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SMPS help needed - Philips VCR

Discussion in 'Electronic Repair' started by [email protected], Aug 6, 2008.

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  1. Guest

    Hi,

    I'd be grateful if someone could give me some clues as to where to go
    next with a dead Philips VCR (VR750) SMPS.

    The PSU died when the mains dropped to 30-60v for about half an hour
    (in the UK so it should have been around 230v) before going away
    completely for a few hours. I was on holiday at the time so don't
    know how cleanly the power came back, although the VCR was on a surge
    protector so it's unlikely that a surge has killed it.

    Neither fuse is dead.

    There is a short (~0.5s) squeal from the PSU when mains is applied.

    I've found a circuit diagram and spent some time reading the various
    FAQ notes.

    I've checked the large electrolytics and the resistors I guess to be
    the "startup resistors" and they all check out OK with a DVM (having
    removed each component in turn). One electrolytic capacitor was a
    little low (67% of its nominal value) and was replaced but this made
    no difference.

    I have measured various voltages around the circuit and quite a few
    are different to those shown on the diagram. There is no voltage
    present on the output side of the transformer. The PSU is integral to
    the main circuit board so will be difficult to test in isolation with
    no load.

    In my limited knowledge, it would appear as though the switching
    control transistor is holding the switching transistor in a non
    conducting state but I would appreciate some clues as to why this
    might be.

    I have annotated the diagram with the measured voltages (in red) and
    indicated the components I have pulled and checked; the diagram can be
    found at http://www.ajuniper.f2s.com/psu.jpg (2MB image).

    I would appreciate it if someone more knowledgable than myself could
    give me some clues as to what to look at next.

    Thanks,
    Andy
     
  2. Ulrik Smed

    Ulrik Smed Guest

    Could it be Q003? It should not be possible to have 1.2V at its base when
    the emitter is grounded, if it works.
     
  3. Franc Zabkar

    Franc Zabkar Guest

    Check for shorts on the secondary outputs. I'd suspect a shorted 18V
    protection zener (DO15) on the AL+15V rail.

    - Franc Zabkar
     
  4. Quite possible.

    Some of the voltages don't make any sense around Q003. For example,
    1.2 V across B-E of a normal NPN transistor, and +1.5 and -1.5 on the
    IC001. But this might be due to there being some AC as the PSU
    repeatedly tries to start up, not just DC voltages, and this is
    confusing your meter.

    While you're at it, it would be worthwhile to check the electrolytic
    caps for ESR and uF. If the zener is shorted, the cause may be bad
    caps.

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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  5. Franc Zabkar

    Franc Zabkar Guest

    Thanks for the followup.

    I just remembered that I've seen faults in Sharp VCRs (Philips VCRs
    are often rebadged Sharps) where the zener isn't actually shorted, but
    is protecting the outputs from an overvoltage caused by a small
    electrolytic cap on the primary side of the supply. In this case I'd
    change CO11 just for good measure.

    - Franc Zabkar
     
  6. Guest

    Thanks for all the suggestions, I'll work thru those tonight and see
    what comes up.

    Andy
     
  7. Good idea to change them all if you find one that is marginal or bad!

    --
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  8. Guest

    Early days yet but it would appear as though both Q003 (infinite
    resistance both ways across one junction) and D015 (short circuit) are
    both dead. So a good call on both counts.

    Unfortunately I can't get a replacement diode or a close transistor
    match until tomorrow so it'll be after the weekend before I find out
    what else has blown.

    Aside from experience, what else led you guys to the above
    conclusions? Why do the symptoms point at these components?

    Thanks,
    Andy
     
  9. Franc Zabkar

    Franc Zabkar Guest

    A squealing sound from an SMPS often indicates a short on the outputs.
    A protection zener across one of the outputs, in this case the AL+15
    rail, will sometimes sacrifice itself to prevent an overvoltage
    condition from destroying more expensive components.

    An overvoltage condition can arise when the main switchmode regulator
    loses control. When everything is working correctly, the optocoupler
    (IC001) senses the output voltage on the secondary side and uses this
    feedback to control Q003 and Q002 in such a manner as to reduce or
    increase the drive to the chopper (Q001). If, for example, IC001 or
    Q003 fails, then the supply thinks that there is no output, which then
    causes the chopper to "run away".

    In your case I would check all the active components (transistors,
    diodes, optocoupler). If any are faulty, then there is a real
    possibility that the supply will fail catastrophically and you'll end
    up changing the exact same components again. 2SC1815 transistors are
    cheap, as are signal diodes, so a shotgun approach may be advisable. I
    wouldn't change the chopper, though.

    - Franc Zabkar
     
  10. Couldn't have said it better myself. :)

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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  11. Guest

    My thought is Q002 has been switched on by the Diode OR Gate formed by
    D007 and D008.
    If D021 does not operate, then I think 6.4 volts will not be generated
    for IC001.
    My 2 C.

    - Ravi.
     
  12. Guest

    Hi,

    Thanks for all the help - turned out it was just D015 that had died (I
    initially messed up checking the transistor) - the video now lives
    again.

    (although the video did get me worried when it wouldn't accept, or
    would just spit the tape out, before I'd put the lid on, but that
    would appear to have been my bench spotlights confusing the tape
    detection beam breaks...)

    Thanks for the explanations of what went wrong.

    Andy
     
  13. Guest

    Another quick question on this - are all Zeners created such that they
    go short circuit like this - or is it just luck - or do I need to
    ensure that the replacement will fail in the same way under the same
    circumstances?
     
  14. Diodes do tend to fail shorted when abuse.

    About your repair: Don't be surprised if the same thing happens again.

    The zener likely didn't die on its own. Probably, some electrolytic
    caps in the supply are marginal. I wouldn't be surprised that
    if you unplug it for a day and plug it back in, the same zener
    will go belly up again.

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
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  15. Guest

    I did also replace the electrolytics anyway, since I'd already done
    that to find that they weren't completely at fault. :)

    They may have been at fault (although only around 5 years old), but I
    bet that the mains voltage dropping down to 30-60v for half an hour
    also had something to do with it...

    Andy
     
  16. Franc Zabkar

    Franc Zabkar Guest

    A friend's Sharp VCR died exactly the same way (shorted protection
    zener) after a brown-out. The only problem I could find was that
    zener, so I assumed that it must have died when the output of the
    supply went into overshoot when the mains supply returned. I usually
    change the small electros on the mains side in any case.

    - Franc Zabkar
     
  17. Yeah, that could do it.....

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
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    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
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