Visual Sound One Spot dead? [behavior explained + pictures]

[Cortex]

Hi folks, I am having a problem with my One Spot which I love btw! Yeah, it's cheap and if I could I would just buy a new one [not available in my country] so I am trying to get this one salvaged if it's even possible...

It was extremly possible to open this thing without damaging it even more











nothing seems to be burned. Anyone seeing something I cant?

The unit behaves strange, when I measure voltage it's oscillating from 9.5V to about 7.8V and all the values in between. I don't think it's some sort of broken contact or something, though once I managed to get a steady 9.5V on it without changing at all, but as soon as I moved the device slightly it started behaving like described. Strange.
Is this salvagable or should I throw it away?

Thanks!

[R.G.]

Your description seems to indicate that the feedback setup that lets the controller make 9.4V is acting up and not letting it regulate properly.

Your comment that "once I managed to get a steady 9.5V on it without changing at all, but as soon as I moved the device slightly it started behaving like described." indicates that it's a cracked or broken solder joint, component, or copper trace. It could also be a failure in the controller chip.

However - opening that case up is as dangerous as letting your pet cobra out of its cage to crawl around in the house. It is major dangerous. If the controller chip has failed, it might also disable the protection mechanisms that keep it from damaging pedals as well.

The up-side on fixing it is small, and the potential for further problems is big. I'd recommend not tinkering with it, just based on the potential dangers.

If you would, PM me about this.

[Mark Hammer]

Great pics, though.  Just goes to show you how complex switching supplies are on the inside...and why we shouldn't monkey with them. 

[R.G.]

They are good pics. A couple of things that aren't obvious:
- The gap with no traces in it on the bottom side, with the blue epoxy dipped device in the routed-out gap beside it. That gap is a bit more than 8mm, as required by safety regulations for minimum creepage and clearance distances for withstanding 4000V transients on the AC line
- The routed-out gap is for keeping the same thing for the transformer pins, not giving high voltage transients a path to short between primary and secondary.
- The yellow tape surrounding the power transformer on the top side serves much the same purpose, because the transformer body bridges the PCB trace gap.
- The windings inside the transformer have multiple insulation layers and creepage/clearance distances of 8mm from primary to secondary **inside** the transformer.
- The bright yellow box-style capacitor on the component side is rated for several thousand volts, and for self-clearing in case it punctures, and for not burning if it's shorted, being rated for across-the-AC-line use.
- The tiny transformer-looking thing beside the cap is a common mode inductor to keep RF switching noise from being broadcast back out using the AC power lines as an antenna. It has to be oriented so its magnetic field does not re-radiate this noise into the power transformer or the secondary outputs.
- There are two L-shaped aluminum heat sinks. The one closest to the yellow across-the-line cap has a high voltage MOSFET attached. If it's not a full epoxy-dipped MOSFET, that heat sink has 400Vdc on it if the power supply is being quiet, and around 600-800V peaks when it's working.
- The big black electro cap beside the main power transformer has 150-400Vdc on whenever the thing is plugged in. There's enough energy there to stop your heart, and this is a tiny, tiny power supply.
- That same cap has the word "vent" on it. This indicates the position of the weak spot in the cap that is designed as a safety valve so the cap's insides will spew out there instead of everywhere, and instead of exploding. It has to be oriented a certain way to maintain this safety provision.
- The exact positions of the traces on the bottom of the PCB (and on the top side, where they're not visible) matters to both performance, low noise, and safety. There are a couple of those that will raise output noise by 10-100x if they're moved a millimeter or two, even if they still connect to the same places.
- The component choices matter. There are often contradictory requirements for component choices and trace placement, so that making something better makes something else worse, and so getting a good design can be a several-dimensional optimization.

I'm probably overly amazed by things like this. Having been around for the infancy of switching power supplies, it still amazes me that this kind of thing can get done at all, let alone cheaply. In my time we struggled and struggled to learn the things that went into this - and made a lot of nasty-smelling smoke along the way. 

[gjcamann]

I also love my onespot and am amazed at such a cheap and good and small power supply  Good work there R.G., and thanks for the great inside tips. Sounds like quite a chore to make the UL people happy, And get it small and cheep.

I'd recommend you harvest the xformer, larger caps and rectifier for a future custom power supply.

RG, is that nice little (and presumably affordable) xformer (in the US version) readily available from mouser or digikey. I have dreams of making a super whoopy power supply with multiple isolated outputs all adjustable from 5 to 18v?

[R.G.]

RG, is that nice little (and presumably affordable) xformer (in the US version) readily available from mouser or digikey. I have dreams of making a super whoopy power supply with multiple isolated outputs all adjustable from 5 to 18v?

Sadly, no, it's not. The transformer is fully custom. This is true of most magnetics other than output inductors on most switching power supplies. There are a few generic switching power supply transformers available from electronics distributors, but not many. And there's not a USA version of the transformer. All the current-manufacture 1Spots work from 100Vac to 250Vac input from the wall. What makes it a USA, Euro, Japanese, etc. device is the special setup of the wall plugs to fit the unique country style of sockets. I'd have to go look through the design stuff to find out if the input wiring connection is different on the AC power rectifiers between 100-140Vac and 208-250Vac; the input rectifier may be a full wave rectifier or a doubler that may be used if it's going to a specific country.

One of the bits of secret sauce that goes into getting any high frequency power supply to be quiet is that the positions of all the windings inside with respect to each other affects how smoothly or not the switching device and high frequency rectifiers turn on and off. This is in common with the idea that where every trace goes on the PCB matters. 

The frequencies are so high that you can no longer assume that the electrical signal will stay in the wires. Most switchers run over 100kHz these days now that we have MOSFETs that will operate at those frequencies. Back when I was designing switchers (I didn't design the 1Spot     ) we were trying to use fairly crude and slow bipolar switching power devices, and so we were fighting to get things to run at 25kHz. Today, some of them are running at a 1MHz clock. The primary switching is usually 300-400V switched on and off in something like 100nS, which is both bigger and much, much faster than any signal in audio. The harmonic content of the main switch and transient switching spikes can easily transmit out of the thing or be tuned by the inductance and capacitance of the PCB traces, a resistor lead, or a wire into a transformer.

It's easy these days to get a switching power supply to work and make voltage. The tough part is getting it not to pass through hum (the parts are all close together so AC hum comes through easily) and whine (low frequency heterodyning of the RF switching with *everything*) and to not transmit lots of RF. Most switching controller chips and MOSFET power devices will work first time and make an output with some care in getting the connections and parts values right. Making that behave politely in a high-gain audio environment can be challenging.

Don't get me started on safety and EMI certification. Certification is remarkably expensive even if everything is perfect and it's just a look to see if everything is OK. We're talking tens to hundreds of thousands of dollars here. And you get to re-certify if you change anything, including the manufacturing line you make it on. When I take a few deep breaths I can more calmly contemplate that everything should be safe and non-interfering; I know the drill, but it's a touchy point.

[Cortex]

Two things:

If I shouldn't have posted this photos I apologise, or if you wish I hadn't. not that anything you said made me think so, just...this complex a device should have remained a mystery to us

Second:

I buried it Ordered a new one.
Actually what I think resulted in it's fail, is the fact that the plug, which was glued to the holes, broke and started to move freely, I could move it inwards and outwards as I pleased. It almost certainly made a short circuit somewhere, or whatever, so I think the malfunction was caused by a mechanical event. Such a strange weak spot of this fantastic power supply, there has to be some sort of solution better than gluing the plug to the rest of the plastic enclosure?
Nevertheless I hope my new one will arrive in time, I have a gig in a week

[Hemmel]

I buried it

I hope you salvaged components first ?

[R.G.]

If I shouldn't have posted this photos I apologise, or if you wish I hadn't. not that anything you said made me think so, just...this complex a device should have remained a mystery to us

Oh, no, no problem at all. The device is yours when you buy it, and you can break it open and take pictures, whatever. One reason I was going on about safety is that it really is quite dangerous to poke around in such a tiny device with high voltages in it. We need all the guitarist/electronicists we can get and I didn't want to lose one - that being YOU! 

And I'm kind of against mysteries. That's another reason I went on about the internals. The insides of that device are not mysterious at all - but the things that can matter are not at all obvious.

Actually what I think resulted in it's fail, is the fact that the plug, which was glued to the holes, broke and started to move freely, I could move it inwards and outwards as I pleased. It almost certainly made a short circuit somewhere, or whatever, so I think the malfunction was caused by a mechanical event. Such a strange weak spot of this fantastic power supply, there has to be some sort of solution better than gluing the plug to the rest of the plastic enclosure?

Hmmm. How old was this unit? And what country-version was it? I'll go do a look into whether we've had other such issues.

Nevertheless I hope my new one will arrive in time, I have a gig in a week

I hope it gets there too!

[bluebunny]

I know your intention won't have been to advertise, R.G., but I have just taken delivery of a 1Spot!  Your thorough (and modest) explanation of how it all works and what all the parts do, in this and many other threads, shows that a good amount of care has been put into this product - perhaps sadly lacking in many things, these days.  It's a very reassuring approach by VS, and I'm always happy to spend my hard-earned on quality.