Vox MV50 AV - supposed standby switch fault...

[nickb]

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

Agree, it's a TL084C.

[jondoe]

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.

[nickb]

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.

[jondoe]

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?

[nickb]

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

[jondoe]

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.

[jondoe]

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

[g1]

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?

[jondoe]

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".

[jondoe]

Quick update, I've swapped out the ST 084C for a TI TL084c and the fault returns. I did power it up with the IC removed and the 18v rail stays stable switching between Standby and On. Once installed the new chip heats up just the same as the previous one.

[jondoe]

I finally got around to removing the CM053B from the board, this stopped the TL084 over heating and kept the 18v rail stable, a new one arrives this week so will pop it in and see if the fault returns. I might try and lifting legs on the IC until the fault goes away and see if I can trace the fault that way.

[Canadiandad]

Hello. New to this thread. Just wondering if you made any progress on figuring out the nature of the fault? I have a Vox MV50 Clean with the same issues

[jondoe]

Hello. New to this thread. Just wondering if you made any progress on figuring out the nature of the fault? I have a Vox MV50 Clean with the same issues

This is still in my broken pile I swapped out the CM053B with a brand new IC and the fault remains. The only other over heating part on the board is a tiny tiny SMD transistor, but I've been unable to identify the part. I might post some close ups of it in the hope someone might be able to help. I even swapping out a load of the bigger caps, just on the off chance, but that made no difference.

[mozwell]

Here are some pics of the amp http://www.voxshowroom.com/uk/amp/mv.html
It uses one of those nutube things....
I would be worried about 18.9V on the power supply for a CD4053, those cmos chips are 18V absolute maximum, so even replacing it may just damage the new one.
Is the external 19VDC power supply putting out the correct voltage ?

A TL084 or CD4053 should not run hot, there must be a short or fault elsewhere causing this issue

It cant hurt, maybe contact the vox distributor in your area, (or vox direct) say you are a service tech, and was wondering if it was possible to get a service manual, or if there was a fix for the standby problem. If the answer is no, you are no worse off, and even if you have to sign a NDA (non disclosure agreement), its still worth while.

[jondoe]

Here are some pics of the amp http://www.voxshowroom.com/uk/amp/mv.html
It uses one of those nutube things....
I would be worried about 18.9V on the power supply for a CD4053, those cmos chips are 18V absolute maximum, so even replacing it may just damage the new one.
Is the external 19VDC power supply putting out the correct voltage ?

A TL084 or CD4053 should not run hot, there must be a short or fault elsewhere causing this issue

It cant hurt, maybe contact the vox distributor in your area, (or vox direct) say you are a service tech, and was wondering if it was possible to get a service manual, or if there was a fix for the standby problem. If the answer is no, you are no worse off, and even if you have to sign a NDA (non disclosure agreement), its still worth while.

The CM053B has a maximum supply voltage of 20v according to it's datasheet, so it's just within safe operation. The standby switch only *appears* to turn on/off the 18+v feed to the CM053B, if the amp is powered on from cold, not in standby, it functions perfectly with no over heating. Only switching too standby causes the fault to appear, you cannot not make the fault go away until you remove the power completely and switch standby off. The external supply is putting out the correct voltage.

I did email Vox and ask for assistance with a repair but they informed me there were no schematics available and no serviceable parts.