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Crate vtx65 - strange fault

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  • #1

    Crate vtx65 - strange fault

    Hi, I have a crate vtx65 on the bench at present. First fault was relatively simple being a couple of pots with broken tracks which after a good deal of research manages to track down replacements. After doing the bench test with everything appearing to be functional - utilising the good old 100w lightbulb short circuit tester and a bench speaker, I re-assembled the unit to try it properly. Still plugged to the short circuit tester (thankfully) switched on to a loud hum and a bright bulb!
    I connected an external speaker through the socket on bottom of amp and bulb went dim and carried out a quick test with a mic. All working A1. Disconnected ext speaker, switched on again - Bright bulb, loud hum !!!. Disconnected internal speaker and switched on - no problem, reconnected int speaker - all works A1 so eliminate speaker fault.
    Having powered up yet again with int speaker connected, again bright light and loud hum - however plugging a bare jack plug to ext speaker socket then removing it again allows the amp to work A1.
    I am missing something really simple here.
    Has anyone come across such a fault or hazard a bit of knowledge - I have attempted to Email Crate but their website takes me to another which is closed.
    I do not have a schematic for this amp either which would help.
    Please can anyone help me out ???
    Thanks
    In life there are no problems, only challenges but in my present situation I may have a problem
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  • #2
    Originally posted by jbmdisco View Post
    I connected an external speaker through the socket on bottom of amp and bulb went dim and carried out a quick test with a mic. All working A1. Disconnected ext speaker, switched on again - Bright bulb, loud hum !!!. Disconnected internal speaker and switched on - no problem, reconnected int speaker - all works A1 so eliminate speaker fault.
    Having powered up yet again with int speaker connected, again bright light and loud hum - however plugging a bare jack plug to ext speaker socket then removing it again allows the amp to work A1.
    The two speakers are wired in series, so there seems to be problem with the extension speaker jack wiring. Perhaps an intermittent solder bridge on the pc board to the jack.

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    • #3
      Look at the output, R97 10 ohms. Is that open?
      Education is what you're left with after you have forgotten what you have learned.

      Comment

      • #4
        Originally posted by Enzo View Post
        Look at the output, R97 10 ohms. Is that open?

        Hi, I've tried both ideas. Checked over board with magnifyer. no stray solder.
        metered R97 at 10ohms.

        I took the following readings : Internal speaker connected : Switch on : Short detected : Voltage @ speaker terminals 9.25vdc
        Disconnect speaker : Reconnect speaker : Short disappears : Voltage @ speaker terminals 0.1vdc
        Switch off
        Connect internal speaker with inline 10w 24v bulb : Switch on : Bulb glows briefly : No short : Amp works A1

        I have metered out various resistors around the outputs etc all within specs with exception of 2 5w .47ohm which both meter @ 0.8ohms at which I don't think there is any concern as it is when I either break and make the speaker circuit or apply an in series resistor that everything begins to work. I am wondering if either of the output transistors (tip42, tip47) are beginning to fail but when I have output there is plenty of clear and undistorted power.
        With the lack of any diagrams I am going around in circles
        The addition of the 10w 24v bulb adds about 58ohms - I'm almost tempted to swap out R97 for a 100ohm resistor to see what happens but this would only hide the fault.

        Any ideas ??

        Thanks
        Joe
        In life there are no problems, only challenges but in my present situation I may have a problem

        Comment

        • #5
          You are just guessing, and that is dangerous.


          What remotely tells you changing R97 to 100 ohms would help? The 10 ohm R97 is part of the amp stability network, it prevents RF oscillation.

          Your amp is currently unstable, a stable amp will power up and down with or without a load.

          The Crate amp website links do not work. So contact Loud Technologies, they even have a UK branch. They are the makers of Crate. They generally supply schematics if you agree not to put them on the internet.
          Education is what you're left with after you have forgotten what you have learned.

          Comment

          • #6
            Appologies for the way my reply to this was read. This was a 'tongue in cheek' state
            ment which, now, i realise could be taken the incorrect way by in-experienced techs. Of course I would only perform such 'modifications' as experimental procedeures, and I realise that this forum may be read by amatuers. PLEASE IF YOU ARE A HOBBIEST or AMATEUER do not attempt to perform any of the previous 'Ideas' as they, along with my perplexity, are only frustrated ramblings!!!
            The fault has now been traced to a defective Output transistor which, until isolated, managed to elude detection.
            The said output transistor appeared to function correctly when when coupled with an additional resistive load ie 24v 10w lamp in series. This provided enough resistance to allow the amp to perform as it should. However! when the lamp was taken from the circuit a 'short' would be detected. I metered all resistors in the output stage of the amp and also tested the caps. All showed up within spec +/- 5%. The only component left following this was the output transitor TIP147. I replaced this with a new part. The problem of 'short circuit' disappeared and now I have an amp which is working 99.9% (0.01% because I have been unable to obtain a schematic/circuit diagram/data).
            Thanks to all who have read this, and I know many will be shaking their heads in disbelief of my comments, however this is the way some folks think A+B=C !!!
            I have been providing repairs to West Central Scotland UK for 11 Years, without any adverse publicity. Unfortunately this is the first 'Crate' amp I have been asked to repair and it has previously been 'tampered' with, being the best description. (tracks burnt from pcb, bridging wires added etc, etc.)
            Loud Technologies have disappeared from the 'map' as far as contact is concerned. I have tried every telephone number and email addy suggested without any luck (number not recognised, emails returned undelivered etc etc)
            I apologise for making comments that are un-professional. I was frustrated!!! One fault, followed by another with an un-understanding customer calling 4 times a day for progress reports. Crate products will now going to be refused due to lack of technical back-up and 'run around the block' dead ends.
            Fender,Peavey + Marshall products are un-rivaled as far as tech support is concerned. This may sound like a 'Plug' and favouratism . . . . . . . . . . . . It is !!!
            In life there are no problems, only challenges but in my present situation I may have a problem

            Comment

            • #7
              Just called them at this number last month and asked for a schematic. They got me connected to a really nice gentleman that got me the schematic by email by the time I hung up the phone.


              1-866-858-5832
              Technical Support, Service and Parts departments are available Monday-Friday, 8am-4pm Pacific Standard Time
              When the going gets weird... The weird turn pro!

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