Hum issues on Guitar into Audio interface

if it's not bad, use a gate. 
 
I was having a problem with a loud hiss on my recordings when performing live? Unfortunately, I learned it was coming from the audience.

Only hardware plugged into the mains is the laptop. PSU says
Input 100-240V 1.6A 50-60Hz
Wide range input
So I think that's fine
 
It also says,
Connect only to grounded outlet
but we don't have those, so may explain the Hum when the laptop is plugged in. When the laptop isn't plugged in, I'm on headphones and the Quad runs off USB, so disconnected from the grid entirely

He's got humbuckers so the pickups shouldn't be picking up noise.
 
My guess is that he has a loose/broken wire in his guitar wiring.  I had a similar issue, one of the solder joints in my guitar failed and I would get intermittent/poor/no contact which resulted in noise issues.  A 15 minute repair later, and I was back in business.
 
Regarding, just having the cable plugged in, your guitar cable can pick up electrical noise (touch the open end of it and see how the noise changes).  If you really want to rule out the guitar, you need to plug another guitar in that you *know* is not noisy.

Unfortunately I do not have access to any other electric guitar shops, and as far as I am aware, no one sells brand new ones in any of the shops in this city.
 
I'm using both the High-Z selector on the Quad and the TH2, but thanks for checking, because on the TH2, it's hidden away a little.

So you don't think a different quality cable would make a difference, assuming their is no shield breakage and it's been well soldered.

The noise drops some when touching your strings?
The noise drops some when touching the metal on the cable?
That's all pretty much "normal" in my experience.
T

If you do not have Voxengo's SPAN (free), it is worth downloading and throwing as an effect onto an audio track. This will give you a look at that noise with the guitar plugged in and the PSU disconnected. I suspect (and hope) you are seeing a nice clean spike at 50Hz without nasty harmonics.
 
Using just this setup (not playing), you can fiddle with the noise gate, HP filter, and notch EQ to knock that spike down and save the effect chain you make into a preset for use with other tracks in the future. A noise gate alone should take care of any harmonics, but if you have a spike, it should be below the Low E (Low E = 82.4Hz).
 
An instrument cable is unshielded so it will act like an antenna on you. There is not much you can do but minimize its length, but you already have a 3m cable and I would not go any shorter since it would impede using the guitar.
 
I also own an Epiphone 335 Dot, and that selector switch should come out of the f-hole easily... however, if you are unfamiliar with this I would blow it off for now. The wipers inside that switch may be corroded (esp. if sat for a long time unused) which can many times be mitigated by cycling the switch rapidly several times. If the switch works in both up and down positions, but noisy in the mid, I assume the internal wipers are fine, just corroded. Try cycling that switch first and see if it helps.
 
Let Voxengo's SPAN guide you on what is going on with your efforts to minimize it.

mettelus
An instrument cable is unshielded so it will act like an antenna on you. There is not much you can do but minimize its length, but you already have a 3m cable and I would not go any shorter since it would impede using the guitar.

That's not true, an instrument cable should be shielded, but it is not balanced which is why it can act as an antenna.

The cable is shielded, I noticed yesterday. If I see a quality 1 metre cable here (which I doubt, because of the lack of options, especially for electric guitars (though I due a visit to the British Embassy in Hanoi or Ho Chi Minh soon)) I may try. Because I play seated and the quad is on the desk in front of me, I may be fine with a shorter antennae..er lead.
 
Before going to work yesterday, I started to suspect the switch was beginning to function again, so maybe playing and switching between has begun to help the switch. I can take the switch out, I checked it a few weeks back when I first discovered it wasn't working. NO visable signs of corrosion, but a small layer off oxidisation may be enough to fug it up I guess. What approach do you suggest to remedy this. I haven't sen contact cleaner here, no idea where I would get it. I'm wondering it running a piece of paper through the plates, to rub off the oxidisation would be enough.
 
I downloaded Voxengo yesterday, and I'll have a look today
 
   

Thanks for taking the time to do that, it's a fiddly job getting it back in so appreciated. That's the same design as mine too.
 
I was a little confused to why it wasn't working when I looked, as going by your last picture. When the switch is pressing that right plate away, it breaks the signal for the pick up on that side. And vice versa. In the middle position, it presses neither plates away away, so breaks neither signal, allowing both pic ups to be heard. So here is my confusion. If the contact plates work when the opposite is pressed away, then the contacts are working, so why wouldn't they work when neither plate is pushed away, as both sets of contacts are working on there own? 

That makes sense, as when I thought it was returning yesterday, it seemed as if the bridge pick up wouldn't kick in until I turned the volume over 1 and a half.
 
Thanks again btw fore pulling your knob out for me earlier (errr that doesn't sound quite right does it)

Scoot, the way to cure this is to bend the two middle connections ever so slightly outwards so that they apply more pressure to the outer switch half.
If you can make it so that the switch toggle feels very slightly loose in the middle position then you've got it about right.  If the toggle is firm in the middle position then it is applying pressure to the switch halves and breaking the signal.
 
I had to do this on a LP for exactly the same reason, no sound in middle position.

Hum on a guitar...... something near and dear to my heart.
 
I have a classic Gibson SG with humbuckers.... the good ones... and if I get too close to a monitor it will hum. Distance, direction, angle, all affect the level.  The electronics and or wiring in the guitar should also be shielded.
 
A lot of the cheaper guitars.... the Epiphones, and the Fender MIM & MIC ...are all cheaper guitars. They have to use cheaper pickups and likely cut corners on shielding to sell at the price they do and still make a profit to stay in business. I haven't taken my Fender Modern apart to look..... but at $450 vs $1000 for a Fender Telecaster..... there has to be corners cut and cheaper parts used. I'm absolutely certain one of those corners is cheaper pickups.
 
On my Gibson, I have total silence...well no hum anyway with my amps and modelers. However, the Tele Modern is a whole different story. Depending on the pickup selection I use, I can get silent operation or hum that's pretty significant. And... the hum is also dependent on the gain settings I'm using. 
 
I have to choose which selector setting I use and when I use it. I often simply record the parts I want with the guitar and whatever noise is present..... and then use surgical envelopes in the track to cut the noise/hum to silence.

I've got a Gibson Classic 57+ in the bridge and a 490R in the neck of my Epi 335; both sound sweet. I don't recall having any issues with hum while recording. As Karyn has mentioned though, manny things can be the cause...dimmer switch, wall outlet, computer monitor, etc.

Touching the case does make a difference, problem it, I'd have to run a wire from the case into the bathroom and to the electric showers earth. No way am I touching touching Vietnamese electrical system in this hotel. I've already experience an apartment fire when a neighbour's electricity meter went up in flames, taking the other 6 floors electrics and melting the plastic piped plumbing with it.