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Electronic keyboard repair (I hope)

Discussion in 'Electronic Repair' started by Puddin' Man, Dec 12, 2007.

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  1. Puddin'  Man

    Puddin' Man Guest

    New here ...

    Yamaha PSR-12, has maybe 72 keys, around 24 voicings, flexible-speed
    percussion, canned tunes, etc. About 20 years old.

    I turn it on lately, it works fine for 2-10 min., then freezes.
    The little light that is supposed to come on for beat 1 (when
    percussion accompaniment is running) locks on. That and the power led
    are the only things functional.

    The case has 2 halves. I workbench the thing turning the business
    end (with keys and most PCB's) up 90 degrees to tinker, and it plays OK
    for up to 3 hours (presumably lots longer). I figured the problem
    has to do with either gravity or heat buildup, but I just left it
    normally assembled and on for several hours and it didn't freeze up,
    so maybe heat isn't the problem.

    There are PCB's for power and switching, and 2 big PCB's presumably
    for tone generation, special effects, etc. I tried some ribbon
    cable connections: wouldn't budge. I'm afraid of breaking a
    board or connector or ?.

    It's gotta be repairable. I think. :)

    Any/all help/advice much appreciated.

    Thx,
    Puddin'

    "Well, there's two trains runnin'.
    Ain't neither one goin' my way.
    One run at midnight,
    the other run just before day."
    - from "Still A Fool", Muddy Waters, maybe 1949
     
  2. Arfa Daily

    Arfa Daily Guest

    Stuff with micros locking up, often is a heat problem to do with the power
    supply, especially if the item really is 20 years old, and more especially,
    if it makes use of a switchmode power supply. As a first move, I would feel
    inclined to pop a meter on the main 5v rail, and see where it's at, then
    leave it on there, and see if it's any different, when the unit locks up. A
    'scope on there wouldn't hurt as well. The rail should ideally be no lower
    than about 4.85v and no higher than about 5.2v. The 'official' limits are a
    little wider than that, but it won't have been designed to run outside of
    the limits I've said. The rail should also be 'clean' with hash or ripple no
    higher than say 20mV.

    If there are any issues with voltage level or noise, look to caps and
    resistors in the regulator circuit, for your trouble.

    If the power supply turns out to be ok, the next place I would turn my
    attention, is to the system control processor's clock crystal. Old xtals
    sometimes get reluctant to oscillate, and will just stop after a short
    while, in just the way you describe. You can often 'prove the point' by
    taking the xtal out, and refitting it reveresed to its original orientation.

    Finally, I have a dim recollection of a keyboard having a problem like this,
    some years ago. As I recall, it too had a couple of large logic boards, and
    ribbon style interconnects looping the boards and power supply together. The
    'launch' level of the power supply was fine at 5v, but by the time it
    reached the second board, having passed through tracks from one end of the
    board to the other, and the in and out board connectors, it had dropped to
    perhaps 4.8v, due no doubt to age-deterioration of the connector plating,
    and the large current that all this logic draws on the older stuff. Anyway,
    it was enough to cause the thing to keep locking up. Eventually, I decided
    that there was no particular need for the supply to the furthest board to
    loop through the nearest one, so I gave it its own heavy gauge wires
    straight back to the power supply, which cleared up its problems completely.

    Arfa
     
  3. Ron(UK)

    Ron(UK) Guest

    It`s just possible that a reset to factory settings might help[1].
    There`s a list of Yamaha reset procedures here
    http://www.yamaha-europe.com/yamaha_europe/uk/service/030_support/40_resetlist/index.html

    I dont see one for the PSR-12 but it`s possible that one of the other
    PSR resets would work.

    [1] It`s a long shot but it might work!

    Ron(UK)
     
  4. Puddin'  Man

    Puddin' Man Guest

    Here's some additional info:

    The specs say "rated voltage - 9v dc".

    I have only 19 yr-old $10 meter. I don't expect accuracy, but rather
    ballpark readings.

    Directly from the 120v converter connector, it *looks* like 15v dc.

    The switch/volume board has a 2 element wire that appears to feed
    the 2 main boards. There I measure 2v dc both with the unit
    functionaing -and- when it is frozen.

    Did that make any sense?

    Thanks,
    P

    "Well, there's two trains runnin'.
    Ain't neither one goin' my way.
    One run at midnight,
    the other run just before day."
    - from "Still A Fool", Muddy Waters, maybe 1949
     
  5. Puddin'  Man

    Puddin' Man Guest

    Anything's worth a shot.

    But I'm having trouble interpreting such as:

    PSR220 Press [+/YES], [-1/NO] + Power on

    There's no +, -, YES, NO on the keyboard. There are 2
    pairs of buttons with only up/down settings: pitch and
    beat. When the unit freezes, these buttons have no
    effect.

    Thx,
    P

    "Well, there's two trains runnin'.
    Ain't neither one goin' my way.
    One run at midnight,
    the other run just before day."
    - from "Still A Fool", Muddy Waters, maybe 1949
     
  6. jakdedert

    jakdedert Guest

    Yes...and no. The 2 volt reading is meaningless unless you can
    determine that it was measured at the point where supply voltage enters
    that particular board. There might be some sort of marking at the
    terminal...perhaps "+5v" or "B+" or "Vcc". That the voltage does not
    change when frozen is irrelevant.

    That said, if it *is* the main supply terminal, it is probably low, and
    the unit is just barely limping along with reduced voltage even when
    functioning.

    The 15vdc reading from the 'unloaded' (IOW not connected to anything)
    supply is also irrelevant. The relevant reading is taken when the
    supply is actually supplying current to the keyboard.

    You need to find the main power supply board, and trace forward from
    where there. That board (or portion of the main circuit board where the
    DC adapter connects) is where the excess voltage is regulated down to a
    level that the actual circuits use.

    jak
     
  7. Puddin'  Man

    Puddin' Man Guest

    Power supply board:
    Takes input from batt. or converter
    Connections:
    Black wire - screws to alum. kb frame
    "to HP" - 3 element, goes to speaker
    "to DM" - 4 element, goes to 1684 PN1 (lg. board)
    elements are labeled DG +5D AG +5A
    +5D and +5A measure 5v DC to black wire
    "to SW" - 8 element, goes to switch/volume board
    elements are labeled E 5v B T M S E E
    M measures 15v DC to black wire

    The two large boards (2 x 12+ ") are labeled 1684 PN1 and 1684 PN2
    with markings for the voicings, various effects.

    Any help?

    Thx,
    P

    "Well, there's two trains runnin'.
    Ain't neither one goin' my way.
    One run at midnight,
    the other run just before day."
    - from "Still A Fool", Muddy Waters, maybe 1949
     
  8. Arfa Daily

    Arfa Daily Guest

    "+5D" will be the 5v digital supply - ie the 5v supply for the digital
    electronics, and "+5A" will be the 5v analogue supply. "DG" and "AG" are the
    corresponding digital and analogue ground returns for those two supplies, so
    will be the references to measure the supplies against. You really need to
    be able to measure the +5D supply with a reasonable amount of accuracy - to
    at least 0.05v.

    Arfa
     
  9. Puddin'  Man

    Puddin' Man Guest

    That's a big help. I had no idea how they code such stuff.
    To what extent will such an accurate measuring device empty my po' wallet?

    Goes without saying that if I get 5v from +5D when it plays and 0v when
    it freezes, I've likely found my problem?

    Thanks,
    P

    "Well, there's two trains runnin'.
    Ain't neither one goin' my way.
    One run at midnight,
    the other run just before day."
    - from "Still A Fool", Muddy Waters, maybe 1949
     
  10. Arfa Daily

    Arfa Daily Guest

    Weeeelll, yes ... But more likely, it would be something like 4.9v when
    it's working ie just limping along on the lowest threshold that the micro
    can work at, and 4.7v when it's wrong. With digital electronics,
    particularly older garden variety such as this, a few decimals of a volt are
    enough to make the difference, which is why you need a meter that can read
    to half a decimal. Such a meter is not expensive, and if the one you have,
    even if it is a $10 job, is digital, then it should be capable of this level
    of resolution. The trick is how *accurate* it actually is, and that is
    something that you can't really know without comparing it to a 'standard' or
    someone else's much more expensive meter. I am shortly to be away for a few
    days, so will only get back on here another time or two. There are lots of
    others on here who should be able to help you through this. Good luck with
    it, and hope you manage to get a result.

    Arfa
     
  11. Puddin'  Man

    Puddin' Man Guest

    It's analog.

    I spotted a Sears Craftsman dig. meter on sale $30 but sez
    "DC accuracy to 0.7 percent for accurate measurements."
    Not sufficient for the job?
    Your help is -very- much appreciated.

    I continue to try to measure differential voltage between the ekb when
    working and when "frozen". Just measured both putting the meter under
    the microscope of my eyeball. If there was any difference at all, there
    was a snatch-hair more voltage when "frozen". Reading was like 4.9 v.

    Dunno where to go from here. Any help most welcome.

    Cheers,
    Puddin'

    "Well, there's two trains runnin'.
    Ain't neither one goin' my way.
    One run at midnight,
    the other run just before day."
    - from "Still A Fool", Muddy Waters, maybe 1949
     
  12. N Cook

    N Cook Guest

     
  13. Smitty Two

    Smitty Two Guest

    I think the 0.7% meter will be accurate enough. Arfa mentioned .05V at a
    5V level; that's 1%. As others have mentioned, loose or corroded cable
    connections and cracked solder joints are usual suspects in any
    electronic failure. Beyond that, not only are your diagnostic skills
    going to come into play, but also your soldering skills in removing and
    replacing defective or suspect components. A voltmeter may or may not be
    the only piece of test equipment you need to troubleshoot this. You may
    want to formulate a flow chart to help yourself maintain some clarity
    about when you decide to take it to a professional. If you're dedicated
    to shooting it yourself as a learning experience, a schematic will be
    helpful.
     
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