Need service manual or schematic for Williams Allegro digital piano

Thread Starter

alanmcrae

Joined Dec 19, 2018
21
My wife's Williams Allegro digital piano powers up okay but first lost sound output to its speakers then recently lost sound output to headphone jack too. I suspect a simple component failure on the audio amp circuit board, which I might be able to fix with a little careful testing.

If anyone has a copy of the Williams Allegro digital piano service manual or schematic diagram it would be really helpful.

P.S. Any links to online articles about the basic subsystems found in digital pianos might help me to understand where to focus my initial troubleshooting.
Especially if it has block diagrams and maybe suggestions for basic troubleshooting test procedures.

I am psyched to try this repair as I repaired an LCD tv that I got off the street on garbage pickup day and still use it as my master computer display & entertainment tv. Repairing things and putting them back in service is so satisfying.

Thanks for any help or guidance,

Alan McRae
 

wayneh

Joined Sep 9, 2010
17,153
Well can we assume you’ve exhausted the search engines? You might get a little better results if you include a model number. Sometimes you can find things printed on the circuit board that are helpful in identifying which schematic you need.

Without documentation, you’ll have to rely on the trained eyeballs that hang around here. Take some good quality photos of your circuit board and include, if you can identify it, the area you suspect has a problem.
 

Thread Starter

alanmcrae

Joined Dec 19, 2018
21
Well can we assume you’ve exhausted the search engines? You might get a little better results if you include a model number. Sometimes you can find things printed on the circuit board that are helpful in identifying which schematic you need.

Without documentation, you’ll have to rely on the trained eyeballs that hang around here. Take some good quality photos of your circuit board and include, if you can identify it, the area you suspect has a problem.
Yes, I exhausted the search engines. The "Allegro" is the model number apparently.

I took some photos of the circuit boards earlier today. Is this the proper place to upload them, or is there a different forum area for a troubleshooting thread?
 

wayneh

Joined Sep 9, 2010
17,153
Yes, I exhausted the search engines. The "Allegro" is the model number apparently.

I took some photos of the circuit boards earlier today. Is this the proper place to upload them, or is there a different forum area for a troubleshooting thread?
This is the preferred spot. New members might have some restrictions on uploading. Just let us know if you have any trouble.
 

Thread Starter

alanmcrae

Joined Dec 19, 2018
21
Thank you for your guidance. I have uploaded my annotated photos.

Piano status: it powers up, voice selections button works, LED display shows voice selected, but no audio thru speakers or headphone jack.
Piano history: first the speakers stopped working, then a few months later the headphone jack stopped working.

UPDATE: I just went to test the function keys and now EVERYTHING IS WORKING FINE!

I haven't disconnected anything or changed anything, so I don't know how it has somehow started working just fine.

NEXT STEPS: I will jiggle a few wires just to make sure that it doesn't cut out again. If that goes okay, then I will put it back together again and see if it keeps functioning.

One possibility: I am a computer technician so maybe the cpu got scared that I would start dismembering its limbs and desoldering its supporting chipsets ;-) Silicon-based life is so easily intimidated! (Ah... open the pod bay doors Hal. Hal?)
 

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wayneh

Joined Sep 9, 2010
17,153
I haven't disconnected anything or changed anything, so I don't know how it has somehow started working just fine.
Just a shot in the dark - any chance it works after disassembly because it gets better cooling? Also, any idea what caused that "scorch" mark in the upper right of the photo of the I/O board? It doesn't look too serious but it's odd.
 

Thread Starter

alanmcrae

Joined Dec 19, 2018
21
Just a shot in the dark - any chance it works after disassembly because it gets better cooling? Also, any idea what caused that "scorch" mark in the upper right of the photo of the I/O board? It doesn't look too serious but it's odd.
Wayne, yeah that scorch mark looks real suspicious!!!
The component nearest the scorch reads 47 D3 1N4738 (see photo "Scorched_Area)
(There appears to be a surface mounted tiny component right on the other side of this component's mounting hole, but I'll have to unmount the PCB to get a better look at that side. Could be our Prime Suspect at the moment?)

Earlier when I said "hooray" my wife said "but it used to play for a few minutes and then go silent. You told me not to turn it on/off a zillion times a day, so I've just been keyboarding with the piano off to keep my fingering limber."

So, I had her play for a few minutes with the power on & the cover off and sure enough it totally stopped any audio output after just 3 or 4 minutes.

Also, I noticed a lot of audio hum while it was working and recorded that. This hum got much louder for a few seconds just before complete audio cutoff.

So, it does seem like some component may be heating up and then failing after it reaches a certain temperature.

Note that there are two heat sinked IC's on the Power/Audio/I/O board:

The larger one is marked (I believe): TDA7377 CZDHS337 V6 CZ MAR
The smaller one is clearly marked: 7805CT RBXC42 G

Do you think one of these might be the culprit, and if so, how could I test them?

(I have an old Harmon Kardon oscilloscope that I inherited from my Dad, RIP, and a VOM multimeter. Funds are limited but I could purchase an inexpensive IC tester/logic probe thingy if needed, as I plan to be doing more digital electronics projects in the near future. (I am a newbie at this stuff, but I learn fast & have good overall troubleshooting skills in many areas of applied technology. So buying the right tools, I know, is often vital to successful troubleshooting.)

Photos attached

What do you recommend I do next Wayne? (Should I remove & look at the printed circuit side of the Power/Audio/I/O PCB to see what component is by the scorched hole in the board?)
 

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Thread Starter

alanmcrae

Joined Dec 19, 2018
21
D3 is a Zener, 1N4738. Probably a current limiting resistor on the back side of the board.
Ylli, I think you might be right - it looks like a surface mounted miniature resistor to me, or something else with a tiny clear plastic shell and what looks like tiny red "guts" in it. I would need to remove the PCB to look at that side and see what that component is and if anything has leaked.
 

Ylli

Joined Nov 13, 2015
1,053
I would need to remand ove the PCB to look at that side and see what that component is and if anything has leaked.
Don't remove the PCB yet. Do you have a voltmeter? Place that across the Zener diode - positive lead to the top - and measure the voltage at that point. Watch it until the piano goes silent. Did the voltage change? A 1N4738 is nominally 8.2 volts.

In addition, measure the voltage between the lower end of D3 and the top pin on the 7805. Mease it when it is working, and when it is not. If the 7805 is configured as a basic voltage regulator, you should be seeing 5.0 volts.
 
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wayneh

Joined Sep 9, 2010
17,153
Note that there are two heat sinked IC's on the Power/Audio/I/O board:

The larger one is marked (I believe): TDA7377 CZDHS337 V6 CZ MAR
The smaller one is clearly marked: 7805CT RBXC42 G

Do you think one of these might be the culprit, and if so, how could I test them?
I'm thinking those are fine. If either had failed, the thing would not work at all. The 7805 is a 5V voltage regulator. It's job is to reduce the incoming voltage (lefthand pin while facing the label) to 5V (righthand pin), both measured against the middle common pin. You can check it with your VOM.
What do you recommend I do next Wayne? (Should I remove & look at the printed circuit side of the Power/Audio/I/O PCB to see what component is by the scorched hole in the board?)
I think that's worth a look, yes, but on the other hand I'd say it's a low probability thing. Hard to know until you see what's there.

The hum you mentioned, was it good old 60-cycle hum? That implies power supply noise breaking through to the output. Failed electrolytic capacitors are a key suspect. I don't see any obviously burst open but they can and do certainly fail without any visible change. Replacing electrolytic capacitors - shotgun fashion without regard to testing or analysis - is a valid repair tactic. It fixes a lot of things and isn't too bad a job if you have soldering experience.

Another repair trick is to cool suspected components to see if operation can be maintained longer when that particular component is prevented from overheating. They used sell aerosol cans of cooling spray for this but that may have been before the freon ban - I haven't looked lately. I hate to recommend laying your fingers in the components while it's live, but you can sometimes identify an overheated part just by touch. Maybe feel (and sniff) around immediately after powering it off after a failure.

Probably another shot in the dark, but is that a battery on the logic board? Have you checked it? It's probably more for saving settings in memory when the power is removed, but who knows.
 
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Thread Starter

alanmcrae

Joined Dec 19, 2018
21
Thanks Wayne and Ylli,

A short break while I tend to some urgent business and then I'll do some VOM testing at the points you recommended.

I'll report results back here as soon as I have them. Won't be long.

Thanks for your guidance on my troubleshoot. Back soon.

Alan
 

Thread Starter

alanmcrae

Joined Dec 19, 2018
21
REPORT: turned on piano but no sound. I expected sound because piano had been unplugged & off for several hours.
Jiggled cables, jiggled power brick input plug, power switch on/off cycling - nothing. With power on, Zener diode voltage was around 8.2V.

Went to measure voltages across 7805 and got a spark at the probes. Oops! But then sound came on. Yeah!
But sound will work for 30 seconds or so, then fade, cut out, then cut in again. Hum is present at all times whenever there is any sound at all.

Due to my difficulties with holding probes steady to the IC pins with chips in a vertical position, I didn't get useful readings on this first attempt.

I think I may need some sort of probe clips that will make good pin contact and not cause shorting. I will research this tonight and see if I can get some suitable clips tomorrow at the electronics store.

Looking forward to tooling up for this troubleshoot & repair. Microelectronics is really interesting, but I can see that measurement tools need to prevent accidental pin shorting - especially for fat-fingered newbies like me.

Good night for now, and thanks for your expert guidance. More when I've got my measurements act together.

Alan
 

Ylli

Joined Nov 13, 2015
1,053
On the 7805.... you can put the neg voltmeter lead on the mounting tap (not the screw, that might be insulated) and the pos on the top pin. Might be easier than trying to use the D3 anode as ground.
 

ebp

Joined Feb 8, 2018
2,332
Troubleshoot intermittent faults can be a real pain.

If you got a spark from the 7805 without having shorted anything, it may simply be that there is a bad solder joint that managed to be adequate until it wasn't anymore. I've seen this several times. A joint that has always been marginal eventually opens up after a number of thermal cycles - but sometimes re-makes with changes in mechanical force or just temperature. Visual inspection usually is adequate to find them. I'm afraid you'll need to pull the board.

The tab on the 7805 will be almost certainly be "ground." It looks like it isn't isolated from the heatsink, so that whole heatsink can be used as place to clip the negative lead for your meter. Most things can be measured relative to that point.

The "scorch" mark isn't a big concern. That is a "phenolic-paper" circuit board and they do discolor like that in areas where something heats them over a period of time. I've seen far worse than that.

Pianos never know the words, but humming along isn't a good sign and almost always means an issue in a power supply. Going silent is not expected, as a rule. If an electrolytic capacitor in a power supply "wears out", the result is typically noxious persistent hum. However, there might be circuitry that mutes the output under certain fault conditions. I would be more suspicious of a connection problem somewhere. There are obviously some more circuit boards buried in behind. One of them is probably a power supply board. A flakey cable connection or possibly a bad solder joint on a rectifier diode might account for the symptoms.

The big IC on the big heatsink is the audio power amplifier.

==
Many years ago my place of servitude had a fancy daisy wheel printing terminal that kept misoperating. I'd take it to my lab, hook a scope to look at some things and it would work fine. Back to the terminal room. Back to the lab. And again. It turned out to be a self-tapping machine screw with a cup point like a set screw (by default, not by design). The point would cut through the insulation on a ribbon cable and short things, depending on how you leaned on the case of the terminal.
 

ebp

Joined Feb 8, 2018
2,332
There's a 'Main Power Input' connector on the audio amp board. https://forum.allaboutcircuits.com/attachments/power_audio_io_board-jpg.166191/ .Doubt that there is any other power board.
D'oh! I didn't even notice the power jack.

Yes, it would be helpful to have a pic of the power supply. There is a chance it is the culprit. The power input would be the first thing to check when everything goes dormant. Presumably the power comes in via the common mode choke (green doughnut with two wire windings), the big diode and the cable. The power supply must provide DC and the big capacitor in the corner of the board is either the only filter or additional filtering.
 

ebp

Joined Feb 8, 2018
2,332
Alan, don't spend any money on any extra tools at the moment. The meter you have has proven itself in reading the zener voltage and there is good chance it will be all that is required for troubleshooting.

One thing to test with your meter is whether the AC ranges are AC or DC "coupled." Some meters will read DC plus AC on the AC ranges and some have a capacitor that blocks DC. For certain things like checking capacitors in a powered-up circuit it is useful to measure just the "AC component." This might be helpful in tracking down the hum. If you have the manual for the meter, it may say, otherwise ...

Try measuring any convenient "pure" DC source with the appropriate AC range. Any handy battery of any sort will do. If the meter reads nearly zero (may take a few seconds to settle) nothing else is required. If it reads some significant voltage (possibly not terribly close to the battery voltage), then an external capacitor would be required to block DC. In case we need one, do you have any capacitors in the range of say 0.1 to 10 microfarards with a voltage rating of at least 25 V on hand?
 

wayneh

Joined Sep 9, 2010
17,153
I meant to ask earlier, can this thing run off batteries? That would likely eliminate the hum. It might not prevent the shut down issue.
 

Thread Starter

alanmcrae

Joined Dec 19, 2018
21
Wayne, Ylli and ebp,
I will be answering all of your proposed questions in this one response:

Power supply brick is 12VDC 1000mA. It is NOT the original power brick and I don't know if it has the proper specs. Still researching this.

Voltage measurements across components:

Zener+ to Zener- 8.26V
Zener+ to 7805-Output 4.97V
7805-Input to 7805-Common 21.3V
7805-Output to 7805-Common 4.97V

I will next try measuring voltages continuously to record what happens during the failure transition period.

Photos:
Annotated Audio board diagram
Power Brick

Observations:

The 21.3 volts at the 7805-Input to 7805-Common was a big surprise to me, since the power brick's output is 12VDC. Is it likely that the power supply would bump up the input voltage to 21.3V then lower it down to 5V?
 

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