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Thread: Vox AD120VT Problem

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    imf
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    Vox AD120VT Problem

    Hey all... I should say, I'm not a tech and don't really know amp circuits or electronics all that well. But I have a DMM, soldering iron and determination to try and learn if that counts for anything. I can somewhat follow the schematics.

    I have a vox AD120VT valvetronix from circa 2001. I liked it a lot actually, but after a few years it crapped out on me. I took it to a tech here on Long Island a while back, and he told me I shorted it out by dropping a pin onto the board (through the vents on the top of the unit). He wanted nothing to do with this thing is what I got from it... he told me go pick up a simple tube amp.

    I have done some research (searched this board, which has a few threads on it) and some other forums, and I get why he didn't want to work on it... nobody does, it's basically a computer lol. I'll try to keep this short, I know you guys like it to the point here.

    I took it out of the closet a week ago and turned it on. Everything lights up perfect as it did when I last powered it on. So for shits and giggles, I plugged in and took the effects loop send out of the amp into a small Roland cube 30 I play on for practice. It sounds great - so obviously the preamp section is fine.

    I plugged headphones into the phone out - nothing just light humming.

    The speakers and phone out have a low cycle hum which gets a bit louder as I move the power select switch, but overall it's not THAT loud.

    The speakers are fine and there is no DC current on the speaker out wires. (I've read to check that from someone on this forum).

    I have read that the headphone and other jacks can cut the signal it if the contacts are dirty. I have deoxit d5 on the way to try an clean all the jacks, but since the tech guy told me I shorted something.. I'm not counting on that being the problem. I never gigged the amp so it has no road abuse either.

    I don't have a scope, but I can build a small signal tracer probe to see where the signal cuts out if that's worth it. Maybe I could check continuity between all the switching. Not sure where to start.

    I opened this thing up and there is no burnt anything anywhere, looks clean as far as components, but I've also read the solder joints are garbage on this as well.

    Just looking for advice on what I could/should check out first, as maybe it's worth investigating since it seems to be in the power section.

    here's the schematics link that I'm looking at:
    http://www.valvetronix.net/docs/AD12...l_Complete.pdf

  2. #2
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Plug your guitar into the return jack and see if you get anything out of the phone jack and speaker.

    If it works the return jack is the issue - simply plugging into it a few times is often enough for it spring back into life.
    Last edited by nickb; 11-19-2017 at 11:19 PM. Reason: Edited - was looking at wrong schematic
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    imf
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    Quote Originally Posted by nickb View Post
    The line/phone out jack sits right is fed from the return jack after a single chip buffer. Either the return jack contact is bad or the buffer is. Plug your guitar into the return jack and see if you get anything out of the phone jack and speaker.

    If it works the return jack is the issue - simply plugging into it a few times is often enough for it spring back into life.
    Hey nickb, thanks for checking it out.

    I just tried plugging into the return jack, nothing. I plugged the jack in/out many times... still nothing. before I do anything I'll be getting the cleaner to try with also.

    It seems the signal is gone maybe well before the line/phone jack, although I'm not great at reading this schematic. It seems the problem may start from Master Volume onward. Power Section is fed from the Master Volume W09 into CON1(R-IN) + CON2(L-IN) to the power PCB where it goes to the VN2410L's and on... looks like signal to the line/phone outs are after the valves right?

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Standby... I looked at the AD100 schematic....
    Experience is something you get, just after you really needed it.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    OK.. Ahem.. Looking at the correct schematic now

    Yes the return jack goes thru the master volume VR15 and on to the Q3/Q4 and Q1/A2 differential pair amps. Now since this is stereo and both outputs are dead it suggests something common like a power supply. I suggest you check the voltage on the center tap of the transformers for about 218V.
    Last edited by nickb; 11-20-2017 at 06:05 PM.
    Experience is something you get, just after you really needed it.

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    imf
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    Quote Originally Posted by nickb View Post
    OK.. Ahem.. Looking at the correct schematic now

    Yes the return jack goes thru the master volume VR15 and on to the Q3/Q4 and Q1/A2 buffers. Now since this is stereo and both outputs are dead it suggests something common like a power supply. I suggest you check the voltage on the center tap of the transformers for about 218V.
    Yep, I'm thinking check the output transformers as well, will do. I was just looking them up, seems hard to find new ones (vox part# V904050013). Don't want to jump the gun though.

    Thanks for looking it over, much appreciated. I'll update once I go digging later this week.

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    imf
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    Ok I tried cleaning the jacks and as expected, that wasn't the problem. I propped the PA board up and started probing around underneath to look for some voltages and I'm not getting much on the output transformers, not even a volt. These are PCB mounted OT's with 6 pins on each side so maybe I'm doing it wrong, I just checked each of the 12 solder joints on each one. The Tubes are glowing though if that means anything.

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    Supporting Member The Dude's Avatar
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    If you don't have voltage to the output transformer center taps, check that 100mA fuse just off the orange wires of the power transformer. It should be near "TAG2".

    FYI: I don't think Nick was saying your output transformers were bad. He was just wanting you to check voltage there. That voltage comes from the rectifier circuit off the orange wires of the PT.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    imf
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    Quote Originally Posted by The Dude View Post
    If you don't have voltage to the Output transformer center taps, check that 100mA fuse just off the orange wires of the power transformer. It should be near "TAG2".

    FYI: I don't think Nick was saying your output transformers were bad. He was just wanting you to check voltage there. That voltage comes from the rectifier circuit off the orange wires of the PT.
    Just checked it, those were the first things I looked at when I took the chassis out. They seemed fine but I just checked continuity on the F1 and got the beep.

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    Supporting Member The Dude's Avatar
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    If the fuse is good, check the voltage on the cathode of D11 or D12 (same point). It should read about 216V.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    imf
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    Quote Originally Posted by The Dude View Post
    If the fuse is good, check the voltage on the cathode of D11 or D12 (same point). It should read about 216V.
    Reading barely anything on D11 or D12, R84 gives me 195V right before them.

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    Supporting Member The Dude's Avatar
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    Your meter is set for DCV and not AC?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    imf
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    AC on R84, DC on rectifiers right?

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    Supporting Member The Dude's Avatar
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    Yep.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    imf
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    Quote Originally Posted by The Dude View Post
    Yep.
    ok yeah, 13.1mV on the cathode side (side with the stripe).

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    Supporting Member The Dude's Avatar
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    Do you have AC on BOTH sides of R84 all the way to the anode of D11?
    Last edited by The Dude; 11-22-2017 at 06:55 AM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    imf
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    Quote Originally Posted by The Dude View Post
    Do you have AC on BOTH sides of 84 all the way to the anode of D11?
    195V on the top, .2V on the bottom of 84.

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    Supporting Member The Dude's Avatar
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    Check R84- 1K/1W. I believe it's open. You should also check those rectifier diodes for shorts. Check the 216V rail for continuity to ground to make sure there isn't something shorted on your rail. There's probably a reason that resistor went open.

    Edit: Also check that F1 is the correct value. It should have gone first. Maybe someone put a larger fuse in there? (assuming I'm right about the resistor)
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    imf
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    Quote Originally Posted by The Dude View Post
    Check R84- 1K/1W. I believe it's open. You should also check those rectifier diodes for shorts. Check the 216V line for continuity to ground to make sure there isn't something shorted on your rail. There's probably a reason that resistor went open.

    Edit: Also check that F1 is the correct value. It should have gone first. Maybe someone put a larger fuse in there?
    Will do ASAP tomorrow. Thanks a lot for your help The Dude.

    The tech I brought it too over 10 years ago said I dropped a pin onto the board and it shorted... so it's starting to make sense here now that you say this. He just had no intention on trying to mess with the insides of this amp, I don't blame him.

  20. #20
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by The Dude View Post
    FYI: I don't think Nick was saying your output transformers were bad. He was just wanting you to check voltage there. That voltage comes from the rectifier circuit off the orange wires of the PT.
    Quite so. It was just a convenient test point.

    We're having a late night by the looks of things

    The tech I brought it too over 10 years ago said I dropped a pin onto the board and it shorted... so it's starting to make sense here now that you say this. He just had no intention on trying to mess with the insides of this amp, I don't blame him.
    I was very, very surprised he didn't attempt some basic trouble shooting - having no 218V is a pretty simple thing to test for.
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    imf
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    Quote Originally Posted by nickb View Post
    I was very, very surprised he didn't attempt some basic trouble shooting - having no 218V is a pretty simple thing to test for.
    What's up nickb... Yeah I don't know, maybe he did test it I'm not sure. He was a nice guy though, seemed like he was in his late 70's, old school electrical engineer, so I didn't argue. I figured he just didn't wanna mess with it. He showed me the pin it was one off of a dress shirt lol. Didn't charge me a thing of course.

    Anyway I tried my next feeble attempt at diagnostics in regard to what The Dude told me to check.

    Not sure how to check if R84 is open, I guess that means take it off the board and test it, but I might as well just get a few new ones if it's even suspect. There is not much AC getting by it at all, like .2Vac before the rectifiers so something definitely wrong there.

    I checked the rectifiers and they seem fine - I'm getting .523 V on diode check and 0L when I flip the meter leads around.

    And for the ground continuity check on the 216V line... (which I'm only getting 195V really, straight off that orange wire before R84) I don't get continuity to ground until I check after the D13 and D14 rectifiers on their anode side, not if I check right at TAG4 connection. Is that how it should be?

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by imf View Post
    What's up nickb... Yeah I don't know, maybe he did test it I'm not sure. He was a nice guy though, seemed like he was in his late 70's, old school electrical engineer, so I didn't argue. I figured he just didn't wanna mess with it. He showed me the pin it was one off of a dress shirt lol. Didn't charge me a thing of course.

    Anyway I tried my next feeble attempt at diagnostics in regard to what The Dude told me to check.

    Not sure how to check if R84 is open, I guess that means take it off the board and test it, but I might as well just get a few new ones if it's even suspect. There is not much AC getting by it at all, like .2Vac before the rectifiers so something definitely wrong there.

    I checked the rectifiers and they seem fine - I'm getting .523 V on diode check and 0L when I flip the meter leads around.

    And for the ground continuity check on the 216V line... (which I'm only getting 195V really, straight off that orange wire before R84) I don't get continuity to ground until I check after the D13 and D14 rectifiers on their anode side, not if I check right at TAG4 connection. Is that how it should be?
    The ground is fine. To check R84 just turn off and measure it's resistance. I agree with the Dude - it's odd that should blow before the fuse.

    195VAC is fine too.
    Experience is something you get, just after you really needed it.

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    imf
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    Oh ok cool thanks. Yep it's open, read 0L. I tried a couple other resistors and they read fine. I'll check the fuse for correct value now...

    here's a pic, it says "carbon resistor" on material list, is that carbon film?

    ad120vt-r84.jpg

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by imf View Post
    Oh ok cool thanks. Yep it's open, read 0L. I tried a couple other resistors and they read fine. I'll check the fuse for correct value now...

    here's a pic, it says "carbon resistor" on material list, is that carbon film?

    Click image for larger version. 

Name:	ad120vt-R84.jpg 
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    From the apparent physical size of it, I'd think metal oxide. What's the wattage? Probably a 1W CF?

    Standing the resistor a little off the PCB will help if this has chronic heat issues. Catastrophic over-current, not so much.
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    imf
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    Quote Originally Posted by eschertron View Post
    From the apparent physical size of it, I'd think metal oxide. What's the wattage? Probably a 1W CF?

    Standing the resistor a little off the PCB will help if this has chronic heat issues. Catastrophic over-current, not so much.
    Hey eschertron, it's 1K/1W.

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    imf
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    BTW, you guys are good... The Dude was right... this is a T125mAL fuse in here... I'll be damned. Should be a T100mAL.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by imf View Post
    BTW, you guys are good... The Dude was right... this is a T125mAL fuse in here... I'll be damned. Should be a T100mAL.
    I think Dude had money on it being a 20A fuse

    1W carbon film aught to be right for that size resistor. What's its function? I haven't looked at the schem.
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    imf
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    Quote Originally Posted by eschertron View Post
    I think Dude had money on it being a 20A fuse

    1W carbon film aught to be right for that size resistor. What's its function? I haven't looked at the schem.
    lol - i'm worse than an amateur here, sorry guys.

    It's the resistor right after the 216V PT lead hits the board, right before the rectifier diodes.

    I got 1k/1w Xicon carbon film and some of the same in metal oxide at close to the same size at mouser. The one I pulled out is 3.2mm x 10.5mm something like this. Is there a preference for metal oxide in the power section over carbon film, or really not much of a difference?

  29. #29
    Supporting Member The Dude's Avatar
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    The resistor composition makes little difference assuming the proper fuse is installed. Any 1K/1W would work. As eschertron suggested, I would leave the leads a little longer when you replace it and get it standing up off of the circuit board a bit to keep heat away from the board.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    imf
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    Quote Originally Posted by The Dude View Post
    The resistor composition makes little difference assuming the proper fuse is installed. Any 1K/1W would work. As eschertron suggested, I would leave the leads a little longer when you replace it and get it standing up off of the circuit board a bit to keep heat away from the board.
    Cool, will do as suggested. Thanks a lot everyone, I'll let you know how it goes after I get the components.

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    Supporting Member eschertron's Avatar
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    Is R84 even sized right? I calculate about 32mA is the failure point for this device. Why try to protect it with a 100mA fuse?

    P = I2R
    I2 = P/R
    I = SQRT(P/R)
    I = SQRT(1/1000)

    Am I off here? What's R84 even doing there?
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  32. #32
    Supporting Member The Dude's Avatar
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    FWIW, it's a T100mA (slow blow). I imagine the fuse is only there to protect the PT in case of catastrophic failure. Otherwise, there are lots of amps that don't have an HT fuse at all. Without measuring, we don't know what actual current is in the circuit.
    Last edited by The Dude; 11-23-2017 at 12:59 AM.
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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by eschertron View Post
    Is R84 even sized right? I calculate about 32mA is the failure point for this device. Why try to protect it with a 100mA fuse?
    Quote Originally Posted by The Dude View Post
    FWIW, it's a T100mA (slow blow). I imagine it's only there to protect the PT in case of catastrophic failure. Otherwise, there are lots of amps that don't have an HT fuse at all.
    Worded poorly, I know. I guess I meant to ask, how is the fuse even going to blow first? I know the resistor will take a few seconds (or fractions of a second) to heat to damage, but so does the fuse. A direct short across the rectifiers, or some other catastrophic current failure, I see the fuse protecting the transformer winding. But a slower high-current failure (between 32 and 100mA) will always take out the resistor first.
    So my point is, if the bean counters didn't want to pay for a resistor sized to live through the short (yup, needs 10W plus to do that), why not ditch the fuse and leave the resistor as the sacrificial element? Would it not blow fast enough / reliably enough to save the tranny?
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by eschertron View Post
    Would it not blow fast enough / reliably enough to save the tranny?
    And having said that, since the resistor was probably collateral damage, are we any closer to finding the fault?

    edit: OK, I read back and see that a foreign object was dropped into the chassis? Was that the root cause?
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    imf
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    Quote Originally Posted by eschertron View Post
    And having said that, since the resistor was probably collateral damage, are we any closer to finding the fault?
    Could it have anything to do with me dropping a metal pin onto the board (fell through the vents on top)? No idea where it could have hit.

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