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Fixing an Aune T1 (amp/dac) TUBE DAC Board

Discussion in 'Repairing Electronics' started by bittgata, Mar 25, 2017.

  1. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Excellent, pfofit! Thank you. Thought that's what it was (in a general sense), but I just couldn't make out the part number(s)

    bittgata, see above post (#39).

    This is my interpretation of the circuit we're investigating (It is only a very basic circuit example, not the actual circuit):
    upload_2017-4-8_15-43-53.png
    The "bad" DAC appears to have a fault in the V+ side. Don't know why at this time.
     
  2. bittgata

    bittgata New Member

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    I didn't have a solder wick and I can't find my solder plunger but I managed to improvise. The plate voltage seems to be unaffected, it changes a bit but still around -0.4V. Also the light never comes on with the Vin desoldered.
    Thanks for the info pfofit, I don't know long I would I have had to search for it.
     
  3. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Ok.

    Sounds unlikely, but it does appear that the bridge (1/2 of it, anyway) circuit has failed. We'll start with the output of the bridge:
    upload_2017-4-10_19-4-1.png
    Check the voltages from ground to the points below:
    upload_2017-4-10_19-14-5.png
    Both under "AC" should be ≈16VAC, + ≈ +22VDC and - ≈ -22VDC.
     
  4. dave

    Dave New Member

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  5. bittgata

    bittgata New Member

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    On the plus pin I get 23.5VDC and on the minus -23VDC.
     
  6. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Ok. So the bridge appears to be working as it should. Something in the circuit after the bridge is causing the problem.

    We're going to attempt to isolate what that "something" is. We'll test the KIS-3R33S PS's load(s).

    TEST 1. Momentarily re-connect Vin to the trace.
    upload_2017-4-11_19-18-40.png
    Disconnect the Vout pins of the KIS-3R33S PS from the PCB traces (as you did with the Vin pin):
    upload_2017-4-11_14-31-7.png
    Re-power the PCB and note the voltage on tube pin 1.
     
    Last edited: Apr 11, 2017
  7. bittgata

    bittgata New Member

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    In the picture below Test 1 the arrow is pointing at the ground, I think that's a mistake but if it's not a mistake I have no idea you mean to show me.
    Also Regarding Test 2 you say " If, pin 1's voltage drops to previously noted very low levels", the voltage on pin1 was always low with or without the vin connected so it can't drop to the very low level because it's already there. Do you mean if temporarily reconnecting the Vin while the dac is powered on doesn't do anything to change the voltage on pin1 then proceed with test2 or did I get that wrong?
     
  8. Les Jones

    Les Jones Well-Known Member

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    I agree that the arrow is poining to the wrong pin. The Vin pin is the one to the left of the arrow. You also say in post #42 that disconnecting it does not change the plate voltage
    I think CBB may have miss read post #44 as I did when I first read it. Where you said "On the plus pin I get 23.5VDC" I initially read it as "on pin 1 I get 23.5VDC"
    You will have to trace the track on the board from the positive output of the bridge rectifier until it gets to the plate pin on the valve. Show all of the components connected to this track.

    Les.
     
    • Agree Agree x 1
  9. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    bittgata, please excuse that error.

    I rewrote Post #45 to just test the loads of the KIS-3R33S PS and how their absence affects (if at all) the bridge PS circuit. Please re-do the test in Post #45.

    Les is right, although, at the moment, I'm working backwards with traces and components we can see.

    The traces for the ±22VDC supply are very hard to follow. I think it's only a double sided board but can't really say for sure that there's not a third layer between the two.
     
  10. bittgata

    bittgata New Member

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    I managed to disconnect the both Vout connections and then I temporarily reconnected the Vin. The plate voltage still has the same very low value.
    PS:
    I've mentioned this before but I imagine you have other things to do besides reading the entire thread every time I post something.
    Though there is a large difference between the very low plate voltage of the "bad" DAC and the ~22V of the "good" DAC the Vout is also lower ~16V on the "bad" DAC vs ~22V on the "good" DAC. I also checked the Vin voltage after I desoldered the Vout and it did not change, still at around 16V.
     
    Last edited: Apr 14, 2017
  11. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    OK. It's looking like the problem is in the primary, ±VDC supply circuit.

    I've been attempting to "reverse engineer" the primary power supply (pps) and it's slow going, especially without the actual board in my grimy hands. Many of the traces disappear under components as well as pass between the top and bottom through a "PCB via". This is not going to be easy and might very well end in failure.

    I'd like to avoid just a willy-nilly parts replacement approach, but that may become necessary.

    But first, how comfortable are you with exposing (by removing the dark green cover film) a PCB trace? And then severing that trace? The idea is to isolate the pps, going backwards from the loads (as best as I can identify them) until the "bad" components can be discovered. Part of this process is to restore the trace if that particular change proves nothing.
     
  12. bittgata

    bittgata New Member

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    First thing first, I am not at all uncomfortable severing a trace but there is a high chance I will screw that up somehow so I would rather not. Regarding the PCB film, probably the blue coating in this case, I did not even know that could be removed but would it help if I just removed the film without severing any traces? How and can this be done with components connected to the board? Would this have long term effects/ would I have to apply another coating?

    Without doing any of the above I've been thinking the reverse engineering might take a lot more time so it might be worth to wait for the ESR meter and start testing every component that I can, might be most or just some of the 2 and 3 pin components, before doing anything else.

    Could you direct me to the areas or some of the components that look like they might have anything to do with the ±VDC supply circuit? I will test outside of this if I don’t find anything but it’s better to have a place to start.

    PS: Is it likely or possible that board itself has developed a defect?
     
    Last edited: Apr 14, 2017
  13. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Sorry for the delay.
    I understand. But I have exposed (the blue comes off rather easily) and severed many a trace. If you choose to do so, I can guide you. We would only sever traces between components (to isolate them, or additional down stream circuits) in easily accessed areas of the PCB. There are no long term effects I am aware of or have experienced. And replacing the coating (after removing any flux) is recommended after you've reconnected the trace edges.
    Works for me. Although, understand that the ESR meter is only used for testing capacitors, not many of which, anymore, have more than 2 pins.

    I'd start with the two electrolytics (silver top with the embossed cross, note polarization - gray stripe should be minus\Gnd) and the two polyesters (C33 & C34, red rectangles - NOT polarized).
    upload_2017-4-15_17-16-35.png upload_2017-4-15_16-58-8.png
    At the moment, I very curious about the cap above with the red X (the smoothing cap for the +22VDC side of the PS.

    Don't forget that the negative VDC side of the PS seems to be working just fine.
     
  14. bittgata

    bittgata New Member

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    I've ordered one of these. Not sure If it's gonna give me a reading for the 3 pin components in circuit but if it does I can just compare them to the good dac so it shouldn't even matter if it's accurate or not.
    I ordered it at the same time with a few things I already received so hopefully I'll have it soon.
     
    Last edited: Apr 18, 2017
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I don't really see as an ESR meter is going to help you? - electrolytic's go high ESR due to poor design, misuse, high temperatures, and old age - none of which apply here.

    What you need to do is post the schematic - if you can't get hold of it, then draw it out. Without that everyone is just making totally blind guesses, valve technology is crude and simple, and dead easy to repair - but you can't do it with a blindfold on wearing boxing gloves (which is pretty well what's happening here).
     
  16. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    True, Nigel.

    But the problem seems to be in the (also simple) ± VDC PS, for which there are, at the least, some identifiable traces and components that shouldn't be too difficult to remove/replace (if, of course, that's where the problem lies).

    I think it's a worthwhile exercise for the OP, even though it may very well get stopped dead in its tracks if anything else, other than the PS's is/are the problem. Back engineering the board would be, essentially, impossible and not worth the effort. Especially at a distance.
     

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