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Help with Sony SLV-N750 VCR

Discussion in 'Electronic Repair' started by G B, Jan 23, 2009.

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  1. G B

    G B Guest


    I have a Sony SLVN750. It failed such that it had no time display and
    would no longer power up when pressing the power switch. I tore it apart
    and "shot-gun" replaced the nine capacitors in the general area of the power
    supply with capacitors of the same value. It now will power up and even
    play a tape BUT when it is powered down (in standby) the display blinks on
    and off. It seems the power supply is cycling on and off. Any idea what
    would cause this? Any help is appreciated...

    Greg (gberry--AT--frontiernet--dot--net)

  2. Maybe just need to set the clock?

    Mark Z.
  3. b

    b Guest

    with weak or pulsing displays there's usually some relatively high
    value caps (2200uF) on the line to the panel need changing.
  4. Arfa Daily

    Arfa Daily Guest

    My first thought, too ...

  5. G B

    G B Guest

    Thanks for the reply...

    This VCR will automatically set the clock, so no blinking --:-- forever,
    when it works correctly. The problem is, even if you set the clock, when
    the unit is put in STANDBY mode. The power supply appears to be unable to
    keep its standby power stable. The VCR will reset itself. (again and again
    and again) That means if the clock was set while in the powered on state,
    when it shuts down, the
    clock is reset. Problem is, I have no idea how this VCR power supply keeps
    standy power working. I assume it shuts down the regular switcher but has
    some sort of regulator that provides enough to keep system memory and
    bootstrap working. Any other ideas?
  6. G B

    G B Guest

    The display isn't flashing per say...

    The display goes blank after the supply apepars to shut itself down. It
    returns after say 500 msecs.. then stays on for about a second.. then goes
    blank and starts again. Also every time the display blinks, the carriage
    motor appears to go through its position check...
    I did replace the all the "big" electrolytic caps.. the largest were a
    couple of 1000uF... I suspect that section of the circuit is working
    properly.. I think there must be a standby power section of the circuit
    which is toast... any ideas?
  7. Arfa Daily

    Arfa Daily Guest

    Have you replaced any electros in the front end or were they all secondary
    side ? This sort of odd behaviour in switchers can be symptomatic of small
    caps - often only from 1 to 47uF in value - situated around the oscillator /
    control IC having gone high ESR. You will often find them located close to
    some other component or heatsink, which runs hot. Do you have an ESR meter ?
    Just as an aside, it makes a thread and the replies much easier to follow if
    you bottom-post rather than top. :)

  8. G B

    G B Guest

    I replaced 9 caps all in the area of the switcher, or at least where I think
    the switcher is. I replaced the large regulation cap (82uF @ 200V) and I
    replaced a small ([email protected]) on what I would consider the primary side.
    (prior to the large transformer). I replaced all the electorlytics: 10uF,
    100uF, 330uF, 470uF, and 1000uF values on what I think is the secondary. Do
    you know if the switcher provides the boot-strap voltage in standby mode?
    Or do they have another regulator somewhere off the line regulation
    circuitr? I have also what appears to be a hot-spot on the circuit board
    around QIP107, RIP110, QIP108, ZDIP05, and DIP109. It doesn't feel warm to
    the touch after it has been running for a while.. so the hot-spot is a bit
    of a mystery. I don't have an ESR meter here. I guess I could probe the
    removed components at work... of course I don't know what the ESR of the
    original caps. I thought I could use a DMM to get an indication, but it
    must be marginal enough I can't tell which one is the bad actor.

    As for the posting at the top vs bottom... thank Outlook for always starting
    at the top rather than the bottom of the post ;)

  9. Likely scenario:
    The hot spot on the board was caused by the over-voltage at that point,
    caused by the bad caps. You replaced the caps - now the voltage there is
    normal. This presumes the zener diode, which is what got hot, did not just

    Mark Z.
  10. Arfa Daily

    Arfa Daily Guest

    Yeah, that's a problem with it. Nobody gets too upset on here, but on some
    groups they get *really* arsey if you do it. Top posting can sometimes
    confuse a thread, especially when specific questions are being answered, and
    the majority are posting one way, and someone is posting the other ...

    As to your question, I would expect that the standby supplies for the system
    control micro are derived from the switcher, but I couldn't say for sure.
    It's been a while since I did any work on any VCRs, Sony included. Knowing
    what the ESRs of the original caps should be is not really an absolute. As I
    have commented a number of times with regard to using ESR meters, a lot of
    it is intuition, with the meter just acting as a backup. In general, the
    smaller the value of the cap, the higher its ESR will read on a meter. The
    small ones, such as your 4.7uF, should not read higher than a few ohms. 47uF
    up to say 220uF should not be higher than a couple of ohms. Above this
    capacitance value, ESRs will generally be in decimals of an ohm, and a
    factor of 10 lower than that for the 'big' values of 2200uF and up. In
    general, the higher the voltage working, the lower the range of ESR values
    that would be 'normal' for any particular cap. Usually, if a cap has reached
    a point where it is causing trouble in a switcher, its ESR value will have
    gone off with the fairies, and be easily spotted as the bad one.
    Interestingly, its capacitance value will often read near enough correct on
    a C - meter.

    What your remaining problem is, I really don't know without looking at a
    schematic for the power supply, but one thing I would say is that it seems
    odd that you have managed to cure the primary problem with replacing
    whichever cap it was amongst the ones that you changed, and then been left
    with this slightly 'odd' problem. A faulty cap is a good call for the
    original problem, and was almost certainly the correct diagnosis, but it is
    very rare, when this is the case, for there to be any other issues. If it
    were me, I think that as a first move, I would go over all my work again
    very carefully, making sure that I had got the right values in the right
    places (for that you would need a schematic), and that I had not
    accidentally put any in backwards (easily done :-\ ). Also, make sure that
    the replacement types are 'suitable' i.e. the correct or very close value,
    similar voltage working, low ESR type. They should also be 105 degree or
    better types for future reliability. Also, I would not trust any caps that
    you have had lying around in a drawer for years, or any radial leaded types
    as being suitable.

    Stating the obvious maybe, but also be sure that your soldering is all good.
    If it's not something which you do all day, it is easy to get a poor joint
    on a component, without realising it ...

  11. G B

    G B Guest

    That was at least part of the theory.. I was thinking this might be the
    circuit that was used to regulate ~150VDC down to a standby voltage. I need
    to trace the circuits to see if I can figure it out. If it is the problem,
    I have no idea what to replace it with. I doubt I can see the numbers on
    the diode, but
    I will yank it all back apart to see if I can determine what it is. Any
    idea what the standby voltage in these VCR's is?
  12. G B

    G B Guest

    I'm going to tear it apart again. I will check the cap's polarity, but I am
    not expecting I messed that up. I did draw a diagram before removing the
    original capacitors so I think I have the right capacitors in the right
    holes. I did substitute one value, I think a 100uF for an 82uF. I tried to
    buy duplicate values so worse case I can try a 2nd version of the
    replacement capacitor. I will reflow all the solder too... I'm pretty good
    at seeing a bad joint, but I've been bitten by that before too.

    Thanks for all your help... This would be so much easier if we just had a
    schematic or a similar schematic to get an idea from :)

  13. Arfa Daily

    Arfa Daily Guest

    Yeah, that's a problem with it. Nobody gets too upset on here, but on
    100uF subbed for 82uF will not be a problem.

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