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AA764 issue with trem intensity knob

Discussion in 'Shock Brother's DIY Amps' started by zoran, Jul 7, 2019.

  1. zoran

    zoran Tele-Meister

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    hey gang

    i got a AA764 build i just completed, and the trem intensity knob seems to work like a second volume in reverse. with the knob rolled off all the way (at 0) i get full volume from the amp, but as i start turning it up the volume turns down, and at full intensity the volume is turned all the way down. is it bleeding the full signal to ground somehow?

    i'll supply voltages and images of whatever is needed - if anyone has any troubleshooting ideas id love some help with this one!

    many thanks in advance!
     
  2. D'tar

    D'tar Friend of Leo's

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    Make sure the "wiper"is connected to the trem tube cathode.
     
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  3. schmee

    schmee Doctor of Teleocity

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    Yeah, double check all your wire routing/connection points. ...new builds always seem to have a culprit somewhere!
     
  4. zoran

    zoran Tele-Meister

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    thanks for the help! i just checked the wiper, its connected to pin 8 of the second tube via a 68k resistor. contacts seems good on that one. is the triode aa764 layout a good one?
     
  5. zoran

    zoran Tele-Meister

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    traced the leads on the intensity pot - good connections as far as my continuity and tests go and visual inspection of the contacts... really curious as to why it's working like an awesome reverse volume lol. i've learned with amp builds, you really gotta embrace these issues as learning experiences!
     
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  6. D'tar

    D'tar Friend of Leo's

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    Measure resistance V1,p8 to ground and rotate the intensity pot. You should have a steady (approx)1.5k If not something is misfired.

    Amp off, unplugged, caps drained etc....
     
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  7. zoran

    zoran Tele-Meister

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    Thanks D'tar - Im getting a steady reading of 2.65k resistance on pin 8 of v1 while turning the intensity pot. its steady so that seems good - but is that resistance too high compared to the expected 1.5k?
     
  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    With Dr. D'tar on the case, you can't go wrong. I hear him closing in on the quarry already. "Mr. Holmes, they were the footprints of a gigantic hound!"

    If you want, you might also post some pics of the board and pot wiring especially in the sectors below. Also, continuity / ground / resistance testing is gonna be extensive, as a grundle of those wires go under the board and their connections are very close together...

    aa764.png
     
    Last edited: Jul 7, 2019
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  9. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    After you fix this issue...

    Remember to plug the footswitch in -OR- a dummy plug in the Trem footswitch jack, otherwise the Trem won’t work.

    I was scratching my head like an idiot after I built my amp. Took me way to long to realize the obvious.
     
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  10. zoran

    zoran Tele-Meister

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    Yup, been there my friend!!
     
  11. D'tar

    D'tar Friend of Leo's

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    I forgot we need to disconnect the nfb wire to get a proper reading from V1,p8 to ground. A few good pictures and a voltage chart wouldn't hurt anything too.
     
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  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If the tremolo is not oscillating and stuck at maximum current flow through the right half of the oscillation tube then the intensity pot will indeed function as a volume control. The problem is probably not with pot wiring but with the oscillator circuit.

    [​IMG]
     
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  13. D'tar

    D'tar Friend of Leo's

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    That would be too "intense";)
     
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  14. zoran

    zoran Tele-Meister

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    Well hope you guys are ready to see my super messy build!

    Thanks for all the help. Not sure if i need to test any of the oscillator caps.. i am getting some pretty great trem from the amp as well


    7CFDC264-C17C-4870-9291-3F92569DF6A6.jpeg 7CFDC264-C17C-4870-9291-3F92569DF6A6.jpeg
     

    Attached Files:

  15. D'tar

    D'tar Friend of Leo's

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    I would like to see a voltage chart.... Each tube every pin, noting voltage change at the cathode of v1b rotating the intensity pot. Pull v2 (trem tube) does it act the same?

    Is that an 820 ohm power tube cathode resistor? Stock would be 470 ohm.
     
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  16. D'tar

    D'tar Friend of Leo's

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    Can you check the ground connection at the eyelet circled below. I don't see a wire or its hard to make out.

    Edit: the black ground wire looks like the 220k grid leak ground, cant tell if it also grounds v1b/v2a cathode at the 47r/4k7/1M eyelet.

    upload_2019-7-10_13-6-10.png
     
    Last edited: Jul 10, 2019
  17. Ten Over

    Ten Over Tele-Holic

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    The 47r NFB resistor appears to be 4700r on my computer. That would explain the 2.65V reading. The eyelet for the ground connection mentioned above looks like it could use a little more solder.
     
    Last edited: Jul 10, 2019
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  18. D'tar

    D'tar Friend of Leo's

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    Agree... should be yellow/violet/black

    Good eye!!!
     
  19. zoran

    zoran Tele-Meister

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    You guys are amazing for the help, thank you so much!

    i reflowed the solder and checked continuity to ground at the above junction and yes, it is grounded thru the black wire to the right. i will post voltages shortly!

    also great eye on the resistor value!
     
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