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Thread: Vox MV50 AV - supposed standby switch fault...

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    Vox MV50 AV - supposed standby switch fault...

    Hi everyone, I recently picked up a Vox MV50 with a standby switching problem, I've seen this problem mentioned online in a few places, it being, when power is applied with the amp not in standby, the amp works fine, if you switch the amp into standby, then out, the amp doesn't come back up, or if you apply power in standby. This has been the reason for many people sending them back under warranty, I thought I might pick up a bargin that just needed a switch, much like Orange Micro Terrors!

    On investigation the problem isn't the switch, in fact the switch itself works perfectly, all it does is switch 19v on and off to another part of the circuit.

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    TP12 on the middle of the picture, left hand side (+19v written above it) is switched on and off with the switch. This is in close proximity to the Ti CM053B, which is some kind of logic gate from what I can tell? The only other observation I've made is the D15 has 18 volts on the single pin side when it's working, but in the fault state this drops to 0.5v.

    Has anyone on here had a poke around one of these?


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    There's an older model SS Vox amp which locks up and if you tie an unused pin of one of the chips to the +5v supply the amp will work properly.

    I thought I had the info but can't find it, maybe someone else has it?

    It may be the same type of problem.

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    I suspect I'll never see a schematic for this so I'm certainly guessing!

    I could get a CM053B from China and try swapping it out, but that might not even be the problem for all I know. I used a little alcohol on the board while it was in standby and there is only one location that warms up and that's a TSV914A OP amp, it doesn't warm up when it's in working mode, only in this standby failure mode, which is a little odd.

    And one more thing, I took the tube daughter board out and had a look under it and found the following;

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    Notice R309 is missing, but what's that resting against the side of C115?! It was like that from the factory as far as I can tell!

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    I removed the resistor hugging the capacitor, placed it back on R309, no change in function. Further testing the op amp shows that it does in fact get very hot when the amp is in standby and only when in standby. When the amp is operating normally the opamp doesn't break a sweat.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    I'd just bypass the standby switch. End of problem.

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    That is the work around solution for sure, but it would be nice to know what's causing it, right?

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    Further oddity, the TSV914A has +19v on the VCC, but it's rated at 5v according to the datasheet. But it doesn't get hot while the amp is working, it only heats up when the amp is in standby.

    Either I have the wrong datasheet or something weird is happening

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    Or the chip doesn't have the other end at ground.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by jondoe View Post
    Further oddity, the TSV914A has +19v on the VCC, but it's rated at 5v according to the datasheet. But it doesn't get hot while the amp is working, it only heats up when the amp is in standby.

    Either I have the wrong datasheet or something weird is happening
    I'd guess that the output is connected to a part of the circuit that is powered down when in standby so leading to excessive dissipation.

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    How about posting all the pin voltages for that IC, standby, and run.
    I've attached the datasheet.

    As far as moving that capacitor, did you check that maybe the pads are going to the same places? Maybe they had some hum issue etc. that required them moving the cap closer to the resistor. If it's the same 'schematic' location, I would not move it physically.
    Attached Files Attached Files

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    These are the voltage readings while the amp is operating normally (powered up while out of standby)

    1 - 9.1
    2 - 9.1
    3 - 9.1
    4 - 18.3
    5 - 9.1
    6 - 9.1
    7 - 9.1
    8 - 9.1
    9 - 9.1
    10 - 9.1
    11 - 0 (earth)
    12 - 9.1
    13 - 9.1
    14 - 9.1

    Amp switched to standby while powered on, or powered up in standby.

    1 - 1.5
    2 - 1.5
    3 - 8.2
    4 - 16.8
    5 - 8.4
    6 - 8.4
    7 - 8.4
    8 - 8.4
    9 - 8.4
    10 - 8.4
    11 - 0 (earth)
    12 - 8.4
    13 - 8.4
    14 - 8.4


    When I looked at the capacitor there is a transistor next to it, one leg of which went to the resistor pad which wasn't populated, but there was that resistor stuck to the side of the cap, so that's why I came to that conclusion it was probably meant to be there.

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    From that data I'd guess that the opamp on pins 1,2 &3 is wired as a buffer and that something is pulling pin 1 down. But, as you say the max supply voltage is 6V so it being operated outside its spec. That is very unlikely unless there is a fault or the part isn't what we think it is.

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    This is the best picture I can get of the chip and it's lettering, I might be wrong, happy to be told so . I'm using the ground tab of the 5volt regulator when taking readings, which is also happily putting out 5 volts as it should.

    Pin 1 of the chip does route back off to the CM053B on the opposite side of the board.

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    I'm seeing what I assume to be a TL084 in that pic?
    Attached Files Attached Files

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    Quote Originally Posted by g1 View Post
    I'm seeing what I assume to be a TL084 in that pic?
    Oh my, so I've probably got that wrong, that'll teach me to use google image search.

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    Quote Originally Posted by g1 View Post
    I'm seeing what I assume to be a TL084 in that pic?
    Agree, it's a TL084C.

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    Thank you g1, nickb.

    Those readings certainly make more sense now. I'll order up a couple of spares and get this one swapped out, it's probably toasted.

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    Quote Originally Posted by jondoe View Post
    Thank you g1, nickb.

    Those readings certainly make more sense now. I'll order up a couple of spares and get this one swapped out, it's probably toasted.
    I wouldn't. It's probably just fine as it can withstand an output short of unlimited duration. Your real problem is elsewhere.

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    Last edited by nickb; 04-19-2019 at 10:59 PM.
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    Quote Originally Posted by nickb View Post
    I wouldn't. It's probaly just fine as it can withstand an output short of unlimited duration. Your real problem is elsewhere.
    So I should probably concentrate of tracing pins 1 and 2 back across/under the board and seeing whats happening?

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    Honestly, without a schematic it's impossible to say.

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    Quote Originally Posted by nickb View Post
    Honestly, without a schematic it's impossible to say.
    I'll have a tinker but I think you're right. Standby switch will have to be hardwired off, then at least it's usable. Fun fact, the pre-amp board has two sections that can be bridged, printed "short fat" and "short bright" on the PCB. I could see people modding these to switch between the different sounds as you'd effectively have 3 tones in one box, I bridged them myself and they certainly do give a thicker and a brighter sound.

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    Last edited by jondoe; 04-20-2019 at 12:15 AM. Reason: added picture

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    Just for my own notes, and anyone following alone at home

    The 084C is tied to pins 1 and 2 of the CM0538

    In normal operation the voltage reading on the CM0538 pins are

    1 13.8
    2 0
    3 18.9
    4 0
    5 0
    6 0
    7 0
    8 0
    9 0
    10 0
    11 0
    12 9.1
    13 9.1
    14 9.1
    15 0
    16 18.9

    in failure mode;

    1 0
    2 0.1
    3 0.5
    4 0.3
    5 0
    6 0
    7 0
    8 0
    9 0.1
    10 0.1
    11 0
    12 1.5
    13 1.3
    14 1.3
    15 0
    16 6.48

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    You mean CM053B, standard like CD4052.
    Supply is pin16, so either the supply is collapsing somewhere on the way, or the chip is shorting. I think you said it was replaced, so maybe it's supply is not holding up. Does it get hot when in failure mode?
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    Quote Originally Posted by g1 View Post
    You mean CM053B, standard like CD4052.
    Supply is pin16, so either the supply is collapsing somewhere on the way, or the chip is shorting. I think you said it was replaced, so maybe it's supply is not holding up. Does it get hot when in failure mode?
    8 vs B, damn it I need to polish my eyeballs. I even wrote down B on my notes, but put 8 on here

    I haven't replaced any of these IC's yet, although I ordered a new 084C as it was very cheap. When in failure mode the CM053B doesn't get warm in the slightest, while the 084C gets too hot to touch for long. I poured some more IPA over it earlier and noted the only other warm item is a teeny tiny SMD transistor marked "NB".

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