Vox MV50 AV - supposed standby switch fault...

[J M Fahey]

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

True enough, but please see it this way:
* apparently you got it as-is , acting weird, for peanuts.
* you have NO data whatsoever, nor help from VOX, who´d love to see you junk it and buy another.

so even if "nice to know" and I agree with you on the satisfaction of digging the truth using the "little grey cells"| (I´m currently reading Poirot ), it does not pay getting too deep (still with uncertain results) , wasting a lot of time and mental energy, while the amp can be made usable and stable with a simple operation.

And the main point: [U]who the h*ck wants a Standby switch on an SS amplifier?[/U]

Oh, the "Nutube" thingie you say?
It´s just a calculator/microwave/VHS player Fluo display, which happens to be wired so as to amplify a little, (10X gain tops ) , which with tons of chutzpah is labelled as "12AX7 improvement/evolution" and which in any case has a direct heated filament/cathode which reaches operating temperature in less than 1 second

It´s a hair thin wire.

[jondoe]

Thankfully I don't work with amps for a living, so it can sit on the shelf and get played with once in a while, it owes me a little money, but I'll consider that the cost of it's entertainment

[nickb]

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.

[ATTACH=CONFIG]54509[/ATTACH]

So when the nutube wears out it's a junker. What a let down. Doesn't compare with a traditional tube well at all.

[Canadiandad]

Thanks for sharing your efforts. I bought this 2nd hand (but not for peanuts) so it owes me quite a bit more. Would love to hear other solutions besides putting the power supply on a power bar and using that to switch it on and off, or unplugging the power adapter from the back

[g1]

There are in-line switches that you could wire into the power adapter cord, or you could rewire the 'standby' switch to disconnect the power adapter jack from the amp circuit.

[tonsound]

True enough, but please see it this way:
* apparently you got it as-is , acting weird, for peanuts.
* you have NO data whatsoever, nor help from VOX, who´d love to see you junk it and buy another.

so even if "nice to know" and I agree with you on the satisfaction of digging the truth using the "little grey cells"| (I´m currently reading Poirot ), it does not pay getting too deep (still with uncertain results) , wasting a lot of time and mental energy, while the amp can be made usable and stable with a simple operation.

And the main point: [U]who the h*ck wants a Standby switch on an SS amplifier?[/U]

It´s a hair thin wire.

that is for legal requirmenents in energy saving i think, see
https://commission.europa.eu/energy-...dby-devices_en

so it shall not wake up again by design. The devices are not broken.

[tonsound]

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.

you better leave the resistor on top of th cap, that is factory default imho to adjust the poweramp. They created first first the ac, then clean an rock and had to modify the poweamp for clean an other a little from what i read in the datasheet of the TI TPA3118 poweramp chip. (a stereo 30/50w (at 21V) design but in in switched to mono mode). see here the rock pcb which has blown the tpa3118. The resistor is on on top of the cap. Later revisions of the board has an extendend layout to place the resistor beneath the cap. Below image from a clean.
Also see my fx loop mod in thegearpage 'mv50 mod tonsound'.