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DOD 460 Mini Chorus Repair

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  • DOD 460 Mini Chorus Repair

    Hi all,

    I've just come across this great forum, and was wondering if anyone could be of help with the project I'm working on.
    I have done a bunch of testing, but would like to get a second opinion on the diagnosis, since I am still quite inexperienced with electronic circuits.

    I'm working on a DOD 460, it only gives the clean signal at the moment when the effect is engaged. I tested all electrolytic capacitors with an ESR meter, and unsoldered and checked the transistors - all good. I've traced the signal with the help of a schematic I found online (sorry for bad quality), and highlighted it:

    Click image for larger version  Name:	signal_trace.jpg Views:	1 Size:	288.6 KB ID:	909165


    As you can see, part of the circuit is only a clicking noise (the dotted line), very faint, coming from ODD OUT and EVEN OUT (pin 3&4) of the SAD512D chip - the INPUT signal (pin 6) seems fine. I can't find a signal in the rest of the circuit.


    I checked the CLOCK signal (pin 1) of the SAD512D with an oscilloscope, and it does look erratic (although I don't have much experience with oscilloscopes):

    Click image for larger version  Name:	IMG-3908.JPG Views:	1 Size:	1.49 MB ID:	909166


    Would you say the SAD512D chip is the culprit here, or did I miss something?
    Last edited by clarisso11; 07-13-2020, 04:52 PM.

  • #2
    Here is a 'better' schematic.
    Attached Files

    Comment


    • #3
      Very cool, thank you!

      Comment


      • #4
        Does anyone have an idea? I should also mention, the voltage at pin 5 and 8 of SAD512D measures as 8.7V, if that helps..

        Comment


        • #5
          If you read the schematic correctly, pins 5 & 8 should be at a high potential.
          So you have good input.
          What does pin 3 & 4 look like on a scope or a signal probe?
          If 3/4 are good but it 'clicks' after the NPN transistor then I would suspect the transistor.
          Here is the SAD512D datasheet:
          Attached Files

          Comment


          • #6
            Originally posted by Jazz P Bass View Post
            If you read the schematic correctly, pins 5 & 8 should be at a high potential.
            So you have good input.
            What does pin 3 & 4 look like on a scope or a signal probe?
            If 3/4 are good but it 'clicks' after the NPN transistor then I would suspect the transistor.
            Here is the SAD512D datasheet:
            Thanks for your help, really appreciate it!

            I just checked pin 3&4 with an audio probe - no signal, only faint clicking. The signal at pin 6 is very clear and loud on the audio probe, so looks like good input, as you said.
            I have unsoldered the NPN transistor and checked it with the diode setting, it all seems to check out.

            Here is pin 3 on the scope:
            https://ibb.co/tJmChKx

            And here is pin 4:
            https://ibb.co/51qH6PW

            Does that look like a "good" signal? This is where I'm a bit too inexperienced to tell..

            Comment


            • #7
              It 'looks' like a good signal.
              So why is it not coming through the audio probe?
              Probably need s a boost, hense the npn transistor.
              If it does appear that the NAD ic is bad, you are out of luck.
              They have not been available for years.

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              • #8
                Thanks, that makes sense.. I will replace the transistor first, and report back with the results

                Comment


                • #9
                  No luck with the transistor, but a new (*old) SAD512D chip did the trick - managed to find one online for a decent price.

                  Thanks for the help!

                  Comment

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