The 21.3 volts at the 7805-Input to 7805-Common was a big surprise to me...

If you can, check the voltage being supplied by the brick while under the load of the Yamaha. I wouldn't be surprised if you see 16V or more. Granted, 21.3 is still a bit of a shock. It might be causing the 7805 to overheat and shut itself down. Double check that the brick output is really supposed to be DC. Better yet, try another power brick if you can.

 

With the unit plugged in and turned on, measure the voltage at the power in connector circled in red. You can use either ground shown, circled in black. Then measure the voltage to ground on both sides of that largish diode just above and to the left of the toroid inductor.

 

Found the specs for the original power supply and it is 12VDC 2500mA! This power supply is only 1000mA!!!
Would I be right to guess that Over Current Protection in this undersized power supply could be the cause of the power shutdowns?

So, it looks like I need to get a properly sized power supply before proceeding with further testing. I will do this ASAP.

(I noted that power supplies listed on eBay for the Williams Allegro had their output specs redacted so that savvy shoppers couldn't find out the specs and buy a suitable substitute at a thrift store. But I kept looking and found a power supply on Amazon where I could read the output specs.)

 

Power supplies like that in the photo are typically unregulated and specified at full load current. With no load or very light load, the output voltage will be near the peak of the AC voltage from the transformer. The ratio of peak to average voltage for a sine wave is about 1.57, so for an average voltage of 12 DC, the peak would be nearly 19 V. When the allowance for the transformer output voltage dropping under load is added, 21.3 isn't too surprising. The power supply was made by Ault for 3COM and I suspect is quite good quality and the rating is conservative.

The 1000 mA power supply is probably OK for testing if you keep the amplifier volume turned down.

The 7800 series of voltage regulators have over-temperature protection and will shut down if they get too hot. That is a possibility, but I think it fairly unlikely. It would turn on again after cooling for a little while and the shut-down temperature is high enough that you wouldn't be able to hold a finger on it.

 

There is behavior here consistent with capacitor "wear out" in conjunction with the under-rated power supply.

The digital circuitry will require a "power on reset" (POR). When power is first applied, the digital circuitry must be put into a known state. This is often accomplished by a fairly simple circuit that relies on the charging of a small capacitor. Unfortunately, the simple circuits don't always cope well with a brief power outage. The circuitry quits working correctly with the brief outage but the POR doesn't generate a reset like it should, so everything just sits there (I have an Ethernet switch with this problem - if I get a brief mains power blip, I have to manually cycle the power for the switch to bring it back to life.)

As capacitors "wear out" the capacitance drops. This can mean that instead of well-filtered or "smoothed" DC, you can get the voltage dropping to something very low each time the AC voltage approaches and passes through zero. This high "ripple voltage" would make serious hum in the audio circuit and if the voltage dropped sufficiently the digital circuitry could get into the brief power outage condition.

When you made sparks at the 7805, you probably short-circuited it long enough to get a POR when the short cleared, and things started to work again.

Evaluating the ripple voltage on the capacitor is what I had in mind when describing how to check your meter to see how it responds to pure DC with on the AC ranges.

 

There is behavior here consistent with capacitor "wear out" in conjunction with the under-rated power supply.

The digital circuitry will require a "power on reset" (POR). When power is first applied, the digital circuitry must be put into a known state. This is often accomplished by a fairly simple circuit that relies on the charging of a small capacitor. Unfortunately, the simple circuits don't always cope well with a brief power outage. The circuitry quits working correctly with the brief outage but the POR doesn't generate a reset like it should, so everything just sits there (I have an Ethernet switch with this problem - if I get a brief mains power blip, I have to manually cycle the power for the switch to bring it back to life.)

As capacitors "wear out" the capacitance drops. This can mean that instead of well-filtered or "smoothed" DC, you can get the voltage dropping to something very low each time the AC voltage approaches and passes through zero. This high "ripple voltage" would make serious hum in the audio circuit and if the voltage dropped sufficiently the digital circuitry could get into the brief power outage condition.

When you made sparks at the 7805, you probably short-circuited it long enough to get a POR when the short cleared, and things started to work again.

Evaluating the ripple voltage on the capacitor is what I had in mind when describing how to check your meter to see how it responds to pure DC with on the AC ranges.

EBP,
Some very useful info in your postings. I will be performing more tests, per your suggestions, very shortly.

UPDATE: I ordered a 12VDC 2000mA power supply that will come with various barrel connector tips of different sizes. This will have plenty of current headroom if 1000mA is just not quite enough. (Info found on internet was not consistent, and Williams was of no help either. They sent me to authorized repair partner and I got voicemail. Authorized repair will probably not want to give me a schematic, and definitely not a service manual - they just want as much money as they can squeeze out of me or convince me to buy a new Williams piano. I really hope Right to Repair succeeds with legal requirement for companies to provide schematics to customers who request it. Especially for legacy devices that have been discontinued.)

I am learning a lot from you guys on this troubleshoot. This is my Christmas present for 2018. So thanks!

 

I'm curious about the voltages on each side of the diode above the toroid to determine if it may be being used as a rectifier, or if it is configured as a protection diode. ...or perhaps something else.

 

I really hope Right to Repair succeeds with legal requirement for companies to provide schematics to customers who request it.

Not to stray too far off topic, but I don't think those two things are connected. They may have to allow you to make alterations or repairs if the law passes, but they won't have to help you do it. I admit I haven't been following this much.

 

Not to stray too far off topic, but I don't think those two things are connected. They may have to allow you to make alterations or repairs if the law passes, but they won't have to help you do it. I admit I haven't been following this much.

There is a little background material on Right to Repair as applied to electronics at https://en.wikipedia.org/wiki/Electronics_right_to_repair.

Since I bought an LG V20 smartphone because it had an easy, user swappable battery, I feel sorry for iPhone users that they chose a phone & an OEM that intentionally does whatever it can to reserve all repair rights to its authorized service partners - even the mere replacement of a battery. So, that is, perhaps, not the best example for Right to Repair in the electronics field.

However, there are, I think, good arguments in the proposed legislation for discontinued products that are no longer supported and would have to be junked if the user cannot repair them. Surely posting the schematic diagrams online in PDF format would not be too much to ask. I would argue that the service manual for discontinued electronic products should also be posted online in most cases. Hopefully there will be growing pressure on lawmakers to come up with a reasonable Right to Repair legal framework that protects truly proprietary OEM information while requiring basic maintenance & repair information to be posted online indefinitely for public access.

Imagine if the car companies told Chilton's that all information on their cars is proprietary and car owners shouldn't be able to get maintenance & repair manuals on any cars. I think consumer electronics should be considered just like cars when it comes to user maintainability & freedom to repair. But, at the moment, we are stuck with the current system of information withholding and repair monopolization, unless we can find ways to work around OEM intentions and enable consumers.

 

I have prepared for my next batch of test measurements based on all of your suggestions.
I will be making those in the morning when I'll be fresh and less likely to short any pins during testing.

NOTE: I do have an old Harmon Kardon Oscilloscope, which might come in handy after the VOM measurements are well documented.
(I'll need to learn more about Oscilloscope procedures to be able to troubleshoot with it, and it doesn't have any memory so the best I'll be able to do is photograph the scope tracings.)

HYPOTHESIS: By the way, I am attaching and updated, annotated photo of the audio pcb, as well as some component closeups. I am wondering if the Main Power Jack was either resoldered or replaced by the previous keyboard owner because of the large pool of glue seen in the photo. I am guessing that they had a power problem and ended up with a new power supply and a new/repaired power jack but when that didn't completely fix the audio problems they then donated it to the thrift store. (I did observe that jiggling the power plug in the jack could cause a power reset.)

QUESTION: can I get a reading from the positive portion of the power jack at the metal tab seen at the back of the jack in the photo?

With gratitude for your help,
Alan McRae

 

Yes, you can certainly measure right at the power jack.

The "common mode choke" (the green doughnut with two windings) looks like it is between the jack and the rest of the circuit (it's purpose is radio range high-frequency noise filtering). There will be a little bit of voltage drop across it for both the positive and negative.

I think that diode is most likely in series in between the positive from the jack and the power switch. It is an ordinary 3 ampere silicon diode, so it will probably have a voltage drop of around 0.6 to 0.8 volts. It would prevent damage due to incorrect polarity.

The glue may have been "original" although it does seem a bit odd when there is a cable tie there to add mechanical support for the jack. Since all the jacks have cable ties and the holes they go through look rectangular (made with a punch), I think they must be original. The heatsink on the audio amp IC and around the capacitor in the corner are glued on with something quite different looking, so maybe the hot-melt glue was not original. But the cable tie on the power jack looks identical to those on the others, so ???
Your assessment of the previous owner's problem sound very plausible.

 

Yes, you can certainly measure right at the power jack.

The "common mode choke" (the green doughnut with two windings) looks like it is between the jack and the rest of the circuit (it's purpose is radio range high-frequency noise filtering). There will be a little bit of voltage drop across it for both the positive and negative.

I think that diode is most likely in series in between the positive from the jack and the power switch. It is an ordinary 3 ampere silicon diode, so it will probably have a voltage drop of around 0.6 to 0.8 volts. It would prevent damage due to incorrect polarity.

The glue may have been "original" although it does seem a bit odd when there is a cable tie there to add mechanical support for the jack. Since all the jacks have cable ties and the holes they go through look rectangular (made with a punch), I think they must be original. The heatsink on the audio amp IC and around the capacitor in the corner are glued on with something quite different looking, so maybe the hot-melt glue was not original. But the cable tie on the power jack looks identical to those on the others, so ???
Your assessment of the previous owner's problem sound very plausible.

Thanks for your feedback edp!

Because of all of this quality feedback I am learning more & more about electronics and gaining some familiarity & confidence with test measurements & troubleshooting procedures. It's like a fast track to becoming literate in one of the most important languages spoken in the world today: microelectronics. (Last night I had several very interesting conversations with my niece's boyfriend who has been doing some very creative work at home with 3D printing. His next project involves deploying an Arduino in an acoustic levitation device, and he said he may need access to an Oscilloscope to tune the transducers. I volunteered my scope & some assistance, so using my scope in the piano troubleshoot at some point will be great preparation for being able to help another maker. This is the circle of mentoring that you guys are helping to support, and the universe thanks you!)

I have some chores to do today, but I expect to be able to perform the next group of test measurements this evening and then post them here.

I have cut & pasted all of the mentoring suggestions & explanations into a digital piano troubleshooting log that is my road map for this repair process. I'm also annotating all photographs with information that ties back to the troubleshooting log. Maybe we'll end up helping others to keep their old electronic pianos alive & well and out of our landfills.

Thank you, and I'll catch you later.

Alan

 

Happy New Year Wayne, Ylli, ebp, and friends!

My little Radio Shack meter seems to have a built in DC power filter when measuring AC power. (Measured a 9v battery in AC mode and it registered 0v.)

That said, I made all requested voltage test measurements in DC mode and the results are in the table pictured in the attachment.

One thing I definitely don't understand: if the no load voltage of the power brick is 15.95vdc, then why is power jack+ to 7805-common 24vdc with the keyboard powered off? (How does just plugging in the power brick to the piano's power jack cause a voltage increase?)

NOTE: now I'm not getting any audio out of the piano. The LED screen & mode select buttons are all working fine, so the Logic Board seems to be working.

 

One thing I definitely don't understand: if the no load voltage of the power brick is 15.95vdc, then why is power jack+ to 7805-common 24vdc with the keyboard powered off? (How does just plugging in the power brick to the piano's power jack cause a voltage increase?)

Sounds like the brick contains a rectifier (to change the AC into DC, but does not have any filtering. The output will behHalf sine waves and your DC voltmeter will measure the RMS (near average) value of this waveform. When you plug the power adapter into the piano, there is a filter capacitor added, and that cap charges to near the peak of the incoming waveform and the voltmeter measures a higher voltage.

You can confirm the output of the brick by measuring both the DC and AC components. Look at the output of the power supply (unplugged from the piano) with the voltmeter set to a DC range. Then try it again with the voltmeter set to an AC range. See how much AC component you have.

 

Sounds like the brick contains a rectifier (to change the AC into DC, but does not have any filtering. The output will behHalf sine waves and your DC voltmeter will measure the RMS (near average) value of this waveform. When you plug the power adapter into the piano, there is a filter capacitor added, and that cap charges to near the peak of the incoming waveform and the voltmeter measures a higher voltage.

You can confirm the output of the brick by measuring both the DC and AC components. Look at the output of the power supply (unplugged from the piano) with the voltmeter set to a DC range. Then try it again with the voltmeter set to an AC range. See how much AC component you have.

Ylli,
Sounds like a good explanation. I'll try the AC & DC VOM tests on the brick and report back.

Hey, I'd love to see the waveforms as well as measure them with the VOM. Can I use my oscilloscope to take a peek? If so, can I just connect my oscilloscope to the brick output plug polarities or do I need to put a resistor or something in that loop to get a good trace?

For the filter capacitor modified wave, what test points on the PCB should I use to see that waveform?

This is a great time for me to learn how to use my Dad's old Harmon Kardon Oscilloscope. The scope has a EICO Scope DIRECT probe on it, as seen in the attached photo. I assume it plays some role in taking scope traces.

Thanks for the feedback,
Alan

 

Don't know what settings are available on your scope, but you will want the vertical set to 5 v/div and the horizontal to 5 or 10 ms/div. If you have the ability to sync on line, you can use that, otherwise sync on your input.

The output of that wall wart will be isolated, so you can out the scope ground lead on the outer connector and the probe tip in the center. Or the other way around. That is not the case in all measurements.

If the power source is filtered, you will see a straight line on the scope. If it is not, you will see a series of half sine waves, but all above zero.

 

Don't know what settings are available on your scope, but you will want the vertical set to 5 v/div and the horizontal to 5 or 10 ms/div. If you have the ability to sync on line, you can use that, otherwise sync on your input.

The output of that wall wart will be isolated, so you can out the scope ground lead on the outer connector and the probe tip in the center. Or the other way around. That is not the case in all measurements.

If the power source is filtered, you will see a straight line on the scope. If it is not, you will see a series of half sine waves, but all above zero.

Ylli,
Thanks for the oscilloscope instructions. I am going to try those scope tests on both the old & new power supplies in the morning.

But for now, the good news is that the keyboard is now operational! My wife was playing it this evening and she is elated. I told her about you guys and the help you have given me, and she says thank you from the bottom of her heart!

Details in a separate posting so that I can thank you all together.

Alan

 

Wayne, Ylli, and ebp:

Thanks to all of you guys the piano is now back in service!

It turned out that the problem was the power supply brick. It simply wasn't delivering the correct voltages to the keyboard and the keyboard just couldn't operate properly as a result.

I have posted a picture of the new power supply and the results of VOM testing with both the old & new power supplies so you guys can see how different the psu's behaved.

TAKEAWAY: the label on a power supply doesn't necessarily mean that it will deliver voltages & current flow as specified. Plus, over time, an overstressed power supply can degrade in performance enough to stop properly powering the connected device and cause it to malfunction.

Many thanks to you all for a fine troubleshoot. I have learned a lot from you all, so now I will try to tackle a couple more repair projects: a Sharp LCD flat screen tv repair and a SONY DASH/Raspberry Pi integration. Time for me to take a deeper dive into the microelectronics world.

P.S. I will bundle all the information from this troubleshoot into a PDF file and upload it to this thread in case it might prove helpful to someone else troubleshooting a Williams Allegro keyboard in the future.

 

Wayne, Ylli, and ebp:

I tested the two power supplies in my oscilloscope, with no load on them. The results are like night & day.
While the bad power supply was a crappy sine wave, the good power supply was a flat line - just a rock steady voltage!

Is there anything else you can tell me about the difference between the two power supplies and the effects they would have on a connected electronic device?

Is the bad power supply typical of a class of power bricks or is it an example of a good brick gone bad?

Is it possible that the 1A rating of the bad power supply meant that it was operating at its peak supply level continuously and, therefore, its components wore out prematurely and its output quality degraded to the point of non-operation of the keyboard?

 

The first one is typical of the old fashioned wallwart. It’s a transformer to change the voltage followed by a rectifier to convert it to pulsed DC instead of AC. There’s nothing wrong with this as long as the device is designed to accept it, as most devices were a couple decades ago.

The second one is a typical regulated supply, probably an SMPS (switch mode power supply). These are more efficient and have replaced wallwarts (by law) in many places.

The 3rd issue is the rating. Any power supply run beyond its rating could produce problems. A wall wart will sag in voltage and overheat. I’m not sure how an SMPS reacts but I would imagine a voltage sag as well.