"you need to make a recording of something else through your soundcard, to eliminate that as the cause."

Thats a very good point. I'll let you know what happens!

It strikes me that, if all the 3k's signal path goes through the headphone circuitry, you might need to be careful to ground yourself before plugging in a set of headphones that might already be on your head or in your hands, and possibly a small static charge built up. Touch the metallic barrel of a jack plug, or the metal on your mixer, or something like that, otherwise it is possible (if however unlikely) that the discharge get into the headphone circuitry.

I know I'm getting ahead of myself, here, just blueskying possible causes...

Do you recall any particular incidents (perhaps the power going out on you while the 3k was switched on) between now and when it WAS recording alright? Strictly speaking, with an external power supply, it should be much harder for power fluctuations to damage your unit, but it IS another possibility...

It's not the sound card or the cabling. Oh dear!

Well, bottom line is, if you can't HEAR any difference...

You say you have the identical file recorded before and after the malfunction? If you set up their levels carefully to be as close as possible (maybe your software has an RMS output meter?), you ought to be able to A/B the two in a good listening situation (maybe good headphones) and see if there IS anything to get worked up about.

Work around available although problem still there!

I've recorded the same midi file using the output from the main jacks, and from the headphones socket, and both look fine.

The output from the phono sockets is still strange (I thought I'd better double check).
I can hear a difference between good and bad, the bad one isn't as crisp and seems to lack a shade of punch. It's not a huge difference though, there's a case to be made that I'm persuading myself there's an audible difference because I know there's a waveform difference there in the first place!

Anyhow, I'm still going to take the lid off, disturb and reseat all connectors, and maybe even put a bit of contact cleaner into the main volume control (it's getting a bit crackly) but that's as far as it goes. If this doesn't resolve the issue with the phono output I'll record off the main jacks.

Thank you all for your assistance, it's made sorting this out much less of a shot it the dark!

John

So, have been inside, had a look at circuit board, nothing obvious, removed dust, squirted contact cleaner into volume pot, re-assembled.

Now have zero crackles volume control, phono output still strange. Which is what I expected to be honest but you have to try these things!

Internally it's pretty neat. All cable runs are taped down or clipped into specific slots in the case moulding. There are ferrite cores on a small number of links, and these are individually supported so they won't move about under their own weight during transit. The major audio-related electronics are inside a Faraday cage and there's also a shield under the main input/output board (which I removed to examine the PCB by the phono's).

Mentioning a Faraday cage reminded me of this video: http://www.topgear.com/uk/videos/car-lightning

Are you sure that the earlier recording you used for comparison was from the phono (RCA) outs? Maybe you had used the headphone or main outs before, and the RCA's were always a bit naff? This one is really a headscratcher. I can't think of why the RCA out would be the only one with the problem. I wouldn't have thought any additional circuitry should be involved. It's just another line out.

And why specifically the negative part of the waveform is distorted, but NOT the positive..? Strange. I'd still like to see another 3k do the same test.

You never know...

I'm stumped to get any logical answer to this as well. I've never used anything other that the phono's for recording so it's definately something that's turned up recently.

Looking at the PCB there seem to be a couple of surface mount transistors and capacitors in the circuit to the phono's but I can't see how a single-point component fault would affect both channels, and even then I don't see what sort of fault would give such an odd waveform. Hairline crack in PCB earth plane somewhere? I looked hard but couldn't see anything remotely suspicious. Oh well.

Well, look on the bright side... two out of three good outputs is still a majority!

BTW, you ever done any quiescent noise recordings from the main and headphone outs, see if maybe the headphone amplification circuitry is introducing a bit more noise than the main outs? I wouldn't be surprised if it did...