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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.... ???
            oc disorder
            Senior Member
            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.


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