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Mojotone Studio One

Discussion in 'DIY Amp Forum' started by LarsN, Aug 2, 2019.

  1. LarsN

    LarsN Strat-Talker

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    So taking the gray wire, and running it to the 22nF cap in the top left corner of the board would eliminate the dirty channel and let me test?
     
  2. LarsN

    LarsN Strat-Talker

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    @Robins -- Just going through the amp checking continuity, and I have a question...

    Should the variable outputs from the output transformer (4,8,16 ohm) all have continuity to ground?
     
  3. Triple Jim

    Triple Jim Senior Stratmaster

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    In case he's busy... They will seem like it because the DC resistance from the three outputs to ground is very low. If you could read very low resistances, you'd see they're not quite grounded, but most meters aren't very good at displaying very low resistances.
     
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  4. LarsN

    LarsN Strat-Talker

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    Scratch that.... I'm dumb, and didn't have anything plugged into the jacks... It's supposed to ground itself if it's not plugged in.
     
  5. LarsN

    LarsN Strat-Talker

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    Well If I put ground on Pin 3 or 8 on the 12BH7 power amp tube, I get a crackle from the speaker. So I think I can rule out the output portion of the circuit. The problem lives in the preamp(s), or between the preamp(s) and power amp.
     
  6. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    Refer to Mutt's thread where I posted a breakdown of the switching board with the schematic. It's simple to bypass to see if that is the problem. The wiring diagram is indeed not very clear.
    The "chip" on the board is actually a relay. Two pins energize the coil, which lights the LED and activates the clean channel. If your LED is not coming on, then you are permanently on the dirty channel or the board is miswired.
     
  7. LarsN

    LarsN Strat-Talker

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    I'll find that thread and see if I can bypass the switch and go clean only channel initially for testing.

    The LED lights and goes out with the push/pull pot (as well as via the foot switch) although I don't hear the relay click, which I thought was strange.
     
  8. LarsN

    LarsN Strat-Talker

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    Ok, so the Gray shielded wire going from the 22nF cap on the right to the relay is the clean pre-amp output....
    Then the gain channel pre-amp output is the blue lead coming off the center lug of the master volume pot?

    making the output to the power amplifier the other blue leg coming from the relay to the 22nF cap on the top left of the board....

    I could take the gray wire, and run it directly to that 22nF cap to bypass the foot switch board.... testing the clean channel
    Or I could take the center lug of the Master volume to bypass the foot switch board.... testing the gain channel

    If neither of those has output, I guess I'm down to looking at the power amp tube and or output transformer?
     
  9. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

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    I was sleeping - it was 2:42 am when you posted the question. I am an old man already I need my sleep xD.
    Yes, put the grey cable to the 22nF and test the clean channel. Put the wiper cable(master volume) to the 22nF and you test the overdrive channel.
    As simple as that.
    Your output transformer is working fine.
    If you you want to heave peace in mind disconnect the power going into the relay PCB. The two green ones coming from the DC heater voltage.

    No noise is not good, strange noise is worse. It is not a big thing. Trust me. Just make sure you test the small things and that is a good way to learn the big(ger) things.

    All the best,
    Robin
     
    Last edited: Aug 9, 2019
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  10. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    Correct.

    [​IMG]
    Studio-One-Switch.gif


    Yes and no. If you have a scope, check for signal on both the clean and gain outputs [1] and [2].
    It is indeed unlikely that, if you have no output in either position, there would be a common problem in the preamp since both gain stages are isolated to the point of the relay, which would then point to a problem with the input to the power stage, but you also have a common connection from the input jack to both preamp stages... plus you have the common power supply, but your voltage readings show that the power supply should not be a problem.
    Think of the preamp section as two amplifiers running in parallel... because it is.
    Bypassing the switching board will tell the story.
    First, with power removed and the caps discharged, test for continuity across [3] and the wiper on the master. Powered down, that should be 0 ohms, and [3] to [2] should be open, since the .22uf caps on the preamp outputs and power amp input should isolate the DC from your meter. Finally, check [1] to [2], that should also be open (or some extremely high reading)
     
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  11. LarsN

    LarsN Strat-Talker

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    So I've bypassed the switching board in both directions, and still have no output.
    That points the problem at somewhere between [3] and the output jacks if I'm following along, which makes sense. Especially after I understood how the two channels connected to that PCB. (for some reason I assumed a knob labeled "Master Volume" would be associated with both channels. Even though I couldn't see how the clean channel connected to the MV at all. :) )

    I've touched up the solder connections in several places, and the Plate voltage on the power amp tube is now at 328v It was previously 308v and the schematic calls for 318v... I'm guessing that's all within the range.

    I'll hook the scope up to points [1], [2], and [3] to see if I get signal there... I guess that'll be easy since I can just put a nice constant 400hz sinewave into the input jack :)

    Thanks for the assistance y'all! I appreciate it.
     
  12. StratoMutt

    StratoMutt Strat-Talker

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    The master volume is lead channel only. It is essentially two amps; clean and lead. Don't expect shimmering bell-like clean as with a Fender amp.

    You have an oscilloscope? :eek:

    Think I mentioned the specified voltages were higher than what I got during testing - similar to yours.
     
    Last edited: Aug 9, 2019
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  13. LarsN

    LarsN Strat-Talker

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    It Lives!!!

    Turns out I had two resistors swapped between the 12BH7 cathode, and a grid resistor...
    a 470ohm and a 470k
     
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  14. StratoMutt

    StratoMutt Strat-Talker

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    Fantastic!! :D

    That is why I tested the value of each part prior to soldering.
     
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  15. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

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    @LarsN Congratulations! great job.
    You not only found the problem you solved it and you learned what it did.
    See? it is "good" having some small bugs in there. To learn and understand what actually goes on in such amplifiers.
    Now, come on make some noise \m/

    Life is good,
    Robin
     
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  16. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    Mine looks like this... built it when I was in high school in my electronics class.
    It's in storage... not sure if it's in my garage or my storage unit.

    365011.jpg
     
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  17. StratoMutt

    StratoMutt Strat-Talker

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    Heathkit! That is serious amp building credentials there!
     
  18. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    Easy mistake...
    Yellow Violet Brown vs Yellow Violet Yellow
     
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  19. LarsN

    LarsN Strat-Talker

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    I miss heathkit, if only because they offered tube amplifier kits for ham radio transmitters and I desperately want to build a 1kw tube amp.
     
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  20. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

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    Wrong resistor is what made Mike SOLDANO became famous xD
    Not that one though.

    Life is good,
    Robin
     
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