Philips (Black Tulip) N5748 HI-FI Cassette Deck Power Outage

Discussion in 'Technical Repair' started by Michael 5of9, Dec 12, 2018.

  1. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    A very good day to you all on All About Circuits;

    I have inadvertently caused the loss of part of the power supply; can someone please give me some help with restoring this back.

    Have lost power to both motors and both solenoid valves (relays 475, 476) this is a Philips (Black Tulip) N5748 HIFI Cassette Deck; the construction is the same as the N2552.

    These are the voltages I have on Board U302 Connection block;

    18. -1Mv
    25. +27v this one is correct
    27. -2Mv
    28. -1Mv

    The full size Service Manual and individual relevant pages can be seen here can be seen hear on my OneDrive; many thanks in advance for your help.

    https://1drv.ms/f/s!AuQDAPiShOjc9RoD2G3xlbzfvUuQ


    U402 Power Supply.jpg
     
  2. LesJones

    Well-Known Member

    Jan 8, 2017
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    Check that the voltage across C730 on the power supply board is about 19 volts. If not check fuse F427. This is to find out why you don't have - 12 volts on connection block pin 27.

    Les.
     
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  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    The psu board gives out -12V, +15V, +25V,
    I would start there first.
     
  4. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    Positive side 21.8v
    Negative side -002.2Mv
    Fuse F427 good

    Kind regards Michael
    Checking Working Voltages accross C730.jpg
     
  5. LesJones

    Well-Known Member

    Jan 8, 2017
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    I suspect IC431 is faulty. I think this is a positive 12 volt regulator. As It's output pin (Pin 2) is connected to ground it's common pin (Pin 3) should be - 12 volts with respect to ground. The fact that on this occasion you did not measure the voltage across the capacitor as requested but you measured each end with respect to ground has saved me asking the next question I was going to ask. Just as further confirmation can you measure the voltage directly BETWEEN pin 3 of IC 431 (Negative probe.) and pin 2 (Positive probe.) of IC431. You should get a reading of about +12 volts. If you do get this reading then the IC is probably OK but there is a break in the path between ground on the power supply board and the point that you used as a reference when making the readings in post #4. If you don't get a reading of 12 volts then the IC is faulty.

    Les.
     
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  6. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    Working voltage across C730; 21.5v

    Working voltage across IC431 L2012C2 + to pin 2 and - to pin 3 gives -000.0Mv
    And working voltages to ground;

    1. 22v
    2. -2.3Mv
    3. -2.3Mv (centre leg)

    Apologies for my misunderstanding, regards Michael
     
  7. LesJones

    Well-Known Member

    Jan 8, 2017
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    IC431 is faulty. Your missundersatnding saved one step in the questions. Dave's advice is good. Checking power supplies is a good place to start fault finding.

    Les.
     
    Last edited: Dec 13, 2018
  8. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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  9. LesJones

    Well-Known Member

    Jan 8, 2017
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    L2012C2 seems to be a Philips in house part number. I think this 7812 regulator will probably work. It is rated at 1.5 amps but we don't know if the Philips item was a higher rating. I am assuming the view of the power board in Fig17 is from the etch side. If that is the case the pin layout should be correct. If not then the position of pins 1 and 2 are swapped over so you would have to find a way to get round that problem. (Possibly make a thick aluminium spacer to bolt it to the heat sink.) Note. There may be other components damaged by the fault that destroyed that regulator.

    Les.
     
    Last edited: Dec 13, 2018
  10. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    That is quite correct Fig 17. shows the etched side; looking at Fig 16. it shows, input to pin 1, output to pin 3 centre pin, and ground to pin 2.

    It seems that this L7812ACV with TO220 packages; does not match, it has incorrect pin sequence, ground being the centre pin, I would have to realign pins 2 and 3, and sleeve the legs with heat shrink.

    I have a query; the input voltage on point 1 is shown as 7v, the specifications shows the minimum input voltage as 19v, to me this looks like there could be a problem?

    I have had a look on Farnell’s website; and this “LM1085IT-12/NOPB - Fixed LDO Voltage Regulator, 2.6V to 27V, 1.3V Dropout, 12Vout, 3Aout, TO-220-3” seems to be a better option, what do you think?
    https://uk.farnell.com/texas-instru...to220/dp/2141368?st=12 volt regulator L2012C2

    Understanding: Fig 16. U402; points 6 and 7 shows 15v~ (ac) is this measurement obtained across those points?

    IC431 point 1 +7v is this measurement obtained to ground?

    Point 3 and (U402) points 11 and 17 -12v are these measurement obtained to ground?

    The attachments shows a close-up of IC431 L2012C2; it looks like the regulator has a loose cover to help hold it in place.

    IC431 L2012C2  a.jpg IC431 L2012C2.jpg

     
  11. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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  12. LesJones

    Well-Known Member

    Jan 8, 2017
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    The +7 volts marked on the input to the regulator and the -12 volts on output pins 11 &17 of the power board are with respect to the ground on your recoder. As the regulator is designed for + 12 volts output and is being used to provide -12 volts output what the data sheet on the 7812 calls ground is - 12 volts on your recorder. This is because the regulatorts + output terminal is connected to your recorders ground. This means that the + 7 volts shown on it's input terminal is actually +19 volts with respect the the terminal that the regulator consideres as it's ground. The pin layout of this regulator will be correct.
    The LM1085 regulator is an adjustable regulator. (The farnell description as a fixed regulator is wrong.) It should be possible to use it but would require two extra resistors to set it's output voltage. See the data sheet on the device. You may also need to add exta decoupling capacitors on it's output as the data sheet shows a tantalum capacitor which is better for high frequency decoupling. It could oscillate without that.

    Les.
     
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  13. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    Many thanks for keeping me right on this Les; so I will fit the new as normally fitted.

    Now that you know my concerns; will it be alright to order and fit the one (7812) you found yesterday or have you since identified a 2 amp one that would be better I think, from the RS website?
     
  14. LesJones

    Well-Known Member

    Jan 8, 2017
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    Yes. As we don't know the specification of the original it is worth going for the higher current rating.

    Les.
     
  15. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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  16. LesJones

    Well-Known Member

    Jan 8, 2017
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    Either of those regulators should work electricaly. The second one seems to be only sold in a minimum quantity of 50. I can't work out if the pinout is correct as the data sheet does not show how they number the pins. The pinout of the first device is wrong The minimum input voltages quoted for both of thes devices must be wrong as it must be greater than the output voltage.

    Les.
     
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  17. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    This is the specifications for the first regulator, kind regards Michael
     
  18. LesJones

    Well-Known Member

    Jan 8, 2017
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    If you look at page 2 the middle pin is not the ground pin as it is on the original regulator and the 7812.

    Les.
     
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  19. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    Yes you are quite right; pin 1 = ground, whereas I am looking for pin 1 = input, I didn’t expect that.

    So I have made a thorough look through all of RS 12v, 2 to 3A regulators and there is only one its the Microchip MIC29300-12WT, Pin 1 = Input, 2 = Ground, 3 = Output.

    This is the same pinout as the 7812; would you kindly give me the thumbs up on this one, I am sure that this one will be the nearest one so far, then I can get it ordered, thank you?
    https://uk.rs-online.com/web/p/low-dropout-voltage-regulators/9101597/

    Kind regards Michael
     
  20. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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