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Re: Replacing a Capacitor

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  • jdb000001
    Some things you might want to check on the 4072 submodule: (The component designation refer to the 4072 schematic) Do you have the +15v and -14v supplies?
    Message 1 of 29 , Jun 6, 2002
      Some things you might want to check on the 4072 submodule:
      (The component designation refer to the 4072 schematic)

      Do you have the +15v and -14v supplies? (Pins 8 and 4 on the LM1458)
      Since the +15v supply is fed through a 100 ohm resistor, R34, a low
      reading could indicate too much current is being drawn, or the filter
      cap, C9, may be shorted. The resistor could be open, but will usually
      appear burnt.
      If the -14v supply is not present, C7 in the "Supply Conditioner"
      could be shorted. Tantalum capacitors are notorious for shorting out.

      Do you have approximately -7v out of the "Supply Rail Splitter"?

      Are the outputs of any of the op-amps near a supply voltage? (+15v
      or -14v for the LM1458, Ground or -14v for the LM3900)
      This will sometimes occur when an op amp goes bad, but not a good
      indication in this circuit because of all the dc coupling - one bad
      part will probably throw everything else off.
    • manifold_1
      Excellent advice. I would like to add that an op-amp output that is close to a supply rail is caused by a problem in the feeback components and not usually a
      Message 2 of 29 , Jun 6, 2002
        Excellent advice. I would like to add that an op-amp output that is
        close to a supply rail is caused by a problem in the feeback
        components and not usually a problem with the op-amp itself. As you
        said, I would suspect the tant caps first. op-amps do not
        usually "go bad" unless something else has gone bad that then causes
        the op-amp to fail.

        Check the trim pot RV1 that supplies the Bias Trim. If the bias
        voltage is way off that could cause it to not work correctly. Trim
        pots are mechanical and can go bad just sitting there.

        I was not able to find C3 on the second page of the schematic, maybe
        it has just been a long day... I see C3, 4700pf on page 1 but that
        is located on the module itself and does not correspond to the
        control board or the first photo.

        Check with an Ohmmeter or check visually and see if C3 is connected
        across one of the slide pots. If so, then it is probably there to
        provide a high frequency roll off so that noise and interference do
        not get into the signal path. Then it can be left out until you get
        some signal through it. It is likely a 10pf and the one you removed
        is probably still good.

        P.S. I like the drum head for a background on the last photo!

        --- In Electronics_101@y..., "jdb000001" <jdb000001@y...> wrote:
        > Some things you might want to check on the 4072 submodule:
        > (The component designation refer to the 4072 schematic)
        >
        > Do you have the +15v and -14v supplies? (Pins 8 and 4 on the LM1458)
        > Since the +15v supply is fed through a 100 ohm resistor, R34, a low
        > reading could indicate too much current is being drawn, or the
        filter
        > cap, C9, may be shorted. The resistor could be open, but will
        usually
        > appear burnt.
        > If the -14v supply is not present, C7 in the "Supply Conditioner"
        > could be shorted. Tantalum capacitors are notorious for shorting
        out.
        >
        > Do you have approximately -7v out of the "Supply Rail Splitter"?
        >
        > Are the outputs of any of the op-amps near a supply voltage? (+15v
        > or -14v for the LM1458, Ground or -14v for the LM3900)
        > This will sometimes occur when an op amp goes bad, but not a good
        > indication in this circuit because of all the dc coupling - one bad
        > part will probably throw everything else off.
      • jdb000001
        It looks to me like what he is calling C3 is labeled C62 on the second page of the schematic. Looks to be part of the feedback/resonance circuit. Another
        Message 3 of 29 , Jun 6, 2002
          It looks to me like what he is calling C3 is labeled C62 on the
          second page of the schematic. Looks to be part of the
          feedback/resonance circuit.

          Another problem I've seen in equipment like this is a dirty switched
          contact inside of one of the patch jacks.
        • BrianJHoskins
          I think you ve already had some brilliant advice from jdb and manifold, and I would simply mirror what they ve said. My first call would be to check that those
          Message 4 of 29 , Jun 7, 2002
            I think you've already had some brilliant advice from jdb and manifold, and
            I would simply mirror what they've said.

            My first call would be to check that those power supplies are there, simply
            because they're easily checked, and if you find a supply missing you know
            that you've found the cause of the problem. The fact that the device does
            power up suggests to me that there are at least SOME of the supplies there,
            but you may have one or more missing so you'll need to check them all.

            When you check them with a multimeter, be sure to have a good ground point
            on the circuit itself otherwise you may get misleading readings (speaking
            from bad experience).

            After you've checked them get back to us and let us know what you found :)
            if you find that all the supplies are there, and they don't look suspicious
            in any way, then jdb's further advice is definately your next call.





            ----
            Brian Hoskins
            South Wales, UK

            Email: BrianJHoskins@...
            ----
          • ethanzer0
            I m amazed at all of the excellent suggestions and help I m getting here! I now have a full slate of tasks to accomplish this weekend. I will try to get all
            Message 5 of 29 , Jun 7, 2002
              I'm amazed at all of the
              excellent suggestions
              and help I'm getting here!
              I now have a full slate of
              tasks to accomplish this
              weekend. I will try to get
              all of the readings suggested
              and report back soon.

              --- In Electronics_101@y..., "jdb000001" <jdb000001@y...> wrote:
              > It looks to me like what he is calling C3 is labeled C62 on the
              > second page of the schematic. Looks to be part of the
              > feedback/resonance circuit.

              You definately right. I
              am refering to the board 3
              layout from the service
              manual which labels that cap
              C3. The service manual is
              a photocopy, so, I'll see
              if I can get it scanned over
              the weekend so I can share it.

              > Another problem I've seen in equipment like this is a dirty
              > switched contact inside of one of the patch jacks.

              I did remove the audio out
              jack and tested all of the
              input jacks.

              Thanks again!

              Ethan
            • ethanzer0
              I want to thank every body who made excellent suggestions for diagnosing my VCF. Using your suggestions I was able to isolate the problem to the LM3900N IC and
              Message 6 of 29 , Jun 12, 2002
                I want to thank every body
                who made excellent suggestions
                for diagnosing my VCF.

                Using your suggestions I was
                able to isolate the problem to
                the LM3900N IC and some Tantalum
                caps. After replacing the caps
                and the LM3900N the VCF is working
                again as expected.

                Big thanks are owed to Brian,
                Manifold, jdb, and Robert Graf!
                You guys are what the spirit
                of the internet is all about!

                I certainly don't have the
                knowledge of electronics you
                all have, but, I hope I can
                help you in the future!

                Thanks again!

                Ethan
              • BrianJHoskins
                ... Brilliant! It s nice to get positive results from things, it makes the work worth doing. Also, you get kind of a smug satisfaction from succesfully
                Message 7 of 29 , Jun 12, 2002
                  > I want to thank every body
                  > who made excellent suggestions
                  > for diagnosing my VCF. 
                  >
                  > Using your suggestions I was
                  > able to isolate the problem to
                  > the LM3900N IC and some Tantalum
                  > caps.  After replacing the caps
                  > and the LM3900N the VCF is working
                  > again as expected.
                  > Big thanks are owed to Brian,
                  > Manifold, jdb, and Robert Graf!
                  > You guys are what the spirit
                  > of the internet is all about!
                  >
                  > I certainly don't have the
                  > knowledge of electronics you
                  > all have, but, I hope I can
                  > help you in the future!
                  >
                  > Thanks again!
                  >
                  > Ethan
                  >

                  Brilliant! It's nice to get positive results from things, it makes the work
                  worth doing. Also, you get kind of a smug satisfaction from succesfully
                  repairing a faulty piece of equipment don't you?? Well done :)

                  I'm sure you can be of help to any of us in the future, everyone is
                  talented in their own ways. It all comes down to your own personal
                  interests and experiences really doesn't it? The experience you've just
                  gained could well be of use to somebody else in the future!!!

                  Well done, best of luck with it!!





                  ----
                  Brian Hoskins
                  South Wales, UK

                  Email: BrianJHoskins@...
                  ----
                • manifold_1
                  Excellent! Your perseverance and skill in solving the problem is commendable.
                  Message 8 of 29 , Jun 12, 2002
                    Excellent! Your perseverance and skill in solving the problem is
                    commendable.

                    --- In Electronics_101@y..., "ethanzer0" <ethanzer0@y...> wrote:
                    > I want to thank every body
                    > who made excellent suggestions
                    > for diagnosing my VCF.
                    >
                    > Using your suggestions I was
                    > able to isolate the problem to
                    > the LM3900N IC and some Tantalum
                    > caps. After replacing the caps
                    > and the LM3900N the VCF is working
                    > again as expected.
                    >
                    > Big thanks are owed to Brian,
                    > Manifold, jdb, and Robert Graf!
                    > You guys are what the spirit
                    > of the internet is all about!
                    >
                    > I certainly don't have the
                    > knowledge of electronics you
                    > all have, but, I hope I can
                    > help you in the future!
                    >
                    > Thanks again!
                    >
                    > Ethan
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