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Kenwood KR-V75R Receiver

Discussion in 'Electronic Repair' started by Chris F., Dec 7, 2005.

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  1. Chris F.

    Chris F. Guest

    The unit has sound and seems to be functioning, but the display is totally
    dead. The filaments in the VFD look OK and are getting about 6V across them
    (not checked for continuity though). The power supply seems OK, but without
    a service manual there isn't much testing I can do. Are these displays known
    for going bad? If not, does anyone know where I can find a service manual
    for this?
    Thanks.
     
  2. Arfa Daily

    Arfa Daily Guest

    6v sounds quite a lot to be across the heater. It is usually in the region
    of 1.5 to 3v AC. Most common reason for failure of VFD to light, is the main
    negative supply being missing. This is typically -32v, but can be anywhere
    from -20 to -40v. The heater supply is usually floated on this negative
    supply, so you can measure if it's there or not, by clipping your meter +ve
    probe to chassis, then measuring on the VFD heater pins with the meter -ve
    probe. You can check on the heater pins at whichever end of the display is
    easiest to get to - it doesn't matter, as both ends should be at about -32v,
    with respect to chassis.

    Assuming that the supply is missing, the cause will be an open circuit
    electrolytic capacitor. The supply is usually derived from an AC coupled
    voltage multiplier, fed straight from one of the windings on the power
    transformer. Most commonly, it is the input capacitor which fails. Value is
    usually around 47uF at 50v working. You will be looking for a small cluster
    of caps ( 3 or 4 ) with a few small diodes scattered around them, in the
    power supply area. The caps will often be very " tired " looking.

    If you have a 'scope to hand, this is often the easiest way to locate the
    caps. Find some likely looking candidates, then check on either end of them
    with the 'scope. Sooner or later, you will come across one which has a dirty
    great AC waveform on one side, and virtually nothing on the other.

    Good Luck

    Arfa
     
  3. Chris F.

    Chris F. Guest

    I went through the power supply and didn't find any bad caps. Seems to be a
    good 30V supply there in the PS area, but without a schematic I can't follow
    it any further. Would it be right to suspect logic problems or a bad display
    at this point?
     
  4. Arfa Daily

    Arfa Daily Guest

    It is very rare to get either logic faults or bad VFDs to cause total
    failure of the display. VFDs normally wear out gracefully over a couple of
    years - very common on VCRs. One notable exception was a particular model of
    Sony, which used to suffer a display destroyed in a matter of days when a
    cap in its heater supply went bad, but that is the only bit of kit that I
    have ever known this to happen to.

    When you say that you have a " good 30v supply in the PS area " are you
    talking a separate minus 30v, and not one half of the + / - 30v supplies
    that are probably there for the output stages ? Did you find the voltage
    multiplier that generates this supply. If so, did you check the caps'
    coupling performance with a 'scope, or their " goodness " with an ESR meter
    ?

    Have you checked that there is minus 30v at the display panel heater pins,
    as I detailed ?
    I'm willing to bet that the problem is still supply related ...


    Arfa
     
  5. Chris F.

    Chris F. Guest

    Yes this 30V supply is separate from the two others that drive the output
    stages. There doesn't appear to be a voltage multiplier, it seems to be
    directly derived and rectified from a dedicated winding on the power
    transformer.
    The customer couldn't remember if the display just quit, or gradually got
    dim over the years, so I don't have that info to go on.....
     
  6. Arfa Daily

    Arfa Daily Guest

    OK. Some models from some manufacturers do derive it direct from a dedicated
    winding. On some that I've seen, they then follow this either with a simple
    regulator, or sometimes a series resistor. Is the supply making it as far as
    the display ? Chances are, if it's present at the display heater pins ( you
    still haven't told me this, and it's an important pointer as to where the
    problem is ) then it's also going to be present at the display controller
    IC. Report this info, and we can determine wher you should next be looking.

    Arfa
     
  7. sck0006

    sck0006 Guest

    In my limited experience with VFD's, they don't usually go bad, just
    start to lose brightness on pixels. This is of course just what I've
    seen in very few things, and really don't know that much about them,
    so they may very well go bad like you say. Actually, I'd like to
    know. Anyone have a more definitive answer?
    Steve
     
  8. Arfa Daily

    Arfa Daily Guest

    Please read the whole thread, where I have given a definitive description of
    how VFDs usually go bad ...

    Arfa
     
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