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Sony VCR blows fuse

Discussion in 'General Electronics Discussion' started by Sam I am, Feb 23, 2019.

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  1. Sam I am

    Sam I am

    4
    0
    Feb 23, 2019
    I just took a VCR out of the basement to play my Blues Brothers movie. Plugged it in, nothing. Fuse was blown. Couldn't find a correct 2amp fuse so put a 5 in. Plugged in and poof. Fuse blew instantly. This happened without trying to turn unit on. Just plugged in. Sony mod.# SLV-N50. My only test equipment is a digital multi-meter.
     
  2. Sam I am

    Sam I am

    4
    0
    Feb 23, 2019
    Unit is removed from cabinet now. No obvious signs (black spot) on bottom of power supply board. Heating up the Weller to remove metal cover now. I was given this unit so I don't know if it worked before it was stored in basement or what might have happened to it. Lightning hit? Power surge after black out? BTW...I have no background in electronics. Only basic knowledge.

    tia
    Sam
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    11,662
    2,697
    Nov 17, 2011
    You can try the lamp trick to check the power supply section of the vcr.
    The lamp in series will limit the current even if there's a complete short circuit within the power supply. This way you can measure voltages at different points of the power supply to localize the problematic component(s).
    Uploading crisp pictures of the relevant part of the pcb may help. Look out for large electrolytic capacitor which may have leaked, bulged or itherwise have become inoperable. Also any dark spots on the pcb may indicate overheating. And, of course, look for obviously broken components.

    Not the best preparation for dealing with power supply issues. If you can't find an obvious spot, I recommend you get assistance from someone with suitable knowledge or get another vcr. Or buy the movie on dvd/blu ray. A few bucks are not worth risking your life,
     
    Sam I am likes this.
  4. Sam I am

    Sam I am

    4
    0
    Feb 23, 2019
    OK, I'll try this "lamp trick". I'm thinking the PS in the VCR is a switching power supply. I don't know anything about them. I built a 5 amp PS to run my CB in the house when I was a teen. Basic transformer, bridge rectifier, fuse and a switch. These SPSs are Greek to me.
    I have the CD. Just want to see if this can be fixed or does it go in the scrap heap.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    Do you have a multimeter?

    If so, I suspect that if you read the resistance across the pins of the power cord (obviously with it NOT plugged in to a power point) after replacing the fuse again, you'll find an almost dead short across them.

    There are a few relatively common and easy to repair faults. The first of these is one or more shorted rectifier diodes where the mains is converted to DC.

    Another common fault is the failure of the capacitor following this, but that is typically quite visible.

    A third is the failure of the transistor used to switch the DC through an inductor (or transformer).

    It is not uncommon to have more than one of these faults.

    I'd be taking a close look at the mains rectifier diodes first.
     
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  6. 73's de Edd

    73's de Edd

    3,121
    1,315
    Aug 21, 2015
    Sir Sam I am . . . . . ( I like green eggs and ham . . . .I do )

    After all of the promiscuous overindulgance and self abuse that you subjected that unit to . . . via the use of that 2X PLUS over rated fuse; I don't think that power supply section has recovered, or STILL even KNOWS what HIT it !
    For your next performance . . . you should not . . . . . Pass GO . . . . nor . . . . Collect $200 . . . . .for your effort, but, instead . . . . .proceed directly to JAIL to be able to have quiet time and consult the the marked up schematic of that unit, which I have now provided you.
    Go directly to locate the Q601 switch mode power transistor and fully expect it to now be crunched from its collector to emitter.

    Take your DVM and place it in its LOW OHMS mode and short the probes togeter to confirm its manner of display,of the reading of a dead "short" and then separate them to see how its display responds with an "open" circuit.
    Expect a close effect of that "short" display reading , when you then read from the collector to emitter connections of Q601 with the meter probes.

    Here is the units power flow pattern . . .
    (With mark up of subjective points of max interest, being made in in RED and pale YELLOW highlighting . )

    Raw AC line power comes into the . . . . (in respect, lets now bow our heads) . . . .vaporized F601 fuse link and then the application of both legs of the AC line power goes into the D600 Full Wave Rectifier Bridge .
    I know not where your home locale is . . . . from the non existant forum locale ID, but expecting you to be Amerikanski .
    In that case, you should then be expecting in the order of 160'ish volts of rectified DC power, which is developed across the right and left nodes of the
    D600 FWB unit and flows initially into the C605 main filter capacitor for its storage and DC smoothing, to make a ripple free DC supply.
    Then it takes the RED arrow path to the right and . . . (mainly flows ***) . . . to the right and passes thru the # 6-8 terms of the switch mode power transformer, to then connect to the collector of Q601.
    A conductive ground return flow path then starts from the emitter of Q601 an then via the BLACK-RED-BLACK arrows path down thru the R629 emitter resistor, which might have been knocked out, BUT , . 22 ohms at 1 watt might have survived that short of a fuse duration time . . . you tell us, after you ohm it out. .
    If it went, the D600 migt have " GONE" also.
    Best, to initially see if Q601 has shorted down . If so, pull it out of circuit, and take off the board .
    Then you can test for developed B + supply voltage, but, THIS TIME . . . . do the INTELLIGENT thing of a series 60-100W incandescant " dim bulb" utilization with AC power and then test to see if there is ~ 160-170 VDC being at the now vacated collector pad connection of former Q601.
    If so , you are then ready for some Digi Key or " Mouse House" parts shopping for some proper fuses and that specific Q601. . . . . 2SC4054P.
    ALSO do a CBE junction testing of adjunct transistors Q606 and Q602 , but I don't think that they were affected by any collateral damage..

    ( mainly flows ***) . . .
    Now at the almost simultaneous power application of that B+ supply loop, the DC power flow condition is ALSO feeding B+ down from the FWB and thru the series string of R601-627-602 - 100K resistors, to immediately have C606 E cap take on an abrupt charge and thereby " bump start " power transistor Q601 into its initial oscillatory state, until forthcoming repetetive feedback from T600's # 3-4 winding keeps the unit running on its own . . . until you ever unplug the unit.

    YOUR COMPLETE SCHEMATIC REFERENCING . . . resplendently complete with its relevant mark-ups . . . . .

    [​IMG]

    73's de Edd . . . . .

    Did 'ja ever notice . . . that when you blow in a dog’s face, he gets mad at you, but, then just let you take him out for a car ride, where he immediately sticks his head out the window, and into the strong wind ? ? ?
    Figure it out ?
     
    Last edited: Feb 25, 2019
    Sam I am and davenn like this.
  7. Sam I am

    Sam I am

    4
    0
    Feb 23, 2019
    10-4 good buddy. I copy that. But at least I didn't put a screw in place and look for the smoke.

    I'm a lawn mower mechanic. But I can read that schematic and will take the two above posts to heart. Got to build a few new slot cars for a local Flexi car race but will get to the Vee Cee Are soon. Thanks for the advice guys.

    Sam
     
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