Vox guitar amp problem

Discussion in 'The Projects Forum' started by efdi, Nov 26, 2011.

  1. efdi

    Thread Starter New Member

    Nov 26, 2011
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    Hello everybody!

    I've got a problem that i hope someone here may help me with. I have a 12 year old Vox amp, a hybrid solid state with a 12AX7 tube Vox Cambridge 30 Reverb which i have modded (i replaced the problematic optocoupling from a LT9914 to a VTC5L3 and i changed the original TDA2050 chip with a much better LM3875). The modding done about a year ago made the clean channel sound superb.

    Lately, though, i've been having a problem. Sometimes, in the middle of my playing or right when i kick start the amp early in the morning, the reverb is gone! Couriosly, it always comes back when i press the "boost gain" button from channel 2, even when channel 2 isn't activated! (i usually only use channel one with reverb)

    I tried changing the 12AX7 tube with a new one, checked the wires leading from the spring reverb to the board and even resoldered some the reverb related connectors from the back of the board (from the reverb knob and from where the reverb wires meet the board + plus any other solder that looked "dry").

    I don't think it's a heating problem since the problem appears even when i start the amp fresh, i talked with an electronic engineer friend which suggested it might be some sort of circuit problem since the reverb is always restarted when i press the "boost gain" button from channel 2, even when channel 2 is not activated.

    Does anyone have any idea what might be causing this or what i might do to fix it? I really love the reverb on this amp and i would be very sad to have to buy a new one, but i can't go on any gig knowing the reverb might shut down randomly.

    If anyone can help, it would be much appreciated! My engineer friend said he would help me, but he doesn't know much about amps so any tips as to where he should look for problems would help a lot. Also, i have attached the service manual with the circuit schematics to anyone that might be interested.

    Thank you very much!
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Wow! That's two in a row today that folks are able to post schematics. That makes trouble shooting so much easier.

    How much experience in electronics do you have?
    Can you read schematics?
    Do you have a digital volt meter (DVM)?
     
  3. Yako

    New Member

    Nov 24, 2011
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    These electrolytic capacitors all over the place don't last forever. It only takes one to go into high ESR for a very complex switch-mode power supply to shut down completely.

    I would be replacing all of those cheap electros personally.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Spray some cleaner in the buttons (and pots) and work them. You may have an intermittant connection in the button.

    Has there been any other evidence like crackling when you turn pots etc?
     
  5. efdi

    Thread Starter New Member

    Nov 26, 2011
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    First of all, I would like to thank everyone for the quick answers!

    @MrChips - Not much experience in this field, i did a bit of soldering and helped with the modding of my amp, but my friend is an electronics engineer and he'll help me out with anything i can't handle. Unfortunately, he hasn't worked in the audio field before, thus the reason for posting here. As for the schematics, i'm meeting with him tomorrow to look over the schematics for the amp and try to pinpoint any potential problems. While i don't have a DVM i might try to get my hands on one, if you tell me how i could use it.

    @Yako - is there anyway to test for bad electros? If the issue is with the electros, is it normal for the problem to get fixed everytime i press the "boost gain" button?

    @THE_RB - no, no other problem. I had some intermittent crackling but the source was a dirty guitar input jack and the issue was resolved when i cleaned it. Still, if the reason for this is dirt, why does the issue resolve itself everytime i press the "boost gain" button from channel 2 even when channel 2 isn't activated?
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    My suspicion is that there is a bad capacitor in the circuit and we may be able to find it by taking DC measurements. When the button is pressed it discharges the capacitor and makes things work again. No reason to replace all the caps. We should be able to nail it down to just one cap.

    I have not had the time to look over the circuit but if I find anything suspicious I will let you know.
     
  7. Yako

    New Member

    Nov 24, 2011
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    Oh there are a few ways and a few tests. I'll leave you in Mr Chip's capable hands to make some DC measurements.

    Could be.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Look at the section of the schematic shown below:

    [​IMG]



    See if you can find on the board the two capacitors outlined, C75 and C76. They are both 10uF/25V electrolytic capacitors.

    I believe the problem is in the JK6 jack.

    If you were to remove C76 the problem should go away completely. You will always have reverb no matter what you do with JK6 and SW2.

    I am not sure about the function of JK6 and SW2. Some combination of inserting a jack into JK6 and closing SW2 is suppose to enable/disable reverb. Maybe you can explain how it is supposed to function.
     
    Last edited: Nov 26, 2011
  9. efdi

    Thread Starter New Member

    Nov 26, 2011
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    MrChips, wow, thank you very much analyzing the circuitry:)

    The reverb has only one control point, the knob. If at 0, the reverb is off, or at least it can't be heard. If the knob is turned, the reverb increases volume. I don't know whether the reverb is always on when you start off the amp and insert the guitar in the input jack (the knob's function being only to increases the volume) or whether turning the knob at minimum actually turns the reverb completely off.

    I have attached a picture of the controls. On top, from left to right, the controls are

    a. guitar input jack
    b. channel 1 controls ( volume, treble, bass)
    c. channel 1 and 2 switch button
    d. channel 2 controls (gain, boost gain button (the button which mysteriously reactivates the reverb if gone), treble, mid range boost button, bass, volume)
    e. universal effects - tremolo and reverb (affect whichever channel is activated) - tremolo speed knob, tremolo depth knob and the reverb knob
    f. foot switch jack
    g. power button

    On the back of the amp there are input jacks for headphones, external speakers, line output and external foot switch.

    Would pictures of the inside board help?:)

    @Yako - thank you again for your help!
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    I think JK6 is the EXT. FOOT SWITCH at the back of the amp. Plug a phono jack in and out a few times and the problem should be fixed. If not, get some spray contact cleaner and gently spray that jack while plugging in the 1/4" phono jack in and out a few times. No need to fiddle with the electronics.

    I think you'll be back in business.
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yep. Buttons, jacks, switch pots etc in amps will often go corroded and give similar symptoms.
     
  12. efdi

    Thread Starter New Member

    Nov 26, 2011
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    Ok, thanks for the advice!:)

    I plugged in the jack in the ext foot switch button a few times and now i am waiting to see if the symptoms resurface. Like i said, the reverb loss is random. So far it's working, i'll have to wait a few days and see if it happens again.

    The jack scenario is probable because i also had some problems with the guitar input jack (crackling and loss of volume) which got solved by cleaning it, but since i never used the other jacks i never bothered to clean them too...

    Tomorrow i'm going to buy some contact spray and clean all the jacks just to be sure. Will come back and post the result.
     
  13. MrChips

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    Oct 2, 2009
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    Try not to use too much and avoid getting the spray on the electronics and the tubes.
     
  14. efdi

    Thread Starter New Member

    Nov 26, 2011
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    Ok, i got some contact spray and cleaned all the jacks just like you said, without using excess spray. So far everything works, i'll have to wait and see if the problem resurfaces. :D
     
  15. MrChips

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    Oct 2, 2009
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    Good for you. Glad I can help out.
     
  16. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    There isn't a switch mode supply in the amp. It is a transformer supply with rectification and filtering.

    Hope the jack cleaning worked!

    When caps fail, they usually don't come back to life by flipping a switch back and forth (unless it is the power switch). If the amp has been in continual use since you've gotten it, the capacitors should be in good shape. When they sit without power and varying temperatures is when it is best to bring the voltage up slowly allowing the capacitors to "reform".
     
  17. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    IC5 is a 4558 which, if I recal correctly, is a dual 741.

    Replace it with TL072 or TL082 for better fidelity (just make sure to get the correct package type).
     
  18. efdi

    Thread Starter New Member

    Nov 26, 2011
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    Well, i hope the jack cleaning worked too. If the problem doesn't come back this week i'll assume it worked and post it here.:D

    @nomurphy - do you think the mod will make a big difference to the sound quality? Also, as far as i've read, the TL072 is better than the TL082. What do you think?
     
  19. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    It will make a difference in the sound at higher frequencies.

    Whether or not you like that sound quality is a different matter. No offense, but Tube type people seem to enjoy a bit of rolloff on the highs, so when it is rendered perfectly, it will sound "harsh" or some other word to that effect.

    If you like the way it sounds now, when it is working properly, don't try to fix it. :D If you don't like how some notes don't sound right to your ear then it may be worth a change, which may result in more changes, etc.
     
  20. MrChips

    Moderator

    Oct 2, 2009
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    Don't fix it if it ain't broke. That's what I say.
     
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