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

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

  1. LesJones

    Well-Known Member

    Jan 8, 2017
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    Those resistance readings are OK. Repeat the voltage test on IC427 pin 7 but also check it at pin 11 and pin 17 of the power supply board U402. These points should be connected together so they SHOULD all give the same voltage readings. (The only thing that would make sense of the readings so far is a broken track on the power supply board between pin 11 and pin 17.)
     
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  2. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    With removing this board so many times the contacts have become dirty again, have clean them and I now have good voltage readings on IC427 pin 7. -11v and - 11v with play pressed.
    Rechecked pin 6 and the two readings are still the same, -19m and -5v, so I will go ahead and renew this IC427 as agreed in Saturday's post #137.
     
    Last edited: Jan 30, 2019
  3. Michael 5of9

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    Dec 12, 2018
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    Replaced IC427, the sound, sounds very fast, play, wind and rewind are only for a second.

    If you would kindly look at the attachment, you can see what is happening to the pins on the board, there is a place where the silver coating has worn through, this has happened with refitting the board, lots of times, I have started to take voltage readings and I am wondering if I should press on and renew the rest of the IC's?
    I am getting some 1.5v on pins 5 and 6 of the same IC427 after pressing play?
    Can the damaged silvering be easily repaired?
    I thought I would see what you have to say.
     
  4. LesJones

    Well-Known Member

    Jan 8, 2017
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    Fast play problem. I seem to remember that you said that you have changed some components in the M1 motor speed control loop. That could be the reason for the speed to have changed. We will look at that problem later.
    I don't think the coating on the connector is silver. It is probably tin. If you try to put a coating of solder on it you could finish up with the copper separating from the laminate.
    I can't think of an explanation for the +1.5 volts on pins 5 & 6 of IC427 on board U301 as the most positive supply to the logic ICs is ground. Can you measure the voltage pins 11, 12 & 13 of IC 437 on that board. Also check the supply pins 7 & 14 in both the stop and play states.
    I think it would be worth checking the solder connections to the connector that board U301 plugs into on board U304 particularly pin 28. Also check the solder connections to conn 2 on that board. I also noticed the in the picture in post #143 the end of C746 above pin 28 looks like it is not soldered.
    I would not replace ICs without having evidence that they are faulty.

    Les.
     
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  5. Michael 5of9

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    Dec 12, 2018
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    The parts replaced after wrongly fitting the 4 transistors TS450 to TS453 on the Motor Control Board 302;

    IC455 – 1 off TDA 1059B; replaced with, (unknown manufacture) TDA1059B Integrated Circuit TO-126

    IC435 and IC447 – 2 off LM 358N; replaced with, Fairchild Semiconductor LM358N OP AMP, DUAL, 1MHZ, 0.1V/US, 8DIP

    2A fuse on F426 on the power supply board U402

    No further parts fitted until you advised after power outage; of which I inadvertently caused a short on the same board between board pin 27 and base or pin 28 and collector of TS457.
    Yes that makes sense; years ago they used to tin copper wire for house wiring, it has worn through on many pins and you can see the copper, see post #143 this is very concerning, when I took the last series of voltages in post #136, I ended up removing and replacing this board 64 times in total moving onto and off these connections, it is important to minimise this wearing and I took the decision to replace the rest of the IC’s on that board, they are all fitted except IC439 MC4001BCP this one came as an LM324AN, I have sent it back as it was incorrectly sent, so I cannot proceed until the correct part is here.

    To prolong the life of the board pins I will put a little Kontakt 61 to help reduce the friction every other time of replacing.
    I will check these if necessary once the last IC is fitted and tested; I’m being optimistic about this being solved.
    Meanwhile I will go-ahead and do a resistance test on (board U301, pin 28.) on the socket of board U304 of pin 28. it goes all the way back to connection block 2 pin 2. on the same board U304.

    Yes; I replaced the electrolytic capacitor C746, it was an axial type, at the time I could only get a redial type and the board allowed me to use a closer connection, hence you can see this redundant entry, see the photo.

    On the motor control board U302 there are several BZX79 series voltage regulators; could I test these and how?
     
  6. LesJones

    Well-Known Member

    Jan 8, 2017
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    I don't think replacing any of those parts would cause the capstan motor (M1) speed to change.
    Shorting pin 27 to pin 28 on board U302 probably destroyed the - 12 volt regulator. Shorting pin 27 to either the base or collector of TS457 should not have caused ant damage.
    If moving between test points involves removing the board each time the replacing all the ICs at once may be the lesser of two evils.
    In the factory or repair centre they would probably have extender boards to make the board that you are working on more accessible

    There is no point in doing this test unless we have the same symptoms.

    The idea of the suggestion was to do a physical check of the solder joints as there seems to be an intermittent connection in that path.

    I think you have that new capacitor the wrong way round. The negative side should connect to pin 29.

    You can just measure the voltage across them. It should be about the voltage indicated by the last three characters of the part number. For example D457 part number finishes with 4v7 so that should be about 4.7 volts. For D476 the reading (7.5 volts) will only be correct when there is a cassette inserted.

    Les.
     
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  7. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    U301 Logic Control Board IC’s are all renewed; so time to test with a C60 (30min each side) cassette ready, fast forward works very well, rewind works, but tires near the end then stops, the tape is free to turn, so that should rule that issue out.

    Play still too fast; I wonder if it is one or more BZX79 series voltage regulators on U302 Motor Control Board?

    I have taken a better photo for you to see the negative side of that C746 on board U301 Logic Control Board as it wasn’t clear on the previous photo that you queried.

    You can see a short video just taken today showing the gears in motion; on my one Drive
    https://1drv.ms/f/s!AuQDAPiShOjc9RoD2G3xlbzfvUuQ
     
  8. LesJones

    Well-Known Member

    Jan 8, 2017
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    Hi Michael,
    The capacitor is the correct way round. (I thought I would get that out of the way first.)

    For the rewind problem first make sure that the pressure roller is not being pressed against the capstan on rewind. (I think this is unlikely but worth checking.) If you can monitor the voltage across the motor during rewind to see if it starts to drop when the motor starts to slow down. Also if you can monitor the motor current you could see if it increased when the motor started to slow down. If it did this would indicate an increasing load on the motor. It is also possible that the fault is mechanical. I think the motor will drive the reels in the cassette via a slipping clutch. I this is the case then the clutch may be slipping too easily. I think the clutch will just be a felt pad between two plates.

    Tape speed problem.
    I have only found 3 BZX79s
    I don't think D476 will effect to tape speed (Motor M1) as it just deals with switching on the -11.8 volt rail B1 which supplies power to many of the logic ICs on board U301. (This must be working for the unit to be working as much as it is.)

    I don't think D457 will effect to tape speed as it is just setting the bias point of IC435. As this is just amplifying the tachometer pulses from the capstan motor the actual level is not critical. (The frequency is proportional to the speed and this is later converted to a voltage which is compared with a reference voltage and the difference controls the motor speed. (This comparison is done by IC447b.)

    Diode D477 does provide a reference voltage for the motor speed so it is worth checking the voltage om both ends of this diode with respect to ground. One end should be -12.0 volts (Rail B) the other -2.9 volts (Rail B2)

    Les.
     
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  9. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    Yes the slide relay that operates both pressure roller and lifts the tape heads up into place; are definitely not activated during wind or fast forward, I have taken a video showing the Slide solenoid, forward wind and rewind, you can hear the motor tiring out, incorrect voltage making underpowered?
    I have tried another good cassette, it almost completed rewind.

    Video Slide Solenoid Rewind; https://1drv.ms/v/s!AuQDAPiShOjc9lqNF1CpUvPvdcyT

    I seem to remember that this could be an old problem; and as a workaround I used to just turn the cassette around and fast forward?

    I have monitored the voltages to motor 2. via connection block 12. pins 15 positive and 16 negative/ground, the reading was the same when I connected to my usual ground connection, so I proceeded from that ground connection, which is more secure and safer, I am telling you this so you know how I proceeded with these voltage readings.

    Wind (fast forward) voltage -168mv, my interpretation, should be -6v to -7v
    Rewind gives -7.22v and rises to -8.22v, my interpretation, should be +6v to +7v
    Play need to acquire this and recheck the above and confirm because they are wrong.

    There is a video showing motor 2. voltages; https://1drv.ms/v/s!AuQDAPiShOjc9l1hvN2UvKrSNBpr

    On the schematic it gives motor 2. voltages for Wind ( button ff = fast forward ) and Rewind ( button rew ) and Play; I should have taken the play voltage as well because this might have something to do with the play speed problem?

    Motor current; if this is still important I have two options, the easy option is to use my Fluke current clamp, this plugs into my Fluke 73, I need to check it will work on DC, or unsolder the wire on pin 15 to make two meter connections?

    Looking from the front of the unit the reel on the left is free as a bird, reel on the right is difficult to check as it connects with other gears.
    Can we get the voltages correct first before investigating current and mechanical for issues; I am wondering if there is a bad connection between a board and its connection block, or have I fitted an incorrect part, IC435 and IC447 – 2 off LM 358N, IC455 – 1 off TDA 1059B see first part of post #145?

    I have checked all (to end any uncertainty) the 3 BZX79 series diode voltage regulators (also see the later part of post #146) the first reading is taken across the diode with the positive lead on the anode side and the negative lead on the cathode side and unit on and a cassette in.
    The second voltage test in two parts; neutral to ground and connect to anode (a) side and followed by connecting to ground and cathode (c) side.

    D467-C4v7 (4.7v): -5.22v, a -11.89v, c -6.65v

    D476-C7v5 (7.5v): (-5.89v without cassette) -7.63v, a -11.03v, c-3.39v

    D477-C9v1 (9.1v): -5.22v, a -11.89v, c -6.66v (not as you expected)

    I will post the voltage readings for play and reconfirm Wind and Rewind hopefully on Monday morning; so replying to this when you are ready, many thanks Les for your help.
     
  10. LesJones

    Well-Known Member

    Jan 8, 2017
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    M2 motor voltage readings.
    Using ground instead of pin 16 of Con 12 is not a valid way of testing, TS450 - TS453 behave like a DPDT switch. Think of the collector and emitter as a switch contact. So in one direction TS450 and TS453 will be conducting (Contacts closed) So the left hand connection on M2 will be connected to ground and the right hand side connected to pin 3 of IC455. In the other direction TS451 and TS452 will be conducting. (Contacts closed) So the left hand connector of M2 will be connected to pin 3 of IC455 and the right hand side connected to ground. There will be a small voltage drop across the transistors when conducting so you can't assume zero voltage drop as you would with a normal switch contact. Summing this up TS450 to TS453 reverse the polarity to the motor and IC455 controls the voltage (And hence the motor speed.) I don't think the current clamp will work on DC (There are some current clamps that work on DC that use a Hall effect device.) but even if it does I don't think it will read such low currents. You will have to break a connection to the motor to measure it's current. You can leave that test for now if it is difficult to do.
    The left hand side will be the supply reel so if the clutch between the reel and motor slips too easily then the reel could be slowing down but the motor could be running at the correct speed. When it is near the end of a rewind the diameter of the tape on the left hand spool will be larger so more torque will be required to give the same tape tension so that could explain why it slows down near the end of the rewind.

    Zener diodes.
    I think D477 could be the cause of the tape speed problem.

    Les.
     
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  11. Michael 5of9

    Thread Starter Member

    Dec 12, 2018
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    Thank you for keeping me on the right path, I have made connections as per schematic drawing, this time, negative lead to pin 15. positive lead to pin 16. these are the voltage readings now.

    Wind -6v to -7v
    Rewind +6v to +7v both of these are correct.
    Play -3.5v (Fluke meter -3.5v these are the same) the reading should be -3.7v, would I be correct that the fault is with Zena diode D477-C9V1?
     
  12. LesJones

    Well-Known Member

    Jan 8, 2017
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    I think those readings are near enough. What you need to check is that the voltage on rewind does not drop significantly near the end of a rewind when you see the spools in the cassette slowing down. If the motor was slowing down I would expect to see the motor voltage to drop. If the spool slows down but the motor voltage does not drop then the fault is the friction clutch between the motor and the supply spool slipping too easily. In play mode the tape speed is controlled by the capstan motor (M1) The take up spool must all ways be winding the tape but as the diameter is increasing as it fills up with tape it can't be driven at a fixed speed (RPM) This is solved by having a slipping clutch and running the spool motor a bit faster than the maximum speed required.

    Diode D477 will ONLY effect the capstan motor (M1) speed.

    Les.
     
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  13. Michael 5of9

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    Dec 12, 2018
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    I have produced a new video giving details of voltage and motor 2 movement and sound; https://1drv.ms/v/s!AuQDAPiShOjc9mCOW2InU3aQvzSy
    The tape is about 2/3rd through rewind and the voltage has increase to just over 8.05v and then as it is tiring-out the voltage decreases to 7.74v (the voltage on rewind does not drop significantly near the end of a rewind); it’s 1 volt over specification, is this will be due to the extra load on the motor?

    Watching and listening to the motor; you can hear that there is something struggling and just before the end if you stop the video at 40 seconds running (and it is running anticlockwise) if you look at the top side of the pulley on the motor can see the torque that has been produced by the belt being stretched in a way that you can see that it isn’t being taken round by the large plastic flywheel because of the friction produced on the running gear and tape, this is the problem area.
    There is more torque/friction being produced on rewind than wind, because of the configuration of all the gearing. I am sure you are right that more friction and torque is being produced at this latter part of rewind by the tape being built-up on the reel, if you look and listen to Video 3 Nylon Spacer Washers Fitted can see all is running normal at this section of the tape; https://1drv.ms/v/s!AuQDAPiShOjc9llGWfqRZ6xFT1wb
    I don’t know how you are listening to these videos Les; headphones give a better hearing?

    I have brought this cassette into a warmer environment from 11° to 21° to warm it up; this might have an effect, I will let you know later today on the outcome.


    Do I need to just order a new one of these to correct the speed?
    Many thanks,
     
  14. LesJones

    Well-Known Member

    Jan 8, 2017
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    I don't think I can diagnose the mechanical fault remotely. The voltage readings seem to be OK. There is no closed loop speed control on motor M2 in the same way there is on motor M1. The speed just depends on the voltage supplied to the motor by IC 455. It supplies about 6.5 volts for wind and rewind and about 3.7 volts on play (As the tape is moving slower on play than it is on wind/rewind. M2 is also protected from overload by PTC thermistor R594
    I suggest taking D477 out of circuit and testing it. It is specified to clamp at it's rated voltage of 9.1 volts with a current of 5 mA. I suggest connecting it to a 20 volt DC power supply via a 2.2 K ohm resistor and measure the voltage across it. (Positive to the end with the band.) Also check the voltage between the two points it was connected to with it removed. It should be more than 9.5 volts. (Probably about 11.5 volts) If it is not C747 or R595 could be faulty.

    Les.
     
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  15. Michael 5of9

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    Dec 12, 2018
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    The warm up hasn’t contributed to fully rewinding.
    Thank you Les
    I will remove D477 and with a cassette in and power on only; give you a voltage reading via the connecting legs, negative side of C747 and positive side of C747, which would then be across this capacitor, this would then give you the voltage from the two connecting points of D477?
    I think it would be best for you to tell me which legs and leads to connect to what, on this one, as it needs to be done right?

    I could also give you the voltage to ground via positive side of C747 or even remove and capacitance test?

    What about R595 for a voltage or resistance test?

    If I report back with the results; would it then be possible test the D477 by soldering one leg and taking a reading from the other leg, finding a power supply could be a problem, I will see what I have?
     
  16. LesJones

    Well-Known Member

    Jan 8, 2017
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    Thinking a bit more about the rewind problem. When rewinding there must be something that provides a small amount of braking on the take up spool so that there is some tension on the tape. Think about the two spools as two pulleys with a belt between them. At the start of the rewind the supply spool will be small and the take up spool large so this would be reduction drive so it would provide more torque at the take up spool. near the end of the rewind it would be the other way round so there would be less torque available at the take up spool. I think it would be worth looking to see what provides this braking. It might also be worth trying to fool the recorder into thinking that there was a cassette inserted and doing a rewind. I can't remember how cassette recorders detect tape in path and end of tape so I can't suggest a way to fool these sensors.

    D477 problem. The ends of C747 are the same points as the ends of the diode so they may be easier to get to. (If it is easier you can measure the voltage between ground and each end of the capacitor and subtract the readings.) I think it is more likely to be the diode that is faulty. I think there is another fault on the speed controller as from a quick look at the speed control circuit I would expect the way the zener seems to be faulty would cause the speed to be slow rather than fast.

    Les.
     
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  17. Michael 5of9

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    Dec 12, 2018
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    Rewind Issue Page 9 of the service manual; Cassette Mechanical Mechanism there seems to be 2 items related to both reels, numbers 174 left and 181 right, see the attached photo, giving a glimpse of these, the outsides don’t appear to go round, the inside could be your answer to finding the braking/clutch/tensioner, shall we name it “tensioner or drag mechanism”, I think one of them has too much hold?

    I think some of the experts that know all about these units are still around today; the story of this repair started some time ago, I found some good advice on a forum belonging to a Thomas Baur, in the Netherlands, I think that there is a good chance that someone is quite familiar with this issue, is that alright with you Les if I inquire, before further investigation on the rewind issue?
    I have added some more videos’ showing 174 and 181 whilst rewinding, into a new folder called Rewind Issue, via the main folder.
    https://1drv.ms/f/s!AuQDAPiShOjc9RoD2G3xlbzfvUuQ

    Play speed too fast D477-C9v1 (9.1v): has been removed.

    Voltage reading across C747: 10.3v (negative leg to ground -11.8v; positive leg to ground -1.5v, without deductions) see the latter part of post #149 for all three diode voltage regulator, voltage readings.

    Hmmmmm?
     
  18. LesJones

    Well-Known Member

    Jan 8, 2017
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    I think using Thomas Baur's forum will be a good idea to deal with the rewind problem. (As he is from the Netherlands he will be able to read the text of the service manual which will be a help.) I have not managed to work out how the drive mechanics works from the manual and you videos so I can come up with no possible solutions.
    I think the zener diode is faulty. Have you tested it yet ?) While I was trying to understand why there was a 1.5 volts drop across R595 I noticed that the B2 supply rail (Which is regulated by the zener) is us to supply IC435, IC447 and the charge pump circuit (Which converts frequency to voltage.) as well as the reference voltage for the speed control. This means that my reasoning for suspecting another fault is wrong.

    Les.
     
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  19. Michael 5of9

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    Dec 12, 2018
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    I have a power supply with outputs 3, 6, 9, 12 volts dc?
    Can you kindly calculate the new size of resistor; I have plenty of 130, 160, 200 ohms resistors and some Through Hole Trimming Potentiometers?
    Is the attached D477 test circuit drawing interpreted correctly so far?
    As the diode is connected in reverse for this test; the voltage test across D477, would the common lead connect to anode?
     
    Last edited: Feb 14, 2019
  20. Michael 5of9

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    Dec 12, 2018
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    I have asked for help on the forum of MFBfreaks.com; this is the start of my repair story; http://forum.mfbfreaks.com/viewtopic.php?f=5&t=8426&p=242883#p242883
     
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