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AB763 Deluxe Reverb (6a20) Build Troubleshooting

Discussion in 'Amp Tech Center' started by guspac88, Apr 6, 2020.

  1. guspac88

    guspac88 Tele-Meister

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    Here are some voltages if it is at all helpful to you D'tar or anyone! This is with standby closed (amp running) and volumes at 0.
     

    Attached Files:

  2. D'tar

    D'tar Friend of Leo's

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    That switch unloads the signal going into the phase inverter. You will still have the oscillation with trem pedal engaged so that sounds right. On that switch contact can you ohm to ground and watch it oscilate? Also do you see any light from the neon bulb? You can usually see something.
     
  3. guspac88

    guspac88 Tele-Meister

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    Both switch contacts read a steady 50k to ground no matter the switch or pot position. I turned the lights off and intensity up but couldn't spot any light in the bug!
     
  4. D'tar

    D'tar Friend of Leo's

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    Ok, so v5a is oscillating

    Is there oscillation on pin 7 and pin 6 and at the neon bulb connected off pin 6 at the top of the 10M
     
  5. guspac88

    guspac88 Tele-Meister

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    I am oscillating 230-330 at the bulb and both pin 6 and 7!
    240ish to 340ish at pin 6? Rough guesses, it's moving fast o_O
    -.5 to -4.3 or so on pin 7

    pin 1 is now oscillating at 180 to 240, maybe because it's at max intensity (220-240 before)
     
  6. D'tar

    D'tar Friend of Leo's

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    Can you confirm the 10M resistor, I cant really see the color bands. And that the red lead from pin 6 goes to the top of the 10M and not the top of the 100k. If good you should be able to watch voltage oscilate with your leads connected to each leg of the bulb. Red on one leg, black on the other.

    Check also the ldr on the other side. Ohm across the legs without light you should have high resistance, I dont recal the value. You will have to disconect either the ground from one leg or the wire off the other leg to the intensity pot at a convenient spot to get a proper reading on the ldr.
     
    Last edited: Apr 7, 2020
  7. guspac88

    guspac88 Tele-Meister

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    I’ll get on these as soon as I can tomorrow! Thanks so much for sticking with me on this one, I’m super grateful for your willingness to share your knowledge on these amps.
     
  8. D'tar

    D'tar Friend of Leo's

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    A good nights rest and a fresh cup of coffee can do wonders!

    If I can help at all, it is my pleasure.
     
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  9. AndyPanda

    AndyPanda Tele-Holic

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    Are you sure you don't have the bug backwards?
     
  10. Paul G.

    Paul G. Friend of Leo's

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    Most aftermarket and replacement bugs have a white dot that should be facing the tube side of the amp.
     
  11. AndyPanda

    AndyPanda Tele-Holic

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    The Weber optocouplers have an "N" on the neon side and in the OP's pictures the "N" is oriented correctly HOWEVER (this is a stretch) it might be marked wrong (they are made by humans who make mistakes) - the bulge in the shrink wrap sure looks to me like the resistor is on the side marked "N" rather than the neon bulb.
    Bug.jpg
     
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  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The PT has dual HT taps. (If the Weber site has given the voltages in AC), you are using the 270-0-270. The layout Rob provided shows 330-0-330. If you use the red wires it will give 340-0-340.

    Power up!;)
     
    Last edited: Apr 7, 2020
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  13. D'tar

    D'tar Friend of Leo's

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    Sure does look that way

    G,

    Put your ohm meter on the legs of the roach and shine a light into the end. If the resistance drops you have the ldr end. Check both sides.
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Very nice work D'tar.
     
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  15. D'tar

    D'tar Friend of Leo's

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    Power up indeed!!!

    Screenshot_20200407-104152_Chrome.jpg
     
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  16. D'tar

    D'tar Friend of Leo's

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    That could be quite versatile PT eh!
     
  17. aerhed

    aerhed Friend of Leo's

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    That's a great PT.
     
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  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You think so too!

    Just watch the filter cap voltages. :cool:
     
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  19. schmee

    schmee Doctor of Teleocity

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    By cap can I meant doghouse. Yours has a cap can like a Princeton?

    The red dot on the opti goes on the tube socket side according to my notes.

    I have had Fender tremelos not work for some reason, then suddenly they will start after playing a while, or the next time you turn the amp on. I always assumed it was something to do with a failing roach or the particular tube in the tremelo tube driver position.
     
    Last edited: Apr 7, 2020
  20. rocksmoot

    rocksmoot Tele-Meister

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    I built a 6a20 a few years ago, I put a Weber 15F150 speaker in it (I had Peavey Delta Blues envy at the time) and it is still my favorite amp, fantastic tone. As I recall I had an issue with the optocoupler being marked incorrectly.
     
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