Vox guitar amp problem part 2

Discussion in 'The Projects Forum' started by efdi, Jun 19, 2012.

  1. efdi

    Thread Starter New Member

    Nov 26, 2011
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    Hello, everybody!

    If you remember me, i posted several months ago regarding a 12 year old Vox Cambridge 30 guitar amplifier which kept loosing it's reverb. The thread is here:
    http://forum.allaboutcircuits.com/showthread.php?t=62541&highlight=vox

    That issue was successfully resolved thanks to the kind people over here (it was only a matter of cleaning some jacks with a contact spray).

    However, now i'm facing a new problem and i came back here, maybe someone can help me out with this one as well:(

    So, here it goes: for about i year i started experiencing a random loss of volume while i was playing. It happened very rarely and it automatically resolved itself after a couple of seconds, so i blamed it on power fluctuations.

    However, the problem got a whole lot worse lately. The random volume drops appeared more often these last few weeks, and once they appear, they will not go away. Now it happens every time i play the amp. The problem usually happens after i play the guitar a bit. If i let the amp rest 30 minutes and start it up again, the problem is gone but reappears after 15-30 minutes or even sooner. Also, i noticed i am able to generate the appearance of the problem if i crank the amp at maximum right after i turn it on.

    I tried cleaning all jacks and volume pots with contact spray and i replaced the preamp tube with a new one but to no effect. Also, the problem is not related to the guitar or the cables as i tested them on my second amp and they're fine

    I though that maybe the volume pot is bad, but i have two volume pots on 2 channels and they both exhibit the same problems (and i very rarely use channel 2, the pot there was almost untouched the past 2 years so no reason for it to go bad at the same time as the channel 1 pot).

    Could it be that a solder got bad and when the amp gets hot or is pushed too much the solder fails and the signal is getting weaker?

    I would be much obliged if anyone could tell me what would most likely cause this problem (what solders should i fix, what components might need replacing etc.).

    I have attached the schematics to the amp. Thank you again!:)
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Glad we were able to fix the problem the last time.
    This time it will be a bit more tricky.
    When the volume drops,

    1) does it quit completely,
    2) does it fade to a low volume or
    3) does it fade to no sound?

    Sounds like a component is overheating or a capacitor is building up a charge.

    If I had that amp on a bench with I scope on it I should be able to find to problem.
    What test equipment do you have?
    Are you able to take the amp out of the cabinet?
     
    Last edited: Jun 20, 2012
  3. efdi

    Thread Starter New Member

    Nov 26, 2011
    23
    0
    Mr Chips, hello and thank you again for trying to help me out!:)

    To answer your question, it goes to low volume, and once it does that, it stays in low volume, meaning that if i turn the volume to 20% i hear nothing or a very faint sound, if turn it to 40% i hear it like it is on 20% and so on. Up until several weeks ago it got back to normal volume on its own after several seconds, now it goes back to normal only after it stays off 15 minutes or more.

    I am able to take it apart completely, i have an ex electronics engineer friend who helps me out with soldering and opening it up.

    I don't have any test equipment except for a voltmeter though i don't know what good will that do. I may be able to find some other equipment if needed, if you tell me what i would need.

    In the meantime, is there anything i could try that doesn't involve testing equipment?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    I don't think so.

    You have to get the amp out on a workbench and get access to the circuit board and components. You have to be able to work on the circuit with the power applied while following the circuit diagram.

    Next, you either need a sound source such as a radio or MP3 player
    OR a sound amplifier such as portable player with a line input.

    What you have to do is find at what stage you are losing the signal.

    You can do this in two ways.

    1) You can inject a signal (guitar or MP3 player) at the input jack and use the sound amplifier to trace where the signal is going.

    2) You can inject a signal from the MP3 player at various stages while listening to the output at the guitar amp speaker.

    In both cases, you will isolate your injected signal or the amp pickup with a 10μF or there abouts in series.

    No need to be intimidated. This can be done quite successfully.
    Take clear photographs of the top and bottom of the circuit boards and I will show you where to probe.

    A good starting point is the point labelled "C" in the schematic.
     
  5. efdi

    Thread Starter New Member

    Nov 26, 2011
    23
    0
    Ok, I have an mp3 player and my second amp as a sound amplifier.

    By 10μF you mean this?: http://www.radioshack.com/product/index.jsp?productId=2103610

    What i don't understand is, after i connect the mp3 to the input jack, how exactly do i "use the sound amplifier to trace where the signal is going". Do i connect the amplifier directly to the circuit board at various stages? How do i do that?

    and

    "inject a signal from the MP3 player at various stages". Again, do i somehow connect the mp3 directly to the circuit board?

    I'm sorry if these questions are silly, but my knowledge in electronics is pretty limited (i am willing to learn though:D).
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Yes, anything from 1μF to 100μF will do the trick for now. This is not a permanent install but just for trouble shooting.

    Get some photos of the amp posted. Also show what cables, jacks to your MP3 and player and sound amp you have. If we get lucky we may not need both. One alone may do the trick.

    We have to be able to identify the components in your circuit.
    Do you have a soldering iron handy. We will have to unsolder some resistors and capacitors.

    Does the guitar amp fail if it is left ON for awhile on its own or do you have to be playing continuously?
     
    Last edited: Jun 19, 2012
  7. efdi

    Thread Starter New Member

    Nov 26, 2011
    23
    0
    Yup, i have a soldering iron handy. And yes, i think the volume drops happens if i leave the amp on by itself.

    This is the second amp: http://www.roland.com/products/en/Micro-CUBE/

    this is the mp3: http://www.samsung.com/ae/consumer/tv-audio-video/mp3-players/flash-memory/YP-K3ZB/MEA

    i have a 3.5 mm jack for my mp3 and a standard 1/4" guitar jack for my amp. I have cables and jack adaptors for every kind of jack mentioned above (i can make the mp3 fit into the the amp input jack)

    Tomorrow i'll post some pictures of the board:)
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    The first thing you are going to do once you have access to the circuit and a voltmeter in hand - you will turn on the power - make sure that nothing gets shorted out.

    With the voltmeter you will make the following measurements all with respect to GND at the following nodes (junctions)

    1) C56, R92 (+HV)
    2) R92, R90, ZD3, C54 (+15V)
    3) R90, ZD1, C52 (+9.1V)
    4) C57, R93 (-HV)
    5) R93, R91, ZD4, C55 (-15V)
    6) R(1, ZD2, C53 (-9.1V)

    Then you will wait for the fault to occur and repeat the measurements.

    Also you will try to locate a component that might be running hot to touch (except of course V1).
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    There is one more thing you can go ahead and try.

    When the amp has warmed up and the fault appears,
    connect a jumper wire to short out LT9914, (exactly where you see those dotted lines,
    i.e. a wire from the R41, LT9914 node to LT9914, R42 node
    and see if the volume comes back up.
     
    telefunkinu476 likes this.
  10. efdi

    Thread Starter New Member

    Nov 26, 2011
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    Thank you, will try this today. Also, this time I forgot to mention (i mentioned it only at the beginning of the previous thread), that 2 years ago i modded the amp. I replaced the problematic optocoupling from a LT9914 to a VTC5L3 and i changed the original TDA2050 chip with a more powerful LM3875.

    These are 2 mods that are very common with the amp and people have no problem with them, but who know what might have happened these past 2 years...
     
  11. efdi

    Thread Starter New Member

    Nov 26, 2011
    23
    0
    Ok, i think the problem is solved, Mr Chips! Apparently, one of the main wires from the power supply (the "BK" one) was never soldered in the factory!:mad: It was just wrapped around the metal holder.... it is a wonder it even worked for so long. I can't believe i didn't spot it sooner.... in all these years..... This explains the dodgy sound i got at times and the problems i've been having these past weeks.

    Anyway, we soldered it and the problem is apparently gone. I have to say i am very relieved it was so simple.

    I would like to thank you again for your help, Mr Chips. You've helped me and my little amp quite a lot.:)
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Geez... too bad. I was looking forward to some serious trouble shooting.
    Anyway, good luck with that amp.

    I have to go now and practice for a gig tomorrow night. Have to learn Landslide, Eternal Flame, Not Ready to Make Nice and Creep all of which I have never heard before.
     
  13. efdi

    Thread Starter New Member

    Nov 26, 2011
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    0
    Well, it seems the serious troubleshooting might still be an option:) The problem appeared again this morning, but it's been a little different.

    The amp started out with an overall volume 30% lower than usual. During about an hour, i experienced random spikes of volume going up, the opposite of what i was usually experiencing. Like something was holding the amps volume down and from time to time it was trying to escape (poetic, isn't it:) )

    Then, after an hour, the amp suddenly turned back up to normal volume and it stayed that way so far. I'd really like to hope the problem went away on its own but i doubt it. My friend suggested it may be an active amplification component that is failing, such as the tube (which was ruled out because i tested a spare, though it was a used 12ax7..... is it possible that 2 failing tubes produce the exact same effects?) or maybe the LM3875 chip i replaced. Also, next time it happens i intend to check the tremolo effect, if it doesn't work then the opto is the issue.

    By the way, you're a musician too, Mr. Chips? May i ask what instrument you play?:) And good luck with your gig!
     
    Last edited: Jun 21, 2012
  14. MrChips

    Moderator

    Oct 2, 2009
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    Try jumping across that resistor we talked about before.

    I play piano, guitar and bass.
     
  15. efdi

    Thread Starter New Member

    Nov 26, 2011
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    Yup, will do, as soon as the problem reappears, which, thankfully it hasn't so far. Now, I've shut down the amp and see if the problem reappears when i turn it on later today. I will report the success/failure.

    Nice set of skills you have there:) I only play guitar myself, though i have a keyboard in the house, i use it mostly for interval training, i find the it is much easier on a keyboard layout:p One day i'll get around to learning more of that...

    Say, may i ask what guitar gear you have? I'm mostly interested in the amp. If i'm not gonna be able to fix my Cambridge i'm going to have to invest in a new amp and since you happen to be a musician I'd love to hear some of your suggestions.
     
  16. MrChips

    Moderator

    Oct 2, 2009
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    My guitar is a Strat clone. Bass is a Fender Jazz.
    The amp is a Fender Super Champ XD. For the bass or guitar I have a home-made battery powered portable amp.

    I am no expert on guitar gear so I can't give you specific recommendations. The bottom line is you have to go to the store and try it out. Fortunately, the store I go to has a very convenient return policy. I originally bought a Fender Mustang I and didn't like it and exchanged it for the XD.
     
  17. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I know the question wasn't for me but as a suggestion; from what I've seen these days the amp is much less critical a choice. Most decent DSP guitar effects units now come with amp simulators built in, they will mimic the characteristics of all classic amps in their frequency response, resonances, reverb styles and overdrive behaviour. It's pretty impressive, even uncanny.

    My suggestion would be to choose and amp based on practicalities rather than sound. If you need a loud amp choose a big reliable one, of you don't need big sound then I'd pick one that's real easy to transport. ;)
     
  18. MrChips

    Moderator

    Oct 2, 2009
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    That is what the Fender Mustang is all about. It is a DSP unit that mimicked every popular amp that existed in the past. This was too much for me. Since I play jazz, I just wanted a clean beefy sound with a little bit of reverb. The Super Champ does that nicely.
     
  19. efdi

    Thread Starter New Member

    Nov 26, 2011
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    Thank you for your suggestions!

    I am familiar with the DSP units built into most amps, and this is what i intend to bypass; not because i don't like the digital options, but because i already own a Boss GT-10 sound processor; thus, any built in DSP unit is useless since i already have a very, very good stand alone DSP unit containing all my tones. Now, I'm pretty much looking for what you said, Mr Chips, an amp with a nice clean and reverb. The Fender Champ looks interesting, they have it at my local guitar store and i will go check it out. I was also thinking of a Vox VT40+, the successor of my amp, but it has a DSP unit with tons of useless options built in, exactly what i want to avoid.

    My main problem is that my tones are created from my GT-10 in conjunction with the Cambridge 30, which makes switching the amp a bit painful since i will have to modify all my tones. This is way i try to fix my amp so much. Besides that, i think it sounds great on its own too!:)

    Anyway, the amp worked great yesterday, all day long. I played for about 5-6 hours with no problems after the initial hiccups. This morning i started the amp, everything was fine for 1 hour, then the problem started again: the volume level dropped, and i got random spikes of "normal" volume every now and then, as if the sound was "trying to get out". It's still the same. Later this evening i will try to do what you suggested, Mr Chips, and i will post pictures of the board.

    One other symptom i wanted to report; When the amp is started, it usually produces a somewhat loud sound, a kind of "pop", unrelated to the volume level (i leave it at 0 when i close the amp). This is its normal behavior. However, when the problem manifests itself and the amp is in "low volume mode", when i start it up, the "pop" is very faint, much less powerful then when it is working ok. This is how i can tell right when i start it up if the volume is going to be ok or not. Does anyone have any idea what causes the "pop" noise in the first place? Whatever it is, it is being affected by the problem. Maybe electricity filling up the capacitors? Because i found some that were pretty warm when i last looked inside. It sound silly, but maybe this little bit of information will help out....
     
    Last edited: Jun 22, 2012
  20. #12

    Expert

    Nov 30, 2010
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    The fact that the start pop is diminished means one of 2 things. Either the start surge is being picked up early in the amp stages or the problem is in the output stage. Does it pop way before the filaments have time to get warm? Check for crusty/loose sockets on the EL84's. Swap the EL84 tubes.

    Don't be overly concerned about the time it takes to find an intermittent. Just 2 days ago I found the ground problem in my Main breaker box after 3 months of waiting for it to repeat itself, and that circuit is very simple. [It was an illegal splice hidden behind dozens of (smaller) branch circuit wires.] The fact that splices are not legal in Servive Entrance wires gave me a prejudice that I wasn't looking for a corroded splice, or any kind of splice.
     
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