Monoprice amp blowing fuses...now what?

Discussion in 'Glowing Bottle Tube Amp Forum' started by JD15, Sep 12, 2019.

  1. JD15

    JD15 TDPRI Member

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    Long winded post, sorry:

    So, I have had a Monoprice 15 watt amp for several months. I have changed tubes (mainly cuz they were rattling/noisy, but also cuz I like to try different old tubes) and swapped out a few speakers in the quest for clarity and a little more volume. All was well - it was dead quiet at idle and sounded good...until it was just dead after the last speaker change.

    I bought a used speaker, put it in and played for about 20 or 30 mins and it sounded good. The next day I play for about 10-15 minutes, giving a little more volume to see how it sounds cranked up a bit and then power light went out and it stopped making sound - no flash, pop, static, smoke, screeching or anything odd that lead up to it.

    I buy new slow blow fuses from Amazon:

    https://www.amazon.com/gp/product/B075SNJLCN/?tag=tdpri-20

    Put one in and pops immediately. I look into troubleshooting, searching for melted stuff or signs of damage, look for shorts in the speaker wires and test the speaker's impedance, hook up a different speaker, take all the tubes out and it still blows the fuse. I try these thinking I got the wrong fuses :

    https://www.amazon.com/gp/product/B008QYBE3O/?tag=tdpri-20

    Again blows immediately.

    Is there something obvious I'm missing?
    Anyone have this amp (or any amp) with this problem?
    Are there special guitar amp fuses? The new ones (left) don't look as beefy as the original:
    upload_2019-9-12_13-28-34.png

    Any insight would be most appreciated?

    Thanks,
    JD
     
  2. nojazzhere

    nojazzhere Poster Extraordinaire

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    You're putting in 0.5A fuses....is that what came out originally? I've never seen a Monoprice, but aren't amps usually in the 3A fuse range? Doesn't explain the very first (factory) fuse blowing, but sometimes it just happens.
    https://www.monoprice.com/help?pn=contact
    Contact them, they can tell you better.
     
    Last edited: Sep 12, 2019
  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    As long as you for sure got slow blow, looks don't really matter.

    @nojazzhere - those look like .5A (500mA) - not .05A.

    If you're comfortable doing tech work we could recommend some troubleshooting steps.
     
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  4. Whatizitman

    Whatizitman Friend of Leo's

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    Fuse breaking is almost always a sign something else is wrong. It's there to protect the amp (and you!) in the event of an overload of current. If you bought the same amp rating of the original fuse and it's still blowing fuses, the fuses are doing what they are designed to do.
     
  5. magicfingers99

    magicfingers99 Friend of Leo's

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  6. nojazzhere

    nojazzhere Poster Extraordinaire

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    MY MISTAKE.....I meant to type "0.5A"......I've changed my post. Sorry to foul up! ;)
     
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  7. 24 track

    24 track Doctor of Teleocity Silver Supporter

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    there obviously is an internal voltage issue either with a blown component like a cap or resistor or something shorting to ground , but without having the item in front of me , the issues may not be limited to the above, you mentioned that you changed the speakerwas the new speaker matched to the amp?
    example Fender supers have a 2-4 ohm load for the speakers if you drive it with an 8 ohm load it may/will cause damage
     
  8. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    If the fuse is still blowing without tubes it's not a speaker issue UNLESS the OT has already been damaged somehow.
     
  9. muchxs

    muchxs Doctor of Teleocity

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    Snookered again.

    Coerced into troubleshooting a Monoprice in virtual reality.

    Oh, the shame.

    :oops: :oops: :oops:

    '80s Honda motorcycles have a little rubber tab on the kickstand. It has a little line molded into it. No one knows what that little tab is for. Joke is when the rubber wears down to the line, it's time to junk the motorcycle.

    :lol: :lol: :lol:

    Tried to find a big legible schematic for your Monoprice. Failed.

    If it blows fuses with no tubes, you don't need to look much further. Seems to me if that fuse is a mains fuse it should be 2 amps slo blo. If it's a B+ fuse .5 amps is too much.

    Nice thing about amps with tube rectifiers (Don't look. You won't find one in your Monoprice) The nice thing about amps with tube rectifiers is it makes troubleshooting a no brainer. Amp blows fuses with the tubes pulled, tube rectifier? That generally indicates a bad power transformer.
     
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  10. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    If it still has warranty, put it back to as it was when it arrived and call Monoprice.

    Is it blowing the fuse ( it -must- be Slo-Blo ) when all valves are removed, or just the power stage.

    Does it smell bad.

    Do you have a light bulb limiter.

    Is the output transformer open or shorted.

    Did you at any time run it with an open circuit, ie, burnt out speaker.

    Have you tweaked the bias ( probably not related at all )

    have you chavved any of the wires when moving the chassis.

    The attached schematic is for the Laney. Close enough. Have fun, but consider that warranty.
     

    Attached Files:

  11. PCollen

    PCollen Friend of Leo's

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    How much did you pay for the amp ? Is it the 12 inch Celestion speaker model ? I ask because it is likely a copy of an existing amp for which there may be a schematic available.
    Do you have the model number of the amp ?
     
    Last edited: Sep 12, 2019
  12. Whatizitman

    Whatizitman Friend of Leo's

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    What the..... o_O

    OP, the amp is probably still under warranty. Put the original speaker back in, return the amp to Monoprice, or whatever Amazon distributor you went through (If you bought it through Amazon). It should not be blowing fuses, period.

    EDIT: Or just send it to muchxs. He'll have it ready for you by tomorrow, 8am sharp. I garontee.

    [​IMG]
     
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  13. muchxs

    muchxs Doctor of Teleocity

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    Bill_the_Cat-jpg.png
     
  14. JD15

    JD15 TDPRI Member

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    Ah, here's where it gets funny - I sold the original speaker (ironically, the only one that sold of several I put on Craigslist :confused:). I was hoping there might be a somewhat simple solution, like the wrong kind of fuse.

    It seems the ones I got just don't look sturdy enough. Maybe I'll try these:

    https://www.amplifiedparts.com/products/fuse-slow-blow-250v-miniature-5mm-x-20mm
    Of course, as stated above, the original blew for a reason.

    I have a multi meter and I'm open to poking around a little to see what the problem could be, understanding the inherent dangers of working inside an amp, of course. My research so far indicates it's usually a tube, the speaker or transformer. A transformer upgrade would probably cost as much as the amp, I'll bet.

    I got this super cheap new, only $150, and it's now a hassle to return with the speaker gone.
     
  15. JD15

    JD15 TDPRI Member

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  16. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Almost never a speaker, rarely a transformer.

    I'd pull all tubes, then pull the power transformer secondaries form the board, and see if the fuse blows. If so, it's either something in the switch, early in the power wiring, or the PT itself.
     
  17. magicfingers99

    magicfingers99 Friend of Leo's

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    sharp suit, where'd you get that?
     
  18. magicfingers99

    magicfingers99 Friend of Leo's

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    yeah and make sure you not grounding out on the speaker frame or something, you may have shorted a wire moving things about. Check your work, I always take photo's anymore especially if its got a million wiring harnesses like a video projector or something.

    try another speaker, take notes, so you don't chase your tail. only change one thing at a time, so you don't introduce new and perplexing symptoms.

    start at the outlet, try a known good outlet or use a decent UPS to make sure your not getting transients or massive ripple, work you way up the food chain.

    consider it an education.. at one time I couldn't tell a resistor from my sister and now I know enough to considerably damage high end gear and occasionally actually repair it...
     
  19. PCollen

    PCollen Friend of Leo's

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    .........
     
    Last edited: Sep 12, 2019
  20. NewKid

    NewKid Tele-Meister

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    Think of it as $150 spent on a fancy dinner for two and you got food poisoning. It’s a waste but you’ll live.

    Just don’t eat at that place again.
     
    Last edited: Sep 12, 2019
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