Yamaha Disklavier power supply help - schematic attached

[VintageValves]

Hello! Long time lurker who has fixed a lot of electronics with help I've found here. My ability level is decent. I can almost always solve problems when there is a smoking gun (burned parts, blown fuse, etc). I can often solve issues with no visual clues by reading a schematic. This power supply is a little beyond my grasp.

Where I'm at: all fuses check out good. None of the protect relays are closing. I've been concentrating on trying to trace out RY1 in the upper left of the schematic. Q1 has 17.5vdc on it No power on the Q2/Q3 darlington pair. I'm having trouble understanding how they are supposed to be powered. It looks like it connects to the 100V rail near Fuse F2 on the right side of the schematic. Does it get dropped down by the 100k resistor (R1)?

I'm only seeing low voltage on the C13 side of the bridge rectifier. I think that should be closer to 100V. It starts out at 15V and slowly charges the caps. That seems to make sense because RY1 is not closing to put power into that part of the circuit.

As you can see, I've worked my way into a circle unable to figure out why RY1 won't close.

Schematic for the PSU is on the lower left of the page.

[Enzo]

Q1 has 17v on it WHERE? Q1 has three terminals. RY1 will never function unless that voltage appears on the collector. We have to turn on Q1 for that to happen. Follow the base lead over to SW+, right next to it is SW-, which leads me to think the power switching shorts them together?

[g1]

Also, if you are not familiar with the special safety considerations for SMPS type supplies, please do some reading about them.

[VintageValves]

Hi Enzo. Thanks for the reply! Q1 has 17'ish volts on the Emitter (connected to D2), and on the base. 0.0v on the collector as you surmise. This power supply is from of my Mother in Law's piano and it is all I have in my possession. The piano, and computer box is a couple hundred miles away, but I'll be visiting the piano and in laws for the holidays.

It was the piano technician who told her/me it was a power supply issue.

I gather that you suppose the SW+ and - are indicative of a SWitch. The SW terminals are on a ribbon connector, which leads to another chassis that I don't have.

Just to be clear, the next step may be jumpering the SW+ and - terminals and see what the supply does?

Q1 has 17v on it WHERE? Q1 has three terminals. RY1 will never function unless that voltage appears on the collector. We have to turn on Q1 for that to happen. Follow the base lead over to SW+, right next to it is SW-, which leads me to think the power switching shorts them together?

[VintageValves]

I definitely would like to learn more about these switching supplies. Do you mean that there are special safety considerations for working on them, or safeties inherent to the electronic design? Or both?! I'm fairly comfortable working around high voltage supplies, having fixed quite a few tube amplifiers.

Where is a good place to start?

[VintageValves]

Just to update, I followed the schematic to another page (not on the small PDF I posted) and the PS terminals do indeed lead to the power switch board. Momentarily touching the contacts does cause at least one of the relays to trip. Time for more checks.

I'll also say that what caused this system to malfunction was a the piano being in a bad highway accident while being transported. It's very possible that this power supply will end up being fine and the problem being with another part of the system. Maybe just a connector unplugged in the box I don't have here!

I gather that you suppose the SW+ and - are indicative of a SWitch. The SW terminals are on a ribbon connector, which leads to another chassis that I don't have.

Just to be clear, the next step may be jumpering the SW+ and - terminals and see what the supply does?

[Jazz P Bass]

I definitely would like to learn more about these switching supplies. Do you mean that there are special safety considerations for working on them, or safeties inherent to the electronic design? Or both?! I'm fairly comfortable working around high voltage supplies, having fixed quite a few tube amplifiers.

Where is a good place to start?

Here is a good primer: SMPSRM-D.PDF

[VintageValves]

Thank you Jazz P Bass. I'll need some time to read that carefully and digest it.


With RY1 closing, I am getting power to more of the circuit now, but not much more.

I find 170V on the filter caps C13 and C17. I get power to the primary side of T2 as follows:
1. 160 vDC
2. 160 vDC
3. 155 vDC
4. 153 vDC
5. 151 vDC
6. 152 vDC
7. 123 vAC
8. 123 vAC

I find no voltage (AC or DC) on the secondary side. Of course the transformer won't pass DC... Seeing all that DC at the input of a transformer is suspect to me. Is IC1 suspect? I pulled the heat spreader off and it looks like a modular device called 'Power Stream.' Working on extracting it to see if I can find a part number/datasheet.

Here is a good primer: [ATTACH]46021[/ATTACH]

[VintageValves]

IC1 is a Powerstream XK413A0 - Can't seem to find a datasheet... Looks like they are available for $50'ish if necessary. Should I be checking something else first?

Is IC1 suspect? I pulled the heat spreader off and it looks like a modular device called 'Power Stream.' Working on extracting it to see if I can find a part number/datasheet.

[Enzo]

Your friend diagnosed a power supply problem, but he didn;t understand the system it is part of. I surmise a switch elsewhere just because Yamaha usually labels things intelligently, and the circuit made sense. If a relay clicked, make sure it is the relay we have been looking at. That relay closing applies power to the primary circuit, then there is right after it a soft start relay, which the triac shunts after power comes up.

[VintageValves]

Yes, the report of a faulty power supply should definitely be taken with great suspicion. The person is an instrument tuner / technician, who likely has zero electronic knowledge. I'm not sure if he has even worked on Disklaviers before. I suspect that the piano did not turn on and he immediately suspected the power supply and gave up.

RY1 is certainly the relay which is closing when I short the SW terminals as evidenced by power being measured in first filter caps. I'm assuming that RY2 is the soft-start relay you have mentioned. Is it correct to say that in order for RY2 to function, there would need to be power on the secondary side of T2? I don't seem to be getting any there as mentioned above. Thanks for your continued guidance.

Your friend diagnosed a power supply problem, but he didn't understand the system it is part of. I surmise a switch elsewhere just because Yamaha usually labels things intelligently, and the circuit made sense. If a relay clicked, make sure it is the relay we have been looking at. That relay closing applies power to the primary circuit, then there is right after it a soft start relay, which the triac shunts after power comes up.

[VintageValves]

I should also note that the schematic posted is not for the same exact model. The service manual for the piano this came from (a Mark IIXG) does not have a schematic for some reason. I believe that it uses the same or nearly the same power supply as the one I shared (MX1Z) because the layout diagram in both manuals looks to be the same. Further, the next generation Mark III piano service manual has the same layout and schematic as the MX1Z).

What is a little puzzling is that all three of the layouts show IC1 being a 16 pin device (which it is on my board), but the schematics for MX1Z and Mark III show 21 pins? I'm also a bit puzzled about C16 - I can't find on the schematic. It's connected to the bridge rectifier (D4) and pin 15 of IC1. Big filter cap the same size of C13. Looks to be a negative rail.

Any thoughts and guidance are appreciated.

[Enzo]

Remember we mentioned the danger? it stems from the fact the mains are directly rectified into DC. Generally that means a result of about +170vDC and -170vDC with respect to earth. Moreso, the control circuits on the primary side are using the -170 rail as its reference or common. In other words the primary side "ground" is really -170vDC with respect to earth.

So one could unsuspectingly clip a scope probe to what looks like "ground" and either short out the supply through the earth connections of scope and unit, or if the scope is isolated, put -170vDC on the frame of the scope. Not saying you did that, but your voltages in post #8 make me think you used chassis ground - earth - as your reference for the readings. real readings would be made against that -170v rail.

The secondary side IS referenced to earth/chassis/ground.

One thing I do is lay my scope probe right against the T2. Not making electrical contact, just picking up any radiated field. I do this to detect if any action is occurring there.

[VintageValves]

Thanks Enzo for that information. I definitely could have made a mistake without it. I did gather that the mains were being directly rectified with no transformer isolation! Thus far all my measurements have been with a DMM, using the PCB earth points as reference. I do have a scope (Analog, Hitachi V-750F), but I mostly have used it for tracing out audio paths in amplifiers. I did try to view a few of the reference traces on the schematic on the secondary side. Of course, the secondary side is not turning on, so I don't think I did any harm! I'll retake my readings in post #8 referencing the -170 rail and post results later. I'll also see if I get anything by laying my scope probe over the coils of T2.

What is the function of IC1 just before the primary side of T2? Stock photo from Google images:

Remember we mentioned the danger? it stems from the fact the mains are directly rectified into DC. Generally that means a result of about +170vDC and -170vDC with respect to earth. Moreso, the control circuits on the primary side are using the -170 rail as its reference or common. In other words the primary side "ground" is really -170vDC with respect to earth.

So one could unsuspectingly clip a scope probe to what looks like "ground" and either short out the supply through the earth connections of scope and unit, or if the scope is isolated, put -170vDC on the frame of the scope. Not saying you did that, but your voltages in post #8 make me think you used chassis ground - earth - as your reference for the readings. real readings would be made against that -170v rail.

The secondary side IS referenced to earth/chassis/ground.

One thing I do is lay my scope probe right against the T2. Not making electrical contact, just picking up any radiated field. I do this to detect if any action is occurring there.

[Enzo]

IC1 is pretty much the whole primary side. It is one I have not worked with, but looking at it and hte schematic, it is a hybrid like one of the old STK series audio power amps. It would have the control circuits as well as the transformer drive transistors all in one part.

Your meter will be OK for DC readings, but unlike a linear supply that works at 60Hz or 120Hz, switching supplies work at very high frequencies, like many kilohertz, and on AC it is likely your meter wouldn't respond to it anyway. This is scope terrirtory.