HELP Technics SU-V60 Stereo Amplifier VC-4 distorted sound

Discussion in 'Electronic Repair' started by PinballDog@comcast.net, Dec 16, 2006.

  1. Guest

    I have a Technics SU-V60 Stereo Amplifier. The left channel is
    distorted and crackling at low or high volume with any source cd, radio
    etc.
    You can still get power out of it but it sounds bad. Flipped speaker
    wire, problem still on Left channel.

    I repair pinballs for fun so I'm not afraid to solder or replace
    componets. Looking for a starting point.
    I read that Technics was natorious for bad solder joints. Maybe I will
    reflow all and see.

    There are 2 8000 uf 66v caps at back of motherboard, is this worth
    replacing?
    or are the c3944 transitors with the heat sink?

    Any lead would be a great help. This amp still sounds great ( in the
    right channel ) I really want to save it.

    Anyone have a schematic they can share?

    Thanks In Advance
    Dave
     
    , Dec 16, 2006
    #1
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  2. Arfa Daily Guest

    <> wrote in message
    news:...
    >I have a Technics SU-V60 Stereo Amplifier. The left channel is
    > distorted and crackling at low or high volume with any source cd, radio
    > etc.
    > You can still get power out of it but it sounds bad. Flipped speaker
    > wire, problem still on Left channel.
    >
    > I repair pinballs for fun so I'm not afraid to solder or replace
    > componets. Looking for a starting point.
    > I read that Technics was natorious for bad solder joints. Maybe I will
    > reflow all and see.
    >
    > There are 2 8000 uf 66v caps at back of motherboard, is this worth
    > replacing?
    > or are the c3944 transitors with the heat sink?
    >
    > Any lead would be a great help. This amp still sounds great ( in the
    > right channel ) I really want to save it.
    >
    > Anyone have a schematic they can share?
    >
    > Thanks In Advance
    > Dave
    >

    The large caps will not be the cause of the problem, so don't consider
    replacing them. It is also unlikely that you have defective output
    transistors, particularly if the amp is not going into any self protect
    mode. Bad joints are common on Technics kit, so it is certainly one of the
    first things that I would be looking at. You can get a good idea of the area
    of the problem by seeing if the crackle is altered by the volume control. If
    it is, then the problem is back up the front end, and you needn't look
    further into the area around the output stage. If it is not affected by the
    volume control, then the problem is either output stage or power supply
    related. Many models of Technics gear, have regulator transistors attached
    to the same heatsink as the output transistors, and it is very common for
    these to go bad jointed. Be very careful with your probing, whilst the amp
    is on. A slip of the meter probe can be disastrous on these DC coupled amps
    ....

    Arfa
     
    Arfa Daily, Dec 16, 2006
    #2
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  3. Guest

    Volume dial has no control of crackle. Low and high behave the same
    amount. I noticed a small browning of the circuitboard under a C263A
    located behind the output transistors. Its the one located farthest
    left. Looks like a group of 4. If the left 2 are left channel (from
    front of unit) might this be it?

    Thanks for the help
    Dave
     
    , Dec 17, 2006
    #3
  4. Arfa Daily Guest

    <> wrote in message
    news:...
    > Volume dial has no control of crackle. Low and high behave the same
    > amount. I noticed a small browning of the circuitboard under a C263A
    > located behind the output transistors. Its the one located farthest
    > left. Looks like a group of 4. If the left 2 are left channel (from
    > front of unit) might this be it?
    >
    > Thanks for the help
    > Dave
    >

    Ok, no control of problem by vol control, says output or PSU. I would
    definitely start by visually examining the soldering on all power devices on
    the heatsink, and any areas where the board is heat-discoloured. If the
    joints look even slightly crystaline, wick or suck them clean, and re-do
    using new solder.

    Arfa
     
    Arfa Daily, Dec 17, 2006
    #4
  5. wrote:
    > Volume dial has no control of crackle. Low and high behave the same
    > amount. I noticed a small browning of the circuitboard under a C263A
    > located behind the output transistors. Its the one located farthest
    > left. Looks like a group of 4. If the left 2 are left channel (from
    > front of unit) might this be it?
    >
    > Thanks for the help
    > Dave


    Don't know what your "crackle" sounds like, or looks like on a 'scope, but
    I'm thinking maybe a bad transistor, either a noisy signal or pre-driver
    type, or if the outputs are flat-packs, which is likely, maybe an open
    base-emitter junction on one of them.

    Mark Z.
     
    Mark D. Zacharias, Dec 18, 2006
    #5
  6. Guest

    Maybe crackle is not a good term to use. I would say distorted
    sounding. Almost like overdriven input to a soundboard or a blown
    speaker (but its not that). Well I resoldered all joints on heatsink
    and many of the others while I was there. No improvement or change at
    all. Re-checked with headphones and left and right sound great! Does
    that eliminate the circuitboard behind the front panel and focus us on
    the board with the heatsinks? By the way what do the 2 big caps at the
    back of the unit do?
    Are they a filter of sorts? In my pinball repair they have used them to
    stabilize voltages.
    Wish I had some info the the test points TP1 and TP2. I wish I knew
    what componets are used for the left channel, I could replace some and
    hope for the best. Any other leads appreciated. Thanks Everyone Dave
     
    , Dec 18, 2006
    #6
  7. Arfa Daily Guest

    <> wrote in message
    news:...
    > Maybe crackle is not a good term to use. I would say distorted
    > sounding. Almost like overdriven input to a soundboard or a blown
    > speaker (but its not that). Well I resoldered all joints on heatsink
    > and many of the others while I was there. No improvement or change at
    > all. Re-checked with headphones and left and right sound great! Does
    > that eliminate the circuitboard behind the front panel and focus us on
    > the board with the heatsinks? By the way what do the 2 big caps at the
    > back of the unit do?
    > Are they a filter of sorts? In my pinball repair they have used them to
    > stabilize voltages.
    > Wish I had some info the the test points TP1 and TP2. I wish I knew
    > what componets are used for the left channel, I could replace some and
    > hope for the best. Any other leads appreciated. Thanks Everyone Dave
    >


    The two big caps are the main smoothers ( filter caps ) for the positive and
    negative supply rails to the output stages. You cannot repair DC coupled
    amplifiers by guesswork and hopeful component replacement, I'm afraid. They
    are a black art that can at times, drive even the most experienced of us to
    distraction and tears ... If you have distorted sound, particularly if it is
    harsh and raspy, as you say " like a blown speaker ", it is likely that one
    half of the output stage is not being driven. This could be for many
    reasons, but in your case, it may be for some very odd reason, as usually,
    any inbalance in the drive conditions, will result in a DC offset at the
    midpoint, which will be picked up by the protection circuitry, causing a
    shutdown. The only way really to proceed with this type of fault, is to sit
    down quietly with a schematic diagram, a good multimeter, and a 'scope. I
    would have to say that a good degree of practical experience is also needed
    to work on this type of amp with any liklihood of a degree of success.

    As an aside, I have a Williams Cosmic Gunfight from 1982, which I spent many
    happy hours restoring. Originally had faults on just about every board, and
    some incorrect wiring on the playfield, where parts had been replaced. I
    became quite adept at the 7 series CPU boards, and mended quite a few of
    them for the dealer that I originally got the machine from for fifty quid.

    Arfa
     
    Arfa Daily, Dec 19, 2006
    #7
  8. wrote:
    > Maybe crackle is not a good term to use. I would say distorted
    > sounding. Almost like overdriven input to a soundboard or a blown
    > speaker (but its not that). Well I resoldered all joints on heatsink
    > and many of the others while I was there. No improvement or change at
    > all. Re-checked with headphones and left and right sound great! Does
    > that eliminate the circuitboard behind the front panel and focus us on
    > the board with the heatsinks? By the way what do the 2 big caps at the
    > back of the unit do?
    > Are they a filter of sorts? In my pinball repair they have used them
    > to stabilize voltages.
    > Wish I had some info the the test points TP1 and TP2. I wish I knew
    > what componets are used for the left channel, I could replace some and
    > hope for the best. Any other leads appreciated.


    Sounding more and more like an open base-emitter junction on one output
    transistor.
    While running, carefully measure the DC voltage across the base-emitter
    junctions of each output transistor. Should read from about .450 volts to
    around .6 volts. The one that measures in the range of several volts is
    defective.

    Mark Z.
     
    Mark D. Zacharias, Dec 19, 2006
    #8
  9. Guest

    Mark I tried to send you mail but it bouces back.
    I have a PDF of the amp.

    Dave
     
    , Dec 22, 2006
    #9
  10. wrote:
    > Mark I tried to send you mail but it bouces back.
    > I have a PDF of the amp.
    >
    > Dave


    You should be able to get me at



    and reverse the domain name.

    If the manual's a PDF, send me a copy. File size may require it be split
    however. I'm not totally sure of the limitations on this account.

    Mark Z.
     
    Mark D. Zacharias, Dec 23, 2006
    #10
  11. Guest

    hi there

    Recently I've got the SU-V60 amplifier. It was in good shape,but it
    didn't worked. I've tried to find out what is the reason. At first, the
    hybrid power stage - dedicated for those AMPs, hard to find spare
    parts. However, there was no burned PCB close to it, so I guess its
    working properly. There were some dark areas near the current mirror of
    the first power amplifier stage. I've soldered some bad joints,but it
    didn't help. I also found some burned resistors, first biasing the
    diode in current mirror and the other series in the positive supply
    rail, which comes to the preamplifier mounted on the front panel. The
    preamplifier board is also darkened, mainly around the two power
    transistors and output transistors driven by the AN7062. I wonder if
    there is a current or a voltage signal coming from the preamplifier to
    the main power AMP. Unfortunately, the resistors are burned so bad,
    that it's impossible to read the colour markings :( The output relay is
    open all the time, voltage at both outputs of the power amp ~ minus
    (50...60) V (depending on the speaker impedance switch).

    Without schematic it would be very difficult to troubleshoot this amp
    any further. I'd like to ask if anyone has got the schematic or service
    manual. I would be very grateful. Any close-up photos from the inside
    would be also helpful.
     
    , Jan 2, 2007
    #11
  12. Guest

    sorry, I forgot my email:
     
    , Jan 2, 2007
    #12
  13. wrote:
    > hi there
    >
    > Recently I've got the SU-V60 amplifier. It was in good shape,but it
    > didn't worked. I've tried to find out what is the reason. At first,
    > the hybrid power stage - dedicated for those AMPs, hard to find spare
    > parts. However, there was no burned PCB close to it, so I guess its
    > working properly. There were some dark areas near the current mirror
    > of the first power amplifier stage. I've soldered some bad joints,but
    > it didn't help. I also found some burned resistors, first biasing the
    > diode in current mirror and the other series in the positive supply
    > rail, which comes to the preamplifier mounted on the front panel. The
    > preamplifier board is also darkened, mainly around the two power
    > transistors and output transistors driven by the AN7062. I wonder if
    > there is a current or a voltage signal coming from the preamplifier to
    > the main power AMP. Unfortunately, the resistors are burned so bad,
    > that it's impossible to read the colour markings :( The output relay
    > is open all the time, voltage at both outputs of the power amp ~ minus
    > (50...60) V (depending on the speaker impedance switch).
    >
    > Without schematic it would be very difficult to troubleshoot this amp
    > any further. I'd like to ask if anyone has got the schematic or
    > service manual. I would be very grateful. Any close-up photos from
    > the inside would be also helpful.


    Sent the OP a PDF of the s/m.

    Mark Z.
     
    Mark D. Zacharias, Jan 3, 2007
    #13
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