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Bad pot?


ScottR

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With the strat wiring, I'm currently working with, I was getting some minor buzzing that would go away when I touched the strings, or any grounded point really. I checked continuity on my jack and found I had it across the tip and the sleeve. When I took the jack out and de-soldered the leads, that was no longer the case. Bottom line is I have continuity from the center lug of the volume pot and the casing when the pot is turned to max volume. The center lug is the lead going to the tip of the jack, which is why I have continuity across the sleeve and tip of the jack.

I'm astonishingly ignorant about the wiring, but I assume that means my volume pot is bad? Or is it normal to have continuity between the center lug and one of the outside lugs?

SR

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With the pot full up or full down there will be continuity between the centre lug and one outer lug (the schematic symbol for a potentiometer helps visualise how the wiper, or centre lug, behaves when moved to the extremes).

One of the outer pot lugs will be connected to ground, as will the sleeve terminal on your output jack, and the pot case (if you've made a ground "blob" on the case itself). If the pot were wound all the way to zero when you made your measurement you will see continuity between the output jack sleeve and tip. It is unlikely that there is a dead short in the pot to the case with the volume full up, as any such fault (assuming the case is grounded and the rest of the circuit is OK) would result in total loss of signal output.

A true diagnosis of a pot failure is difficult to do with the rest of the circuit still in place - your best bet is to unsolder and remove it from the guitar and test it on the bench.

Edited by curtisa
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+1 on all of that. Hum and circuit weirdness can also come from the silliest of errors too. Missing ground continuity (from pot to pot for example) within unshielded cavities is one of them. Another is reversed hot and ground wires on the jack.

I would say, "ask me how I know" but that would be plain embarrassing. <_<

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Yeah, I concentrated on ground to ground and reversed wires on the jack to start with. I was surprised to see continuity between both leads on the jack and thought I was on to something. Thinking it through since last night has given me something that makes more sense. The buzz seems like it occurs with the switch on the neck pick up and somewhat less so with it in the neck/middle position. Anything I hear in the other positions are quieter and don't act the same and are likely normal single coil hum. So, I may very well have not done as good of job grounding the neck pup to the case of that pot. I'll dig into that tonight.

SR

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My thinking, isthat if it were a bad pot, ALL the pickups would be affected equally. since you claim the neck pup is worse, that is exactly where I would start searching. Make sure all the wires are intact, joints solid, etc.

its the "obvious" answer of course, I don't mean to step on your toes. Make sure the ground wire on the pickup is also firmly attached. I've accidently pulled a wire out of a single coil before, much to my immense anger and frustration lol.

Edited by bob123
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Slightly left-of-field possibility, assuming the circuit works exactly as intended and no grounding errors are found:

Is it possible that your bridge pickup is wound hotter than your neck pickup (to compensate for the volume difference between positioning the pickups at different points along the strings), and you've accidentally installed the neck pickup in the bridge position? The pickup with more turns will have a hotter output, but also pick up any stray noise more effectively, hence the neck pickup being slightly noiser than the others if they've been swapped.

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Don't worry about making obvious suggestions or stepping on my toes, this is the one area that I only know enough about to be dangerous.....and I'm pretty sure I only know half of what I think I know. :blink:

I agree with your conclusions about the pots, I know longer think they are a problem. They work exactly the way Curtisa described and just like the pots in my other guitars work. I tested my ground joints and they tested fine, but I went ahead and re-soldered the ground from the neck pup anyway. I used this wiring diagram:http://www.seymourduncan.com/support/wiring-diagrams/schematics.php?schematic=3s_1v_2t_5w

I bought this set of controls pre-wired and only had to connect the pups and the jack. While it is not a 100% given, I figure the odds that those connections are good are pretty high. Everything is wired just like that diagram......I think. I know the pots are but I assumed the switch is, since it does switch from pup to pup exactly as it should. I will trace the connections on it tonight. One thing that strikes me as odd is I do not see that the switch is grounded in the diagram and indeed there is no continuity from the switch frame to any pot cases or the jack. Everywhere else there is supposed to be continuity has continuity. The cavity is shielded with StewMac shielding paint.

Curtisa, the pups are marked clearly and the bridge pup's leads were just long enough to reach from the bridge position, and the middle had a yellow hot wire instead of white. I did not pull them out to double check the labeling, but it was pretty easy to tell from the cavity what was what when I did the connections.

Anyway so far nothing has changed. The buzz from the neck and neck/mid combo goes away when touching the strings or touching the screws holding in the switch-which I find odd since the switch is not grounded. The strings are grounded, I checked that with the switch in each of the 5 positions.

So.......what is the best way to start tracing this (the curse is still with this guitar)?

SR

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My thinking, isthat if it were a bad pot, ALL the pickups would be affected equally. since you claim the neck pup is worse, that is exactly where I would start searching. Make sure all the wires are intact, joints solid, etc.

its the "obvious" answer of course, I don't mean to step on your toes. Make sure the ground wire on the pickup is also firmly attached. I've accidently pulled a wire out of a single coil before, much to my immense anger and frustration lol.

Completely not true depending on how the circuit is configured lol. Sounds like it was your own fault lol, etc.

Is it possible just to pull all of the pickups? Building up an entire circuit and then fault-finding backwards is somewhat annoying because of everything being built up. Removing the pickups from the circuit (and from the instrument if possible) and then analysing everything in isolation helps immensely. This isn't always possible of course (and constantly adding and removing parts risks further accidents) however reducing the number of assumptions you make about what is and is not the cause is valuable.

Slightly left-of-field possibility, assuming the circuit works exactly as intended and no grounding errors are found:

Is it possible that your bridge pickup is wound hotter than your neck pickup (to compensate for the volume difference between positioning the pickups at different points along the strings), and you've accidentally installed the neck pickup in the bridge position? The pickup with more turns will have a hotter output, but also pick up any stray noise more effectively, hence the neck pickup being slightly noiser than the others if they've been swapped.

Nice thought and is worth consideration. There is usually less than 1k difference between the neck and bridge pickups in Strat sets which is certainly not enough to account for amounts of hum large enough after cancelling to be more noticeable than general single coil operation. As with any plausible idea it is worth checking if only to cross it off the list.

Left-of-field thinking usually turns up great ideas in the weirdest of places so you get some kind of prize for that.

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Is it possible just to pull all of the pickups? Building up an entire circuit and then fault-finding backwards is somewhat annoying because of everything being built up. Removing the pickups from the circuit (and from the instrument if possible) and then analysing everything in isolation helps immensely. This isn't always possible of course (and constantly adding and removing parts risks further accidents) however reducing the number of assumptions you make about what is and is not the cause is valuable.

It is of course, but for the reasons you list I'd prefer to leave that to a last resort. I am thinking that if I de-solder all of them and the jack and then using jumpers connect them one at a time, first straight to the jack, then singly to their original connection points, that I might learn something.

SR

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If you have a master volume, and your pickups are in parallel circuit (i.e. going into one source like a 5 way switch) then the master volume should have the same degradation to all switch settings. It cant pick and choose to work properly for one pickup position and not the other. If anything the switch itself or the pickup itself have to be the culprit, or some wonky grounding issue somewhere.

I do agree, i would probably pull everything and start over if i couldnt find it.

Scott, care to post a pic of the wiring?

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The volume switch works the same for each setting. The wiring is like the SD diagram I linked to, with the exception that the ground lead to the jack goes to the last tone pot casing instead of the volume pot casing. All the grounded components do have continuity between them.

I can post a pic of the actual control cavity this evening, if that is what you are asking.

I am curious as to why the 5 way isn't grounded.

SR

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Fender Oak Grigsby 5-Way Switch. But look at the diagram I linked to. Unless I'm totally missing something, it is not showing a gound to the switch, and mine certainly is not currently. In fact here is the pup and wiring set: http://www.kleinpickups.com/p-207-jazzy-cat-stratocaster-pickups.aspx

I did not leave them on the plate: "Prewired Stratocaster Control Plate. The plate is CNC Laser Cut out of Vulcanized Fiber (Pickup Flatwork) & is loaded with all the electronics needed to drop in to your guitar."

SR

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The ground tab on switches is only provided as a means of earthing the metal (non-switching) parts of the switch in situations where the mounting method is not electrically conductive. Us lucky guitarists are one of the few situations where wiring a dedicated earth to the switch can be necessary.

It's possible that the Seymour Duncan wiring diagram is assuming that there is a layer of conductive shielding tape under the pickguard that would be earthed via the pot cases. Fitting the switch to the scratchplate then earths the switch frame via the conductive tape and there is no need to provide a separate earth. Then again, the diagram may also be assuming that your switch simply has no earthing lug. Then again, there isn't an awful lot of metal to touch in a strat 5-way switch when it's installed, so the designer may be lazy and is not bothering to earth the switch frame. The joys of guitar wiring diagrams...

Does your guitar have any cavity shielding? If so, is the cavity shielding conductive to ground in all locations?

Extremely unlikely, but perhaps the switch itself is providing just enough additional shielding to the neck pickup wiring so that when you touch the unearthed switch the buzz is reduced via the conductivity of your body.

Another thing to look at is that there is a tone control hanging off the neck pickup. Poor soldering or faulty components here may account for excessive noise on that one pickup too.

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As Bob suggests, it's not necessarily the fix but it's an easy one to try out. You don't even need to solder it if you just want to quickly try it out, just twist some wire around the switch earth terminal and dab it to the nearest ground in the guitar and see if the buzz changes.

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IME the StewMac shielding paint isn't all that great when applied to timber, but every little bit helps. I typically see readings of 200-300 ohms when measuring between the sleeve of the output jack and any point on the control cavity where I've applied the paint - definitely not a dead short as it should be. More coats of paint help. Applying the paint to smoother surfaces helps. Not using the paint and utilising other forms of shielding such as copper or aluminium foil tape helps ;)

Even so, the fact that the buzz is affecting only one pickup out of three suggests that the overall shielding is probably not the issue. Your suggestion of systematically breaking down the circuit is a good one, even if it is labour intensive.

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Grounding the switch helped a little. Lowering the pups helped a lot, but did not eliminate it completely. It did help to see that it was not just the neck pup.....just loudest on the neck because I somehow had it a little closer to the strings. Grounding the switch also made touching a switch mounting screw ineffective at stopping the buzz. But touching strings still makes it go away. It acts like the strings are not grounded but they certainly are. It is low enough now with the pups lowered that I would almost think it was normal 60 cycle hum, except that putting the switch in the two pup positions does not make it go away like it would if it was 60 cycle hum. Also turning down the volume or the tone pots also eliminate it.

Huh.

SR

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Wow. That's a curly one. I've never heard of lowering pickups to reduce buzz?? That implies that the proximity of the pickups to something is the problem (the strings?).

OK, here's something different to try - take a piece of wire and attach it to your output jack sleeve (ie, ground), use the other end of the wire to dab on to the magnetic pole pieces of the neck pickup, and check if the noise changes when the poles are grounded via the wire.

Edit: just in case I'm misunderstanding you - all guitars will buzz slightly until you touch an earthed part of them (eg, strings), it's just more apparent on single coils.

Edited by curtisa
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That had no effect whatsoever. I got a tiny crackle when the wire touched the mounting screws on the neck and middle pup as they were selected, but not on the bridge pup when it was selected. The sound did go away comepletely when the wire accidently hit both leads on the jack....but so did the pup.

SR

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I touched the wire to the poles and it made no sound at all, may as well have been touching a chunk of wood. Nor did it have any effect on the buzz. I accidently touched it to the mounting screws and got a tiny crackle. I'm starting to wonder if it might be the switch. You can hear the position changes through the amp as you move the switch particularly at each end of the throw.

On the other hand, I fought that problem with my last guitar which has humbuckers. I'm convinced that my house wiring is the culprit in that case, and running the amp through a power conditioner took care of that.....and it is not taking care of it in this set-up. These are single coils though........but so are my P-90s, and they do not have this buzz.

Maybe I'll learn something this weekend by isolating the pups from everything else.

What if I had the leads from the pups reversed? I doubt that is the issue since the bridge pup does not really seem to be affected.

SR

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