ST-1 Pick Up Shielding

[Rabbitz]

I live, just about, under an electrical robot, so 50Hz hum is all over the place.

(If you think it is bad for guitars, I also tinker with radio...)

Anyhoo, my ST-1 is struggling to cope.

It seems to me the issue is the coils picking up the hum - if I position the guitar in free space so the hum is maximised, then turn it so the pick ups are at 90° to the original the hum dissipates. I think the coils are actually acting as a series of parallel arrays.

Cavities, bridge etc etc are shielded and continuous.

Thus, my question is regarding the shielding on the pick ups themselves. Are they shielded? It all looks pretty plasticky to me, but without breaking one open it is hard to tell.

Anyone have any idea if they are?

[wokkaboy]

hey Col, there has been photos on the forum of the cover taken off some PBG pups but I'm not 100% sure. I would guess the pups aren't shielded. Weirdbits has pulled a few PBG pups apart so he can probably confirm your question

[Andy40]

good question. If Weirdy doesn't know, i've got my old ST-1 harness still sitting there and could sacrifice one of those pups? at this stage I'm never going to use them.

[Rabbitz]

Hi Woks and Andy,

I wouldn't sacrifice a set of pick ups just yet.

I might try a couple of other experiments which might overcome the issue.

Basically I will try and shield the scratch plate right up to the edge of the pick up cutouts - I am hoping the magnetic field of the pick up will be strong enough to block where the pick ups sit.

[stan]

no, i doubt the pups are shielded... if the ones on my Squier are not, i doubt, as good as they may be, the Chinese generic pups are...

[WeirdBits]

The stock PBG ST style pickups use a ceramic bar magnet on the bottom rather than individually magnetised polepieces, a plastic cover, a bit of wax, a bit of tape, and that's about it:
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The bar magnet could contribute to the 90 edge rotation hum cancellation. I suspect separate individual magnetised polepieces could help but single coils will always have issues in noisy environments. Do the 2 and 4 positions cancel most of the hum? What happens if you sit a couple of layers of aluminium foil on the strings above the pickups, does that silence it?

Shielding is always the first step, and not just continuity of shielding but also thickness. Sometimes thicker shielding can be required to stop all the noise, like the thick under pickguard shield plates you can get. Also, whenever possible, try to have only one path to ground. e.g. if your shielding is grounding the pots then don't link the back of the pots together, instead just have a single ground link to one pot and have everything else link to that point etc.

[wokkaboy]

nice insight there Weirdy, hopefully your suggestions will help lower the hum

[Rabbitz]

Hi Scott,

Positions 2 and 4 do cancel some of the hum. This one of the reasons I suspect the pickups, as I guessed that the two pickups were approaching 180° phase cancelling. Although I am not sure how the real sound gets past this.

:)

I plugged the guitar into a small desk and fed the USB output to a Oscilloscope (PC Based). The wave forms surprised me.

While there was a fair amount of the 50-60 Hz I expected there was a weird but very regular modulation of the 50 Hz wave. When I angled the scratch plate to 90° it became a modulated almost saw tooth 50 Hz.

This led me to suspect a second (or more) noise source. One culprit was discovered - a noisy CFL globe.

Well the search continues, I need to get some copper tape to shield around the pick ups.

As the scratch plate is off the guitar I might wrap the lot in foil and ground that to see what happens. I'll put a sheet of flexible foam on the underside first, shorting it all to earth will quiet it down OK but will defeat the point of the experiment!

This is really an academic excercise, to learn a bit more about the vagueries of this shielding business...