Yamaha CS50 PSU troubleshooting

[ferrochrome]

Hello!
I have this fantastic Yamaha CS50, been playing it a lot lately until it suddenly gave out a slightly burnt smell, and the -15V rail completely cut out. +15    6,5    and    -8,5    are all fine.

I've been trying to find the faulty part but ut's diffucult because I don't know much about htis kind of circuit. Its a sensing psu so according to the manual it should never be powered up without a load. So I've always had a it connected to the synth when powered up.

So here's what I've done so far (no improvement):
-Replaced all the electrolytics in the PSU
-Replaced one of the 4558 op amps (OP1).
-Thought one of the 470kOhm resistors was dead so replaced it as well.
-Written down some measured voltages, relative to ground. They are not 100% stable, some values drifting +/- 0,3V

I'm starting to think TR6 is faulty but I'm just guessing here, anybody have any idea where to go from here?

Any replies are highly appreciated- I need my CS up and sounding great again.. skål!

 

[PRR]

> it suddenly gave out a slightly burnt smell, and the -15V rail completely cut out.

This strongly suggests a fault in the SYNTH more than the power supply. The power supply failure is a result of some short inside the synth. Not for-sure, but I'd bet 1.5 of my 2 cents on it.

> Its a sensing psu so according to the manual it should never be powered up without a load.

Not "a load"; it needs 15 and 15S connected; -15 and -15S connected. This is normally done AT the far end of the cable (so the voltage is correct at the synth). For testing, use 100 Ohm resistors to connect each Out to its S. The output voltage will be 1% high due to the excess 100 Ohms, but now you can fix it without smoking the synth (more than it may already be).

Tr6 looks bad and by numbers and by way-it-works if Tr6 is toast then Tr5 is surely toast also. At that point I would replace Tr4 also.

It would be better to replace the Tr5 Tr6 pair (also Tr1 Tr2) with a 3-pin regulator which will SHUT-DOWN instead of burning when load goes short. But that may be an un-trivial hack.

 

[ferrochrome]

Hello!
I have this fantastic Yamaha CS50, been playing it a lot lately until it suddenly gave out a slightly burnt smell, and the -15V rail completely cut out. +15    6,5    and    -8,5    are all fine.

I've been trying to find the faulty part but ut's diffucult because I don't know much about htis kind of circuit. Its a sensing psu so according to the manual it should never be powered up without a load. So I've always had a it connected to the synth when powered up.

So here's what I've done so far (no improvement):
-Replaced all the electrolytics in the PSU
-Replaced one of the 4558 op amps (OP1).
-Thought one of the 470kOhm resistors was dead so replaced it as well.
-Written down some measured voltages, relative to ground. They are not 100% stable, some values drifting +/- 0,3V

I'm starting to think TR6 is faulty but I'm just guessing here, anybody have any idea where to go from here?

Any replies are highly appreciated- I need my CS up and sounding great again.. skål!

Thanks for your answers PRR. I followed your tip and connected the sense inputs to corresponding outputs through each 100ohm resistors. Now +15V rail is a bit higher as expected, and -15V is up to approx 6V positive. The other measurements are about the same as before.

Some of the PSU parts are obsolete and hard to find. For instance the power transistor 2SA745. I found some suggestion for repairing Sansui amps, replacing it with MJ21193G. Do you think that will be a suitable replacement for this circuit as well?

Thanks for the tip regarding replacing TR5 and TR6 with a 3-pin regulator. I would have no idea how to modify for this, please let me know if anyone has a recipe.

 

[ferrochrome]

> it suddenly gave out a slightly burnt smell, and the -15V rail completely cut out.

This strongly suggests a fault in the SYNTH more than the power supply. The power supply failure is a result of some short inside the synth. Not for-sure, but I'd bet 1.5 of my 2 cents on it.

> Its a sensing psu so according to the manual it should never be powered up without a load.

Not "a load"; it needs 15 and 15S connected; -15 and -15S connected. This is normally done AT the far end of the cable (so the voltage is correct at the synth). For testing, use 100 Ohm resistors to connect each Out to its S. The output voltage will be 1% high due to the excess 100 Ohms, but now you can fix it without smoking the synth (more than it may already be).

Tr6 looks bad and by numbers and by way-it-works if Tr6 is toast then Tr5 is surely toast also. At that point I would replace Tr4 also.

It would be better to replace the Tr5 Tr6 pair (also Tr1 Tr2) with a 3-pin regulator which will SHUT-DOWN instead of burning when load goes short. But that may be an un-trivial hack.

I'm thinking about replacing all the electrolytics in the synth for future reliability and maybe also solving the problem that damaged the -15V psu parts.

Can an electrolytic cause a short in the circuit, and then damage the -15V supply or is it more likely an IC or diode or something else that would cause this?

 

[ferrochrome]

I've replaced TR4, TR5, TR6. Voltages are approximately the same as before replacement. Before I start desoldering and checking the diodes and resistors, does anybody have any idea what would cause the defective -15V rail, when it's not the transistors, electrolytics or 4558 IC?

 

[newmaj42]

Did you ever solve this? Same issue