Help with shielding?

[sneakymarco]

Hi all. I found this forum through the guitarnutz site and I'm hoping some of you might be able to help me troubleshoot a problem I'm having. First, I want to say that I've been taking my guitars apart for about ten years, however I had absolutely no idea what the hell I was doing until very recently, and basically just worked by trial-and-error, and exclusively with electrical tape (I thought soldering irons were mystical tools reserved for people who'd gone to school for electrical engineering). So, I don't know too much about all this stuff yet.

I have followed the instructions for shielding/star grounding my strat, to the letter (I think), yet I can hear no noticeable difference in the amount of noise I get when I plug it in.

The type of noise I'm getting is mostly directional, increasing and decreasing as I turn around, with some small reduction when I touch the strings/bridge/jack. I've tested the same amp and cable with another guitar which is unshielded, although has metal pickup covers. The noise is present with my second guitar, but only barely noticable.

I am thinking that this may be a grounding issue. My multimeter tells me that all of the interior shielding is continuous, however I do not have a completed circuit between the shielding and my isolated star ground. I've determined that the cap which runs from the shaft of the tone pot to the star ground is breaking the circuit.

Again, my understanding of this is limited, but isn't the point of star-grounding (or grounding in general) supposed to be that all of the grounds are introduced to the cable's negative lead, then to the amp? That's how all of the interference is taken from the shielding and dispersed instead of being introduced in to the signal? So if the shielding/grounding itself is not connected to the (-) lead of the cable (as seems to be the case here), then no actual grounding or shielding is happening; correct? Please correct me if I am wrong.

Anyway, when I discovered that this was happening, I tried bypassing the cap with a wire (just shoved/lightly twisted in to place; not soldered), but that didn't seem to have any effect. I suppose that wire may have come loose when I plugged everything in to test it... I've done way too much soldering today and really didn't want to fire up the iron again right then.

If it matters, the cap is an Auricap .33uf, 450v metal film.



The other theory I have is that it might have to do with my exposed pickups. However, as there is basically no reduction in noise pre- and post-shielding, I don't think this is the entire problem... Even if my pickups were picking up (!) some noise, shouldn't the fact that I've shielded my control cavity have at least some positive effect?


Any help that anyone here can give me will be much appreciated! I started out just wanting to shield my crappy old Mexi strat, and somehow wound up spending hundreds of dollars on a new body, new pickups and hardware and electronics, and now I've got everything assembled and wired and shielded, after literally months of waiting and work, but it's still noisy as all hell.... I'm getting so frustrated I'm about ready to just put the thing in my closet and forget all about it.

[zamzara]

By pot shaft do you mean pot shell? If so it sounds like your pot shells are not grounded against the shielding properly. The pot shells should be permanently connected to the shielding. I'm not sure if you have installed the 400v capacitor in the right place either, but I might be misunderstanding the description.

[michaelcbell]

Why don't we welcome our new friend marco.... HI! Welcome to guitarnuts, where all your dreams come true... ok, just a few of them, but if they deal with guitar tone, this is your place.

[michaelcbell]

marco,

it seems you have a solid understanding of the concepts behind the shielding, so let's try to figure out what's going on. I'm not sure, but your multimeter may be reading no connection through the 450v cap when there is actually a connection, given that a cap blocks certain freqencies and your multimeter may be trying to read at that frequency. Your cap bypass test is a good one, but I'd definitely solder the wire in place to test, since that eliminates another possible problem area. Once that is done, with the guitar open, test to see if your shielding connects to your ground (at the jack). If yes, plug it in, if not, we need to look further (is there connection from the shield to the star ground, etc). Don't dispair, we'll figure this out.

-m

[sumgai]

michaelcbell said:

.......
I'm not sure, but your multimeter may be reading no connection through the 450v cap when there is actually a connection, given that a cap blocks certain freqencies and your multimeter may be trying to read at that frequency.

Whoa there, Sparky, time for Remedial Metering 101!

All capacitors exhibit one trait in common - they block DC. That said, what do you think a multimeter uses to read resistance? There sure ain't no little AC generator inside that tiny little case, bunky! It's a DC battery, and that's all they use to supply current/voltage for a resistance reading. After all, that's the surest way to find a true reading of resistance, shorn of any reactance or impedance that would get in the way, if AC were used. (And let's not get started on measuring the effective values if a non-sinusoidal waveform is used!)

A multimeter should read a capacitor as it is charging from the battery within the meter itself. Reversing the leads after that first reading, you should observe the numbers going in the opposite direction - the cap is discharging back through the meter's internal resistance (comprised partly of that same battery). Once the cap is fully charged, the reading should become infinite (OL, or open loop, on most meters). If it ever says 0 (zero) or nearly so for more than a second or two, it's shorted, and should be relegated to File-13.

Repeat after me: "No frequencies were harmed in the checking of this capacitor!" ;D

HTH


sumgai

p.s. Note: If your meter has a selection for measuring a capacitor's value, then it does have an internal AC generator - but it's still not used for resistance measurements.

[sneakymarco]

Thanks for the welcome and the advice.

I did a proper bypass of the cap just now and it seems to have taken care of most of the problem. I still get some directional interference but I'm assuming that's due to my uncovered pickups.

Also;

I have checked the continuity between the shield and the contact point on the cap and it's definitely fine, so I know the cap itself is connected and should be completing the circuit.

Is it a bad cap, or maybe the wrong value? any idea what's wrong? Is it a problem if I leave it out? I was under the impression that it was included in the circuit for life-saving reasons and I'm not sure whether or not it's ok to exclude.



As far as the shaft/shell thing, the instructions say to put a ring terminal around the shaft of one of the tone pots, between the itself and the shield and solder the cap to that; correct? That's what I did.

[sneakymarco]

In light of sumgai's post I have determined that my cap is nonfunctioning. $8 down the drain! Damn.


EDIT:

Also, just to confirm; should the ground wire from the bridge be connected to the star, or to the shielding itself? I was never quite sure on that point.

[setain]

You connect it to the shielding directly. Doing so has not benefit over connecting it to the star ground, except that you won't need to desolder it to take off the pick guard.

[sneakymarco]

All right, thanks. I have a rear-routed strat so that's not so much an issue for me I guess.