Vox MV50 AV - supposed standby switch fault...

[jondoe]

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.



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?

[drewl]

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.

[jondoe]

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;



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!

[jondoe]

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.

[nickb]

I'd just bypass the standby switch. End of problem.

[jondoe]

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

[jondoe]

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

[Enzo]

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

[nickb]

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.

[g1]

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.

[jondoe]

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.

[nickb]

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.

[jondoe]

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.

[g1]

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

[jondoe]

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.