Weber 5F2A build troubles.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by kawi10r998cc, Apr 24, 2015.

  1. kawi10r998cc

    kawi10r998cc Member

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    Hello there! I'll get straight to the point since I can be long winded...this is my first amp build (not new to electronics though) and it has been fun! I built the amplifier exactly to the layout/schematic (as best as I can tell anyway) and after completing it, it fired right up and sounded great!

    However, after about 15 minutes it got very static-y for a few seconds and then cut out completely. I took the back panel off and began to take some readings and noticed R10 smoking a bit and turning brown.

    Needless to say I turned the amp off and let it cool. I switched it back on long enough to get some readings (they are posted below in the layout pic), but in so doing, R14 fried as well...although that happened as I measured mA across pins 3&8 on the power valve so I'm pretty sure that I caused that, and not the original issue.

    R10 is rated at 2W/10K ohms and still measures 10K despite being burnt. R14 now is open.

    I'd greatly appreciated any help anyone can give me as I've reached the end of my rope on this. I cannot find any good, comprehensive direction on where to start troubleshooting despite scouring the internet for about 8 hours. Even the weber amp kit forum is totally blank, I cannot even pull up one thread on it so the site must be down.

    I've attached all the pics that I have, but feel free to ask for more, ask me any questions, or suggest me to do whatever you think may help figure out what is going on! I have no problem with receiving constructive criticism. Thank you again for your time!

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  2. kawi10r998cc

    kawi10r998cc Member

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    A bit of extra info...upon closer inspection I just noticed that the ground wire on the AC cord, and the ground for the large blue capacitors (C2 and C9, both 16 uF) are both no longer connected to the chassis. Both were soldered on securely prior to initial use.

    Is it possible that they came loose because of heat and caused the failure? Would those grounds being off cause my issue? I assume that having no ground on those caps would definitely cause an issue since they would effectively be out of the loop, and I "think" that all the voltage would then go to R10 causing it to fry...
     
  3. Pedro58

    Pedro58 Supporting Member

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    ^^^^^^^^^^^^^^^^
    That would do it, I think...

    Is the polarity on your caps correct?
     
  4. kawi10r998cc

    kawi10r998cc Member

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    As far as I know the polarity is correct. The large blue caps have a black arrow with a negative sign pointing toward the legs that I attached to ground.

    If the polarity was incorrect, the amp wouldn't have worked at all would it?

    Also, the two yellow caps to the right of the blue ones had no indication of polarity so I assumed those didn't matter which direction they went, although the yellow ones are pretty far from the issue so I wouldn't expect they would cause this issue if they were in wrong.
     
  5. Pedro58

    Pedro58 Supporting Member

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    You're right that the yellow ones don't matter as far as polarity goes. I just couldn't tell how one of the big blue ones was oriented. I still think it was what you guessed. The ground came loose. But it fired up and sounded good for 15 minutes? Hmmm... Someone more knowledgeable than I will pipe in soon.
     
  6. kawi10r998cc

    kawi10r998cc Member

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    I agree with you...in my limited knowledge anyway...it seems as though the ground popping loose would've caused my issue...I've since reattached it and now I can tap it with a screwdriver and it won't move so hopefully that is taken care of now!

    Unfortunately, since I fried the other resistor (which is now open) when I was testing, I have to allocate more components before I can test it again as after I fixed the ground issue, I still have no sound...likely because of R14 going out.

    Thanks for your help and hopefully a third person will concur (or correct me) so I can get on with enjoying the fruits of my labor!

    Have you built any amps? I started with the 5F2A as it was one of the easiest ones they had...I wanted to attempt a Deluxe Reverb but it was a bit too complex...now I'm definitely glad I didn't take on that one!
     
  7. Trout

    Trout Member

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    Both Bypass caps are reversed, the black sprague atoms.

    Likely will now need replaced as well.
     
  8. kawi10r998cc

    kawi10r998cc Member

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    Thank you for pointing that out! I now see the little green (+) on what I wired to ground. Perfect. :facepalm I'll add those to my list of components to replace.

    Do you think the blue caps (don't know the term for them) will require replacement as well? I tried testing them for uF but my meter was giving me odd readings so I assume I'll have to remove them from the circuit to check them...
     
  9. cap47

    cap47 Member

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    The large electrolytics if reversed would short the amp and it wouldn't have worked. Would have fried the rectifier.
     
  10. cap47

    cap47 Member

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    They should be OK were oriented correct.
     
  11. kawi10r998cc

    kawi10r998cc Member

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    Wow, so at least that is one thing I didn't mess up!
     
  12. Avatar Tech

    Avatar Tech Member

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    With a multimeter, when you have it set to measure current, you must break the circuit and send all the current through the meter. The current measuring mode is a low impedance setting through the meter. It's almost like a dead short through the meter, and can be dangerous and destructive. R14 on your schematic is the negative feedback resistor, and the amp should work without it, but the tone will be altered. Generally you have to calculate current in a particular component by measuring voltage and resistance and using ohm's law.
    Your plate voltage seems really high. That's at node A in the schematic. That high a voltage at the plate could mean huge current drawn through the power tube circuitry.

    Colin
     
  13. kawi10r998cc

    kawi10r998cc Member

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    Concerning the reading of current...I remember that...now...ha ha! I haven't done component level troubleshooting for about 8 years so I suppose it is one of those "use it or lose it" things. In my occupation now we just do board level troubleshooting and replace the entire board that is the culprit, never getting down to the actual root of the problem, obviously to the detriment of my skills. I need to go back and read through all my notes.

    And I agree on the plate voltage...I couldn't find an exact measure of what it "should" be but nearly 500 VDC is too high. The caps are rated at 450 so I knew it was too high. I am just hoping that it is only too high at the moment b/c of the error I made previously and will be rectified once I change out the components.

    How would I be able to change the plate voltage if replacing the components doesn't bring it down? There are only three taps on the transformer for accounting for voltage and I currently have the brown wire (for 120) in the circuit so that should be correct. There are also two red wires with white stripes that are on the PT that aren't used and aren't even on the schematic so I don't know what they do...they wouldn't be additional taps would they?

    And it is a bit high going into the rectifier to begin with in my opinion...I think it is supposed to be 350 VAC not 370 as I was reading...so would higher voltage in produce a higher DC voltage out?
     
  14. kawi10r998cc

    kawi10r998cc Member

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    My nearest electronics shop that might have the components I need is an hour away but I am heading there today. I'll post an update asap on the status. With any luck this thread will be marked "fixed" soon and will just serve as a way for people to learn how not to install caps, how not to check those caps, and to make sure the freaking ground attachment is solid!

    I think in my next build I may use a terminal block tied to the ground post for grounding all the wires instead of spot soldering them to the chassis. It would be more secure that way apparently.
     
  15. Diablo1

    Diablo1 Member

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    You shouldn't be soldering ground connections to your chassis! The AC line ground should be soldered to a terminal lug that is bolted to the chassis, and nothing else attached to that bolt. Put a locknut on that bolt so it never can vibrate loose on its own. It's critical for safety. The amp circuit grounds should also not be soldered to the chassis. It's difficult to make a good solder connection to a large piece of steel, and if the chassis is stainless steel, the solder will never wet the chassis. Again, solder your ground connections to a lug and bolt that to the chassis.
     
  16. kawi10r998cc

    kawi10r998cc Member

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    There is only one terminal lug on the chassis, so I will attach the AC line ground to it...thank you for the heads up on that! Not like I won't be working in there anyway!

    I do have one question though, since there is only one terminal lug and you stated that I should attach the AC ground to that only, should I drill a hole and install another grounding lug for the other circuit grounds? Or was soldering them all to the brass plate under the tone/volume pots and inputs ok?
     
  17. Pedro58

    Pedro58 Supporting Member

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    As long as the brass plate is attached to the ground lug, yes. Don't drill a hole and create another ground. You'll get a ground loop and that causes all sorts of noises... Don't ask me how I know that one!
     
  18. swiveltung

    swiveltung Member

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    Weber's chassis are now SS... not a good conductor and solder wont stick. Ground the AC ground to the Power Transformer bolt with nothing else.
     
  19. kawi10r998cc

    kawi10r998cc Member

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    SS doesn't allow solder to stick well you say? Ha ha I can attest to that...
     
  20. kawi10r998cc

    kawi10r998cc Member

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    Good point! The amp was super quiet before it stopped working so I definitely want it to stay that way.

    I picked up all the components I fried so I hope to attempt repair and make the recommended changes to grounding today. I'll update once I'm all done, hopefully with good news!
     

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