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Where to begin checking keyboard?

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Where to begin checking keyboard?

Postby bassdude » Tue Feb 08, 2011 3:46 pm

I have a cheap old Fisher Price keyboard that functioned years ago but has ceased to do so. Id like to get it working, and if thats not too much trouble id like to do some modifying as well. I just bought a multimeter and started by checking the batteries, which seem to have enough charge. None of the components inside seem to be obviously blown out or damaged. From what i understand, i need to start pulling individual components out of the board and testing them individually.

I realize this isnt a Jupiter 8 were talking about here, but there are still quite a few components down there, and id like to keep the time i spend on this project to a minimum.

Where should i start?

Can some components be tested while still soldered to the board?

Can certain sections be tested all at once in order to single them out as working or non working?

Does a battery (like a double A) really need to be under load to be tested correctly?

Thanks in advance everybody.
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Re: Where to begin checking keyboard?

Postby Folderol » Tue Feb 08, 2011 8:10 pm

Oh dear! Where to start?

First off, you definitely need to check batteries under load, especially old-ish non-rechargables. They can often look fine with no load, but sag to a fraction of a volt when any attempt is made to draw current.

Generally you don't want to be removing components. The very removal process is likely to damage them. If you have some understanding of what they are and how they work, you can find out a lot in-situ and powered up.

Obvious checks are the battery holder making good contact, switches working correctly (including key switches), plugs and chips properly seated. Cracks in the tracks on the PCB.

Any sound from it at all? background hiss? If so, does it change with the volume control.

Any schematics around? If not you need to be prepared to spend plenty of time finding your way around.
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Re: Where to begin checking keyboard?

Postby artifus » Wed Feb 09, 2011 12:08 am

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Re: Where to begin checking keyboard?

Postby bassdude » Wed Feb 09, 2011 12:58 am

Folderol:
Ive heard that you can test a battery under load by holding a resistor up to the end and then checking it with a multimeter? Does this work? Does it matter what kind of resistor?

By "in situ" do you mean as the components are on the motherboard? Is it possible to reliably test components, or groups of components that are soldered in place? Unfortunately, the keyboard wont power up so i cant test anything powered up.

How do you tell if a part is "properly seated"? Is it obvious? Can cracked traces on a PCB be seen with the naked eye, or must they be tested?

There is no hiss from the unit, or any sign that its powering up at all. I also have not found any schematics for it so far.

Thanks for the advice.

Artifus:
That handmade electronic music book seems like it might be just what i need. I looked on the circuit bending site and it has some good information, but it seems like many information sources deal more with theory than actual application. Another problem that im finding (especially with circuit bending forums and info) is that the information given is for an instrument that is in working condition. I cant do anything with my board until i figure out whats already broken.

Again, thanks for the advice. Any follow up or additional info is GREATLY appreciated. I feel so lost with some of this stuff!
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Re: Where to begin checking keyboard?

Postby James Perrett » Wed Feb 09, 2011 1:12 pm

If it won't power up, the first thing you do is follow the power lines and use your meter to find out where the volts stop.

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Re: Where to begin checking keyboard?

Postby bassdude » Wed Feb 09, 2011 2:20 pm

When you say power lines, im assuming you mean the wires starting directly at the battery terminals? Ive read that attempting to test individual components on a motherboard wont work because youll be reading the voltage of every component around them .How do i go about testing each component? Do i test in sections?

Sorry for what may be stupid questions and thanks all.
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Re: Where to begin checking keyboard?

Postby *INACTIVE USER* » Wed Feb 09, 2011 7:19 pm

- checking batteries: draw about 1/10th of the Ah capacity when measuring. Let's say 50mA for AAA, 100mA for AA, 350mA for C and 500mA for D cells. About 40mA for a 9-volt.
- next check the wiring if there is any. This you can do with a multimeter. Wiggle a bit with the wires so that intermittent contacts become clear.
- check the print for damage:
- green powder from leaked batteries and capacitors
- charred or burned from overheating
- charred components
- if components are mounted in sockets, push firmly on them to make sure they are well seated. If you have to do this, make sure the print is supported at the backside.
- check switches: you can do this with a multimeter without power on the board.
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Re: Where to begin checking keyboard?

Postby bunker » Wed Feb 09, 2011 7:22 pm

I would spend a while having a look at a few books and websites dedicated to electronics for beginners. Keep looking for the schematics for the instrument as even if you get it working without them, you will be eternally grateful when it comes to working out what's what and eventually modding it!

You seem to be getting a bit hung up with the component testing. It can look a bit daunting when you first look inside something with a few PCB's and a load of different components but the chances are your fault could be something as simple as a corroded battery terminal or faulty power switch. There could be a third option tho....

....Did you mention anywhere that you had bought fresh, brand new batteries? Its a no brainer really, especially if you plan to be messing about with it working for any length of time. Bin the old ones off, they could be your real problem.

Do you know how to use a multimeter on ALL it's settings and specifically what you use them for? Your 'meter will become your eyes and ears for pretty much anything you want to do with your keyboard. It's essential you get to grips with basic testing and interpreting the measurements that you take. There are plenty of sites out there where you should be able to pick up the rudiments pretty easily.

The good news is with your current project you won't be able to electrocute yourself although a stray probe or an incorrectly set 'meter might melt a few things!

Please don't get the idea I'm having a pop, I can't fault your enthusiasm and as someone who started out mucking about with stuff as a kid, pre internet forum days, I'm only trying to help you avoid some of the pitfalls along the way and point you in the direction you should take.

Above all have fun! Electronics ARE fun!
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Re: Where to begin checking keyboard?

Postby Folderol » Wed Feb 09, 2011 8:15 pm

Well, a number of people have made some excellent suggestions here - there's safety in numbers you know

The (probably) good news, is that a total failure is, surprisingly, usually the easiest fault to clear.

Usually the battery negative is connected to the 0V (ground) line of everything and any screening or pot cases etc. You can follow this through visually to confirm, but a quick continuity check on you meter should do (often marked as a diode check). This then gives a useful reference for checking everything else.

If there is broken trackwork then usually there is also sign of damage to the board itself - but not always of course. If you can see no damage but have reason to suspect the board then the simplest way to find out is to go point-to-point along the track testing from each solder joint to the next.

Not only can you test components in circuit and powered up, it is often the quickest way to confirm an entire stage is working correctly.

For example, in an amplifier stage using an NPN transistor the base voltage will be about 0.6V positive to the emitter, and the collector will be somewhere near half way between the emitter voltage and the supply voltage. For this to be true, not only must the transistor be good but so must a bunch of resistors, and any caps there must at least not be short circuit.

Now, I don't expect you to know how to recognise such a stage at the moment, but after doing some of the suggested reading and having a poke around you get the idea surprisingly quickly.

Don't forget, as bunker said, have fun
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Re: Where to begin checking keyboard?

Postby bassdude » Wed Feb 09, 2011 11:57 pm

Ive heard warnings about frying components on a board. What is a safe voltage to set my multimeter at so that i can get a good reading, but not destroy the parts?

Ive looked around at various websites, and have learned some of the basics, but again, many of them focus on theory and not actual application. Im having a hard time identifying some of the components and figuring out what it is that each one does. Does anyone have any suggestions of sites with information that would cover this?
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Re: Where to begin checking keyboard?

Postby grab » Thu Feb 10, 2011 9:13 am

A multimeter won't fry components (with vanishingly few exceptions).

What will fry components is static on you. Do you ever hear a little crackle when you take a sweater off? Do you ever get a little static shock sometimes when you touch a doorhandle? OK, then you've got somewhere north of 10kV stored up in static charge on you. Very, very little current - but an absolute stack of voltage!

Done properly, you should be getting yourself an anti-static wrist strap. An anti-static desk mat is also a good investment if you're going to be doing a lot of this, bcos static charge builds up on other things too. You should get away with touching something metal and earthed (e.g. a radiator) for a few seconds before handling any components. Repeat this at regular intervals, and every time you move around (it's friction between artificial fibres or between rubber soles on your shoes which generates the static). But the gold standard is a proper wrist strap.
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Re: Where to begin checking keyboard?

Postby James Perrett » Thu Feb 10, 2011 10:27 am

Lets start from the beginning...

Set your multimeter to read volts - probably on the 20V range to start with.

With batteries in the keyboard and the keyboard off, hold the black meter lead on the battery negative terminal on the keyboard and the red lead on the battery positive terminal. What does the meter say?

Now switch the keyboard on - does the voltage change?

James.
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Re: Where to begin checking keyboard?

Postby bunker » Thu Feb 10, 2011 6:18 pm

grab wrote:A multimeter won't fry components (with vanishingly few exceptions)

Yeah you're right, perhaps I was a bit vague. I was thinking more along the lines of shorting component legs to +ve or -ve with a stray probe or having the 'meter set to measure current rather than voltage (I have seen this done by an "Electrician" when I was an apprentice. Swanky office in the city centre plunged into darkness and a very broken Robin multimeter, is was across the main switch of a consumer unit after all!) Very good point well made regarding static though. Something I forgot to mention..
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Re: Where to begin checking keyboard?

Postby *INACTIVE USER* » Thu Feb 10, 2011 7:00 pm

That's why you do the first measurements without powering the keyboard. There is no need to put power on it to check the switches, wiring and visual inspection. Those have to be OK before you put power on it! If you can measure or see that there is something wrong why take a chance to make it worse? Put your multimeter in "continuity check" for that. It's when it makes an audible beep when you make the leads touch.
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Re: Where to begin checking keyboard?

Postby artifus » Thu Feb 10, 2011 7:18 pm

how to use a multimeter
how to use a multimeter from ni

i'd recommend removing batteries and starting with continuity testing too.
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Re: Where to begin checking keyboard?

Postby bunker » Thu Feb 10, 2011 7:26 pm

I was generalizing about knowing how to use a 'meter, not specifically about the fault he is presented with. I agree, initial checks with the power off but the way I read the posts from the author I get the impression he hasn't used a 'meter before. My advice was to do a bit more reading up on basic principles and using teat gear.
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Re: Where to begin checking keyboard?

Postby bunker » Thu Feb 10, 2011 7:28 pm

artifus wrote: how to use a multimeter
how to use a multimeter from ni

i'd recommend removing batteries and starting with continuity testing too.

Great links arifus! Thats the sort of reading up I was taking about bassdude.

Let the fun commence!
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Re: Where to begin checking keyboard?

Postby bassdude » Thu Feb 17, 2011 3:06 am

James Perrett wrote:Lets start from the beginning...

Set your multimeter to read volts - probably on the 20V range to start with.

With batteries in the keyboard and the keyboard off, hold the black meter lead on the battery negative terminal on the keyboard and the red lead on the battery positive terminal. What does the meter say?

Now switch the keyboard on - does the voltage change?

James.


Im not sure if i followed your directions correctly, but heres what i did:

I set the multimeter to 20 volts direct current (mine has an option for ac as well).
I held the red probe to the spot where a red wire was soldered to a lead coming out of the battery holder. I held the black probe to the spot where another connection left the battery holder (these were the only two points coming from the battery holder).

The reading i got stabilized at 6.33. Sometimes it read 6.34. The keyboard does not do anything if you try to power it up. The reading did not change even after attempting to power it up multiple times.

What does this reading mean? Where should i go next? Thank you all so much for your input.
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Re: Where to begin checking keyboard?

Postby grab » Thu Feb 17, 2011 9:08 am

So power's getting to the board OK. Next step is the power switch. Assuming it's a clicky or slide switch, you should have +6Vish on one side of the switch all the time (if you don't then there's a fault between the battery and the power switch). When you turn the switch on, do you get +6Vish on the other side too? If not, the switch is duff.

Circuits do exist (and are more common these days) where the power switch doesn't physically remove power from the circuit. Instead it just sets the chip (because everything is run from a chip) into an ultra-low-power mode so it draws practically no current. Pressing the power button to restart puts a voltage on a "wake-up" line which starts the chip up again. Given that it's an old Fisher-Price bit of kit though, I suspect yours probably doesn't work this way.
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Re: Where to begin checking keyboard?

Postby James Perrett » Thu Feb 17, 2011 9:55 am

OK - we know that you have power coming from the batteries and that operating the power switch doesn't change anything.

Keep the black probe in the same place and move the red probe to the other end of the wire (or circuit board track) that you tested before. This should be further from the battery. Do you still see the same voltage? If so, what is connected to this end of the track/wire? I'd guess at either an external power socket or the on/off switch but I may be wrong. You will need to work out how the power should pass through the switch/connector/whatever and see if the power actually comes out the other side.

James.
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Re: Where to begin checking keyboard?

Postby artifus » Fri Feb 18, 2011 9:19 am

if you can't find any info on the keyboard itself something else which may help and/or provide some clues for future modding/hacking/bending if not repair would be to search on line for the data sheets for any on board ic's (chips). if available this will show you what voltages on which pins the ic is expecting to see and often provide sample circuit schematics.

what sort of circuit are we looking at here? is it one big ic with a few external components or a plethora of transistors? late 70's, early 80's or more recent? could you perhaps post a photo?
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Re: Where to begin checking keyboard?

Postby artifus » Fri Feb 18, 2011 2:57 pm

when i say photo i mean a gut shot of the innards showing the pcb, component and underside if possible.

please excuse double post - missed that window of opportunity available via the edit function.
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Re: Where to begin checking keyboard?

Postby Folderol » Sun Feb 20, 2011 10:43 pm

Pssst!
Hey mister, wanna see some dirty pictures?

Photos would certainly be a great help here.
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Re: Where to begin checking keyboard?

Postby bassdude » Mon Feb 21, 2011 5:40 am

Maybe i should have done this from the beginning. Here are some pictures of the inside of my keyboard:

Image

Image

Image

Image


The first two pictures are of the upper right hand side of the board, the third is the left hand side which houses the batteries, and the last is the interior of the board and the underside of the top of the board (keys, etc.) If those dont work, ill include links to my photobucket for higher resolution pics.


I did as James Perrett instructed. I seem to be getting the same reading at the end of the red wire where it meets the motherboard (6.34 volts). Although i havent pulled it out yet, it seems like the next closest thing to the red wire is indeed the input for an AC adapter. The power switch is not in this area: it is located on the long thin circuit board below the main board (the one with all the grey dot buttons for the keys). I guess i should check this section next?

@Artifus: Im almost positive this board was made in 1983. Its certainly no older than 80 and no newer than 90. I would LOVE to find some info on the chips in this board (as well as chips i have from other projects), but have so far been unsuccesfull finding chip info on the web. I wish there was some database where you could just plug in the make and number and it would tell you what it does. Anyone have any suggestions for identifying chips?

Once again, thank you all so much.
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Re: Where to begin checking keyboard?

Postby James Perrett » Mon Feb 21, 2011 10:28 am

I'd investigate the power connector next and try to see if it is the sort of connector that will disconnect the battery if someone plugs a connector into it. It would make life easier if you could see the underside of the board as you could then check for dry joints and see where the red wire goes next.

A picture of the big chip would also help - the other chips seem to either be custom chips (in the case of that AMI chip) or standard op-amps and logic buffers.

James.
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Re: Where to begin checking keyboard?

Postby Folderol » Mon Feb 21, 2011 7:19 pm

Exactly my thought as soon as I saw the pic. The give-away is that the there are pin number 1,2,3 around it. As if that wasn't enough you can just see 'DC' on there as well.

You can almost guarantee that it will be switched (the only other safe alternative is a diode in series with the battery, but that would lose over 1/2 a volt), and that if it's been left any length of time tarnish/corrosion on the contact will be the problem.

Sometimes you can cure this simply by squirting some switch cleaner into the socket then shoving a plug in a few times to get it all moving.

In any case I would advise you not to try to run it off a mains power unit until you know exactly what voltage and polarity it wants.
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Re: Where to begin checking keyboard?

Postby bassdude » Tue Feb 22, 2011 5:01 am

SUCCESS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

The two posters above me were absolutely right. I pulled the main board out, looked underneath, and see a crack right through one of the solder points under the AC power connector. I heated up the joint and added a little bit of solder. My work was pretty half assed and i didnt use a drop of flux, but it was enough to get the power going. As soon as it cooled off, i pressed the power and the thing came to life!

There were a bunch of other spots further down that looked like theyd been damaged. Some of the traces looked broken, and some components looked bent over, but this dosent seem to be harming anything. The keyboard plays just like it did the last time i played it, about 16 years ago. The speaker sounds a little distorted, but it wasnt quality to begin with and was always played with the volume at full.

Anyone have ideas about looking up chips on the web? Id still like to find out what the ones in my board do, as i may begin circuit bending this thing.

Once again, thank you all so much! In retrospect this seems like a pretty simple and obvious fix, but i was totally lost before i came here.
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Re: Where to begin checking keyboard?

Postby artifus » Tue Feb 22, 2011 8:14 am



yay! great news bassdude.

datasheet search simply enter the name of the ic followed by the magic word 'datasheet'. some are easier to find than others.

if there is no headphone or line out on the keyboard i'd recommend that as your first hack - keep it simple and go slow.

audio hacking video tutorials

have fun!
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Re: Where to begin checking keyboard?

Postby Folderol » Tue Feb 22, 2011 1:55 pm

Glad to hear it.

Hmmm, seems we've sucked another victim in

DIY anonymous is that way ->
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Re: Where to begin checking keyboard?

Postby bunker » Wed Feb 23, 2011 1:25 am

Congratulations! That feeling you got when you finally got something out of your keyboard is what keeps me repairing stuff! Hopefully you have been bitten enough to save some more vintage gear from the great junkyard in the sky! (and end up like me, not being able to move for, ahem, esoteric curio's.. )
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