Intermittent static noises on a mojotone princeton build

Discussion in 'Amp Tech Center' started by winstonchurchill2, Aug 5, 2021.

  1. winstonchurchill2

    winstonchurchill2 TDPRI Member

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    Helllo,
    I recently built a mojotone princeton reverb which came out pretty well overall. The only issue I am seeing is that it will intermittently make some static noises when its been on for over 10 minutes or so.
    This can happen when there is no cable plugged into it.
    I attached a google drive link to a sound clip. you can hear it happening for 16 seconds and then it stops.
    Not sure what this could be. Any thoughts on what to try?
    https://drive.google.com/file/d/1TXew4ENJlUytGjHqSbIu4c7PePvCbnMP/view?usp=sharing
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    Gently tap each valve, one at a time to rule out a faulty/noisy valve.
     
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  3. winstonchurchill2

    winstonchurchill2 TDPRI Member

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    none of them make noise when I am tapping on them. Unless it is already making the noise before I start tapping.
    I did notice that the noise will get louder when I increase the volume. And I also turned the reverb and tremolo circuits off and all the way down to rule those out.

    Would swapping preamp tube with tremolo or reverb 12ax7 be a way of narrowing things down?
     
  4. Jon Snell

    Jon Snell Tele-Holic

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    Your fault is before the volume control if the control affects the noise level. The noise is coming from within the box I have drawn.
    Screenshot 2021-08-05 at 21.51.00.png
    Not a lot to go wrong but I would firstly suspect one of the coupling capacitors, probably the 250pF.
     
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  5. dan40

    dan40 Friend of Leo's

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    Did you happen to use a silver mica for the treble cap that Jon mentioned? Was it a black, unbranded one? There have been reports of these generic SM caps being bad in the past so I have stopped using them in builds. Even the name brand micas can cause issues if overheated during the soldering process.
     
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  6. winstonchurchill2

    winstonchurchill2 TDPRI Member

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    This is a picture of my build. and I believe this is the 250pf capacitor. kind of hard to see in this pic. Does it look like one of the dodgy ones?
     

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  7. winstonchurchill2

    winstonchurchill2 TDPRI Member

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    I also wrote to mojotone support and they said this.
    "this is more than likely not a tube. When a tube goes bad, either losing it's vacuum or an internal arcing, mechanical failure, or just the age of a couple thousand heat up and cool down cycles, they almost always fail to where they are not intermittent, it's sudden. Either they work or they don't, and if it's an internal arcing issue, it will blow a fuse. Sounds like this almost always point to a tube socket or board solder joint that is going through heat up and cool down cycles. The solder expands and contracts at a different rate than the board. This will usually get worse over time and will ultimately lose contact. This is easy to fix though. Pull the chassis and after it warms up use a small non-conductive implement and move things around. Move the wires around the tube sockets and components on the board. Also, move the tubes around in the tube sockets. This will usually pinpoint the issue"
     
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  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes, swap tubes around.

    Jon showed a portion of the schematic. Do you read schematics well enough to know where the components are in your amp?
    Pretty much they are near the arrow you showed in the picture. Chop stick all of the components, wires and solder joints on that right side of the board. Include the socket and the volume pot terminals. Most of your solder joints look good but a few look a little dodgy.

    Use all safety protocols. No jewelry. Standing in shoes on mat. One hand in back pocket. Non conductive *poker*. etc.

    It can be anything from a bad solder joint, cracked/damaged resistor or cap, or broken wire inside the insulation. Push on the parts/joints to try to make the noise happen. Sometimes it takes more than a soft touch to identify the intermittent problem.

    Check the resistors on the input jacks. Heck, check the tightness of the jacks/pots while you are at it.

    As you can tell, you will be looking for some poor contact point. You may or may not be able to see it with the naked eye.

    Happy hunting.:)
     
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  9. winstonchurchill2

    winstonchurchill2 TDPRI Member

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    So I think I solved it thanks to your help @Jon Snell @Lowerleftcoast . I believe it was bad lead dressing on the red and yellow wires going from preamp tube to volume pot. Moving those wires seemed to make the most noise.
    But it also could have been some of the other stuff I tried. I reflowed and re-soldered most of the circuit board. I swapped tubes between preamp and reverb and later reverb and tremolo. I found a loose screw that secured board and backer board to the chassis. and I deoxed those tube sockets.
    So far no static crackling. I will check further as I play it over the next few days.
    I did notice the reverb circuit has a bit of noise (but more hum, not crackle) when I turn up the reverb level. But that might be normal as I don't usually have it past 4.
    This is a pic of what the lead dressing was before I twisted them tighter and ran along side of chassis.
     

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  10. winstonchurchill2

    winstonchurchill2 TDPRI Member

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    The good news is no static after playing for about an hour today. So I think its gone. Thanks again @Jon Snell and @Lowerleftcoast
    I swapped the position of the reverb tank, and that helped with the low hum. there is a bit of his when I have it turned up past 4. but better than before. Maybe those circuits aren't perfectly silent overall and that is normal behavior
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Fender pushes that 12AT7 hard. Fender uses a lot of voltage on that tube to achieve reverb. The Princeton Reverb schematic shows V2 has a plate voltage (Vak) of 400. To put it in perspective, tube datasheets show the max Vak as 300 volts. Some Princeton Reverb amps run higher voltages than the schematic so, it may be even higher than 400V. In any case, more noise can happen when the tube is pushed hard. Careful attention to lead dress can help. Later Fender models added a resistor to drop that voltage a little which could help reduce noise.
     
  12. winstonchurchill2

    winstonchurchill2 TDPRI Member

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    I might have spoken too soon. I played for about 3 hours with a drummer today and the amp pushed and I started to hear some of that static noise again, even after unplugging the cable. I am going to monitor some more before cracking it open again and chopstickingi it. Just to see if it only happens when it is on for a while.
     
  13. dan40

    dan40 Friend of Leo's

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    Your micas appear to be a dark red so they are likely not the same type as the ones that I was referring to.

    When this issue has occurred, is it always plugged in to the same circuit in your house? I have chased static sounds like this in my amps before but the problem ended up being caused by something in my house. I only realized this after taking the amp out to a buddy's house for a jam. The static never occurred when the amp was plugged in somewhere else.
     
  14. winstonchurchill2

    winstonchurchill2 TDPRI Member

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    It has always been on the same circuit. I will try on a different outlet and a different location of the house to see if that makes a difference.
     
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  15. dan40

    dan40 Friend of Leo's

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    The fact that it last exactly 16 seconds each time makes it seem as though something in your house may be turning on occasionally. Dimmer switches, lighting fixtures, modems and household appliances can all cause noise on the AC power line that can be very frustrating to track down sometimes.
     
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  16. schmee

    schmee Doctor of Teleocity Ad Free Member

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    -It's a tube. try shuffling some different pre-tubes in. Are they old tubes or cheap chinese tubes?
    Wiggle the tubes one at a time. does one get louder or react?
    -What reverb cables did you use? Are they tight on the rca sockets? Unplug them to make sure it's not the reverb section.
    -Check all the wires going to the tube sockets by wiggling them while it is on. (careful!) Sometimes a solder glob looks good but is loose.
     
  17. guitar_paul1

    guitar_paul1 Tele-Holic

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    Back when I did service calls, I traced noise in a bar's PA system to a dimmer in another bar 2 doors down. Really.
    The only time I've had to bring an electric guitar legitimately on a service call.
     
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  18. winstonchurchill2

    winstonchurchill2 TDPRI Member

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    I did swap all the tubes around. and lightly tapping on them did not cause any change or noises.
    I used self built reverb cables, and the sounds happens with our without reverb.

    I also reflowed and re-soldered almost every pad and filament pin, and ran new wiring in a few locations. And I sprayed contact cleaner in tube sockets (again).

    So after spending several more frustrating hours chopsticking and trying things, I gave up and decided to bring it to a professional. There is a an amp repair tech about 1/2 hour drive from me, that has an amazing reputation. So I dropped it off with him, and I will let you all know what he finds out. If it turns out to be a bad comonent, hopefully Mojotone will make things right. We'll see.....
     
  19. schmee

    schmee Doctor of Teleocity Ad Free Member

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    Cool, let us know. A new build can be frustrating. Forgotten wire, mislocated wire end etc... seems I have one every time!
     
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