lines all together.
That is confusing to me as well, if those points were bridged, I would not get respective 5v readings for each wire prior to connecting the PSU to motherboard..
Unless maybe the motherboard connection is at fault?

Thanks again!

Wonder if the 4 x 5V lines are this way to help faultfinding?
First suspect would be shorted Tantalum caps.

[mij59]

Hi,

You could check the connectors an solder joints, e.g. CN203 etc.

Hi,

Thanks!
I will take a look at where the ribbon is soldered to the PSU board.

It is confusing to me how the PSU output is perfect until I plug the PSU into the rest of the machine.
I wondered if it was normal for those 5v lines to not appear, the schematic shows those four 5v lines all together.
That is confusing to me as well, if those points were bridged, I would not get respective 5v readings for each wire prior to connecting the PSU to motherboard..
Unless maybe the motherboard connection is at fault?

Thanks again!

Yes very strange, as noted by Tautech, could be a different circuit.
Check with the Ohm meter if the pins are connected , and if there's a short on the  motherboard.
Also if there's a short IC201 should get very hot. 
Check the voltage at the input and output pin of the voltage regulator IC201 when the motherboard is connected.