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Multitrack cassette recorder repair problem

Discussion in 'Electronic Repair' started by [email protected], Jul 6, 2007.

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  1. Guest

    Hi

    Can anyone suggest what the problem is likely to be with my Tascam
    Porta 07 (a multitrack cassette recorder)?

    I'm operating the unit with a 12V 500mA power supply (exact equivalent
    to the Tascam brand PS-P2 power supply that goes with this unit) with
    correct polarity. The cassette transport works OK, but nothing else
    does.

    The power light doesn't come on, and neither do any of the many LEDs
    including the level meters. There is complete silence from all the
    outs including the headphone socket. There is nothing working at all,
    except the cassette motor and the cassette mechanism.

    I'm guessing that because everything is out, it's likely that there is
    one faulty component somewhere which is not letting power through to
    the electronics. As I have plenty of spares and could replace any of
    the discrete electronics components, it seems a shame to have to dump
    the unit if it can be fixed. Anyone have any ideas? Thanks
     
  2. Arfa Daily

    Arfa Daily Guest

    Start by looking for fuses ( which may not be obvious glass and metal
    tubes ). Look on the board to see if there are any voltage test points
    marked, or a list of the pin functions of any connectors, where you might be
    able to check for supply voltages. If any are missing, or if you can't tell,
    then look for transistors or monolithic regulators mounted on heatsinks.
    Transistor regulators should have one pin at a higher voltage than the other
    two, which will be at a (nearly) similar voltage. For positive rails, if the
    transistor type starts "C" or "D" , then likely, the middle pin will be at
    the highest positive voltage, the left pin will be at some lower voltage,
    and the right pin at a slightly lower voltage than the left pin. For
    transistors starting "A" or "B", the right pin will be at the highest
    voltage, and the centre pin at the lowest. For negative rails, reverse the
    "A/B" and "C/D" descriptions. Monolithic regulators start "78" for positive
    types, and "79" for negative types. The next two digits specify the output
    voltage, so a 7805 is a +5v reg, a 7812 is a +12v reg. A 7905 would be a -5v
    reg and a 7915, is a -15v reg. The pinning for the 78 series is, from the
    left, tab away from you, IN - GND - OUT. A 79 series is IN - OUT - GND.

    If that information doesn't get you on the track, then it is likely that a
    set of schematics would be necessary to get to the bottom of the problem, or
    possibly having a more experienced servicer's eye cast over it. Good luck
    with it.

    Arfa
     
  3. Guest


    Thanks Arfa. I shall try those things now.
     
  4. Guest

    Unfortunately there are no fuses in this unit. However, upon a visual
    inspection I identified a transistor with visible damage to its
    ceramic exterior. This transistor, a C1815, is in place but almost
    half of its ceramic exterior has been chipped off. Quite a bit of
    copper is showing, but there is no broken connection.

    Does anyone know whether it's likely that this sort of damage would
    stop a transistor from working? I have taken a digital image:

    http://xs.to/xs.php?h=xs217&d=07281&f=transistor.jpg

    (the chipped transistor is circled in red).
    Thanks
     
  5. jakdedert

    jakdedert Guest

    Was that a shill just to get to the 'two free ipod nanos'? In any case,
    I didn't wait for the pic to load. I hate popups, but I hate audio
    popups even more.

    However, when I see a component damaged as you describe, I consider it
    toast and start looking for the cause. It's not the external damage
    that's the issue. That transistor has literally exploded through some
    fault; either an internal fault in the component itself--or more
    likely--a fault in the circuit, which overloaded it.

    jak
     
  6. Arfa Daily

    Arfa Daily Guest

    Agreed. However, it looks like that transistor is in the bias oscillator,
    rather than the power supply. If it's gone short circuit though, it may well
    have loaded up the power supply, and done some damage. As far as there being
    no fuses in there, I bet there are somewhere - but they might well be
    fusible resistors, and if you don't know what you are looking for, you might
    struggle to spot them. If the silk screening on the board has circuit
    symbols at each component location ie a little zig-zag for a resistor, look
    to see if you can spot any zig-zags that have an extra little 'sine wave' at
    the end of them, indicating that they are a fuse type. You can usually spot
    them because they look 'different' from all the normal resistors. They are
    often, although not always, stood on end, and usually have matt finish
    bodies, often brown or grey. You will often see two gold bands on them also.
    The circuit descriptor may be different from other resistors. Sometimes,
    manufacturers will use, for instance, "PR" for "protector" or "FR" for "fuse
    resistor". They are likely to be located near to the bridge rectifier(s),
    smoothing caps, and regulators.

    The transistor type is very common. Virtually any general purpose NPN
    transistor with Japanese pinning ( flat towards you, pins down, E-C-B ) will
    do. The full type number for that transistor is 2SC1815.

    Also, its a bit hard to see for sure, but does that electrolytic cap just
    below and to the right of your red circle, have a slightly domed top ? If it
    does, it may have been responsible for the demise of the transistor, and
    will need to be replaced.

    Arfa
     
  7. Guest

    Thanks for your suggestions guys.

    Oh I see. I didn't know that.
    You have good eyes. Yes, that capacitor is slightly domed and could be
    the reason why the transistor failed. However, there is a large 4700uF
    cap next to the power input, which is very domed (fit to bursting it
    seems), and that could be equally responsible. I think on balance it
    would be best to trash the unit.

    Thanks for your help,

    max
     
  8. Arfa Daily

    Arfa Daily Guest

    Well, before doing that, for the sake of a couple of caps, I'd go ahead and
    change them. If that transistor is in the bias oscillator, you can just snip
    it out for now - it ain't gonna stop the unit working. If you still come up
    dead, then by all means give it up if that's what you want, but you never
    know, it might just come back up, and you'll feel really satisfied that
    you've saved an old friend from landfill ...

    That transistor can always be replaced later. It would only affect recording
    / erasing with it snipped out.

    Arfa
     
  9. Guest

    Greetings, all.

    I may be getting into this message chain way late, but the picture you
    took tells me your transistor is destroyed. Replacement is a minor
    thing, but the more important questions is "what caused it to fail?".
    I'm a Senior Electronics Technician and spent way too many years
    repairing equipment for the music industry. (When I started in
    electronics, tubes were a mainstay.)

    IN FACT, I have several Tascam Porta Ones that I'm working on
    repairing. It's going a little slow at the moment due to not being
    able to lay my hands on the service manual & circuit wiring diagrams,
    but I'll get there. (Yes: it's a cheap plug if someone wants to be a
    big help here....)

    At any rate, please feel free to pursue asking me all the questions
    you want (advice is free, actual repair work is not). The only caveat
    is that I don't monitor newsgroups with any frequency, so you may do
    better trying to email me at (and work to get this because I'd rather
    not have every bot in cyberspace sending me junk) seniortech AT
    carolina DOT rr DOT com and I will do what I can to guide you in the
    right direction.

    Thanks and a hat tip.

    Andrew....
     
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