DIY cables: how to test them (Sommer tweed content)

Yorkie · May 2023

Dear folks,

A few days ago I built a very nice treble booster that apparently sounded a bit too quiet (reference here: https://www.thefretboard.co.uk/discussion/246132/treble-booster-vs-boost-with-3-knob-eq/p1). After testing it with some commercial cables, I came to the conclusion that my nice looking tweedy DIY cable was sucking BOTH tone AND volume. It's not a very long cable (10ft tops) and it's got Neutrik plugs on both ends. I hadn't noticed this issue with the Bandit, but now that I'm spending most of my time playing through valve amps it's become pretty obvious – the DIY cable needs more volume from the guitar to get the amp to edge of breakup nirvana. So there must be something wrong at either or both ends of the cable.

My main worry now, and it's becoming a bit of a panic considering my fucking obsession with detail and inability to 'let go', is that I don't know if the myriad DIY cables connecting my pedalboard are OK.

Is there a way to use a multimeter to 'measure tone suck'? I've also got an audio interface with jack inputs – I might be able to do something with it, but it would be tedious to test every little cable (got many).

Thanks!
Jon

springhead · May 2023

Measure resistance end to end on the tip and the sleeve. Should be close to zero. Measure resistance between tip and sleeve at one end. Should be infinity/open/many meg ohms.

Don’t hold the tips of the probes whilst doing this. Or at least not both of them or you are adding yourself into the equation.

ICBM · May 2023

That's the usual symptom of not cutting back the black semi-conducting layer on the outside of the central core insulation, which is usually clear or white. You need to cut or scrape it back so it can't contact the solder joint.

If often doesn't show up well on a multimeter for some reason - but it's normally much more noticeable than just a little bit of tone and volume suck, so the simplest way is just to compare each cable with a known good one, connecting the guitar to the amp - or if too short for that, a bypassed true-bypass pedal to the amp. If it sounds the same, there's no problem.

springhead · May 2023

When I’ve had that problem, as caused by the semiconducting sleeve, as well as treble and volume drop it’s been measurable, in the 10’s of K Ohms. Annoying when you’ve done a particularly neat job!

Yorkie · May 2023

ICBM said:

That's the usual symptom of not cutting back the black semi-conducting layer on the outside of the central core insulation, which is usually clear or white. You need to cut or scrape it back so it can't contact the solder joint.

SEMI-CONDUCTING!?!?!?!?!

Image: https://images.squarespace-cdn.com/content/v1/5a4291f68a02c77d75e61974/1563909051088-Z42EYC4EM1T7LPS7A5TO/doc+brown.gif

Every day is a school day.

Yorkie · May 2023

springhead said:

When I’ve had that problem, as caused by the semiconducting sleeve, as well as treble and volume drop it’s been measurable, in the 10’s of K Ohms. Annoying when you’ve done a particularly neat job!

I’ve finally had a minute alone with the cables. A good cable had a tip to sleeve resistance of ~2M — beyond the multimeter’s abilities — whereas one of the botched jobs had 3k. Good news!

Thanks everyone.

Yorkie · May 2023

Need to repair 3 cables out of 15. Not too bad!

Yorkie · May 2023

It’s a quick job with a scalpel, no need to re-solder anything. Got one from Hobbycraft — the pointy tip that came with it slices through the black stuff like butter.

Jon

Yorkie · May 2023

Ok, so if you’ve come from a Google search and you’re still wondering what on earth is the black semi-conducting jacket, here is a short explainer.

See how the black stuff in the middle is touching the inner core:

If you measure resistance between tip and sleeve on either end, it will look something like this:

So, to get rid of the contact between the black jacket and the core, I have used a good old scalpel. This revealed the transparent insulant:

And as soon as you peel it back, the reading goes through the roof as expected:

Hope that was useful!

Jon

Danny1969 · May 2023

Nice work, that doubled shielded cable catches a lot of people out. A lot of mic cable also uses a semiconducting sheaf as well and needs to be peeled back a bit.

One thing I always do is remove the semiconductor layer about 8mm from the end, strip it, tin the conductor and then put a bit of heatshrink over it.
What can happen is too much heat can squash and stretch the insulation on the conductor and it's possible then to get a capacitive fault that won't register on you meter as that uses DC to measure but will degrade an AC audio signal.

Yorkie · May 2023

Danny1969 said:

Nice work, that doubled shielded cable catches a lot of people out. A lot of mic cable also uses a semiconducting sheaf as well and needs to be peeled back a bit.

One thing I always do is remove the semiconductor layer about 8mm from the end, strip it, tin the conductor and then put a bit of heatshrink over it.
What can happen is too much heat can squash and stretch the insulation on the conductor and it's possible then to get a capacitive fault that won't register on you meter as that uses DC to measure but will degrade an AC audio signal.

Excellent post. And very useful, because it has helped me diagnose an issue I was having. I’ll try to explain:

I recently swapped my Tu-3 (buffered) for a Korg Pitchblack poly (true bypass with a relay operated switch). I immediately started noticing popping and other strange noises whenever I used the tuner (which mutes the output). Plus popping noises every time I switched between pickups. Sounded just like a capacitor discharging

I have spent a few minutes removing pedals and cables, and there it was: the cable right after the tuner. Subbed it for another one, and now I get basically the same sound going direct vs all pedals on bypass. And the noises are gone.

I will have to fix that cable, but I guess this time the scalpel won’t do

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DIY cables: how to test them (Sommer tweed content)

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