Yamaha SPX90

[lowell]

Need a schem. The unit does not power up. I've done my best w/o a schem: checked fuse, mains voltage, resistors, caps in PS to no avail.

[Enzo]

That has the switching power supply, doesn;t it?

[lowell]

I believe so...

[g1]

Do you have +/- 18 and 5V coming out of the supply board at connector?
The supply board and switcher transformer were already extinct back before the turn of the century!

[Enzo]

OK, remember this is a switcher, it runs DIRECTLY off the mains. DO NOT connect your scope or other ground to ANY point on the left side of the transformer.

Disconnect from power completely. Measure C11 for voltage, it could have 170VDC stored.

If the caps are all discharged, set your meter to diode test and check every transistor in there - three of them. Then check every diode, less than a dozen.

Resistors. Make sure R2 is not open, and also R4,R5.

Rectifiers. we already checked them, but a shorted rectifier on the secondary side will shut down a switcher.

Connect the mains to the power supply, but do not connect the secondaries to the rest of the SPX90. Does the SMPS make voltages? +5, +/-18?

CAREFUL, does C11 have about 170VDC across it? If so, then hold a scope probe NEAR the little transformer T1. DO you pick up any high freq activity radiating?

[jrfrond]

OK, here's the secret to servicing a dead SPX90:

First of all, the cause: Yamaha used an adhesive to lock down components on the power supply PCB. The adhesive carbonizes after many years and becomes conductive, creating phantom resistors in the circuit where they don't belong. What winds up happening is that filter caps dry out, diodes tend to overheat and open up, and even some component leads corrode.

The first thing you must do is scrape away all of the carbonized brown muck (it was originally off-white). Get under the components with a dental pick and get it ALL out. Then clean the board with flux remover and an acid brush. After you've done that, look at the filter caps on that side of the circuit. If any are swollen, they need to be replaced with good, high-frequency 105°C caps. If all look good however, power up the unit. After scraping the glue off, there is decent chance it might actually work. If not, replace the caps and all of the high-efficiency rectifiers. That is usually the problem.

[ne0]

It's the same schema for the SPX 990?

I need this schema for one of my client. thanks in advance!!

[lowell]

cleaned all that crap outa there. Replaced C11 as it is bulging. Bridge rectifier is bad. Didn't check that before. Also R2 is open, D5 tests at .021 Vdrop out of circuit, Q2 bad.

What is D5? It just has an "H" on it. Also Q2 has T6E 01207 on it but can't seem to find it online.

[lowell]

Anyone be of assistance on D5 and Q2?

[lowell]

Ok I found Q2 replacement. I've replaced the above components and used my bulb limiter, it glows as if it's plugged directly into the wall. This is apparently a switch mode supply so I'm not sure if the limiter should glow bright or dim as I'm not familiar w/ switch mode supplies. I don't wanna plug this thing in until I hear back from someone about this.

[Jazz P Bass]

Yamaha SMPS

I do not think the lamp should be brilliant.
Means current draw.
I would go back & recheck all the components.
Strange that Q2 blew. I would think Q1 is the power transistor.
Rant: why isn't there a schematic symbol to indicate transistor package?
Check the complete comparator circuit including the opto PC1.
Especially the transistor side of PC1.
Edit: Had Q3 as power transistor.
Need new glasses.

[zero cool]

I do not think the lamp should be brilliant.
Means current draw.
I would go back & recheck all the components.
Strange that Q2 blew. I would think Q3 is the power transistor.
Rant: why isn't there a schematic symbol to indicate transistor packge?
Check the complete comparator circuit including the opto PC1.
Especially the transistor side of PC1.

Q1 is the main transistor, Q2 is the driver to Q1

Q3 is in the comparator circuit on the secondary side

[Jazz P Bass]

Yamaha SMPS

Thanks for the clarification on the power transistor.

[lowell]

you're right Q1 is indeed bad. It has a "T" on it, then C2555. What is this? I cannot find it.

[Jazz P Bass]

Yamaha SMPS

Lowell
2SC2555
Glad to hear you found something.
Don't forget to check the comparator circuit & the opto.
That is the loop that keeps things under control.