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Obsolete op amp?

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  • #16
    not ignoring

    Originally posted by oc disorder View Post
    And when you have finished drawing that out we have an atomic fission problem you may like to get involved with !
    Juan , it may be rather daunting for him to draw the schematic .. he did say "I am a complete amplifier amateur".

    But... us Aussies are resourceful people and the proud inventors of the stump jump plough! ( )

    Mwint50 you will have to undo all the knobs to remove the circuit board to see the reverse side.

    That way you can see where the tracks go.

    A multimeter with a continuity "beeper" (makes a beep when you touch the leads together) is also a very helpful tool
    when checking what goes to what.

    I recall servicing a similar amp... the pots were shocking ..the parametric eq burst into oscillation and was so loud I had to dive for the power switch !

    I do recall failure of part of the power supply a zener... looks like this

    See if you can find 2 with 2 slightly larger resistors which may look like they get hot.

    They are sorta identical .. one is for a positive supply and the other for a negative supply.

    They hold the voltage to a particular value and dissipate the excess in the form of heat.

    Probably found down near the power switch or fairly close to the transformer secondary low voltage wires.
    Hi all. I just wanted to let you know that I have been away in Melbourne and haven't been ignoring you all. I am amazed at the helpfulness and good humour! Am getting back to my electronics obsession today!


    • #17
      Thanks everyone. Ill put it back on the bench today and post any new info.


      • #18
        Each of the op amps in the circuit are reading +17.3v (pin 8) -17.3 (pin4). Also I have noticed that the Signal path makes it to the reverb tank and then to the speaker????


        • #19
          Pin 4 / 8 resistances are all 640ohms / 830 ohms


          • #20
            "Each of the op amps in the circuit are reading +17.3v (pin 8) -17.3 (pin4)"
            Including the (excuse me) "fried one" ?

            From memory they used a higher voltage rail +&- 16.5 or 18volts ... so that's why it's a little higher than 15, so 17.3 is ok.

            If its not getting to pin 4 of the "fried" one .. should be possible to trace it .. watch for wire links joining one pcb track to another.
            Sometimes they are light brown and look a bit like a resistor but only have 1 stripe !

            Not sure if I quite get this "Also I have noticed that the Signal path makes it to the reverb tank and then to the speaker???? "

            I presume on first look you can hear some reverb coming out the speaker but no direct signal.

            This is where a schematic would come in handy however.... does it have a effects loop eg Effects send Return jacks or an insert point...
            pre-amp out jack and a power amp in jack ?

            I'm not sure whose on duty re the switching jack police here on the forum but this comes up time after time...
            in fact I think JP bass made it a sticky!


            This is when you have jack sockets conveniently arranged so you can insert a graphic or compressor delay - whatever in between the pre-amp and the power amp thus the signal gets interrupted when a jack plug is pushed in and conversely when pulled out a tiny switch contact has to reconnect the pre and pwr together again.
            If the contact is bent or dirty it's possible nothing will get through.
            By simply putting a lead from send to return (from out to in) that bypasses the internal jack switch contact and normal service is resumed !

            So firstly does the chip now seem to have both the plus 17.3 and the negative 17.3?

            Edit: "hope that I am not excommunicated" no not this time J M Fahey is on guard duty and surely will come up with something!
            We got all excited about your chip without taking a step back even though you gave us a clue. "I am a complete amplifier amateur". Usually the switching jack fault is mentioned early on... "the switch" or switching contacts are inside the jack socket.
            I had an advantage as we are in the same time zone ..Stralian .. now that Britain has left the EU we drop our AU.... ???
            Last edited by oc disorder; 07-02-2016, 06:15 AM.


            • #21
              Oh I mentioned earlier but perhaps not clearly enough. I don't feel its a dead op amp any longer. My multimeter technique has improved over the past week thanks to you all. Yes all the IC's seem to have reliable +/- 17.3v at each of their respective pins. The effects insert you speak of would RTS wouldn't it. I can't see any evidence of a switch but I will test it by adding something through the loop. Thanks again!


              • #22
                Tail firmly between hind legs!

                Much thanks and many apologies. OC disorder is the winner with Dreaded switching jack problem. On the upside I really learnt a lot from everyone and hope that I am not excommunicated from the forum!



                • #23
                  Oh, donīt worry,not excommunicated, just a light session with Mistress Mathilda, the Forumīs Punisher, and her trusty "Cat o' nine tails"
                  Juan Manuel Fahey


                  • #24
                    What is that, "I'll whip this country into shape"?
                    Education is what you're left with after you have forgotten what you have learned.