Tone Control Not Working

[mingus]

I have just wired up my first project according to the following diagram and have found that the tone control doesn't work at all.

I have checked my wiring agaoinst the diagram and can't see any fault so i guess there is a short somwhere, but i have no idea how to find it. I have had some experience with electronics and know how to use a multimeter, but having not worked on guitars before i don't really know where to expect continuity. For example, there is continuity between the righthand lug on the tone pot (with the cap on it) and ground, which varies depending on the switch position and volume pot position. I guess this is to do with the changing resistance across the pickups as the switch is moved but have no idea if this is what is supposed to happen.

Also, my volume pot has a large step change in level just near the high end of it's travel, that is it gets gradually louder as i turn it, then suddenly gets much louder just before the end. Is that normal?

[digi2t]

What type of pots are you using? 250 KOhm? 500 KOhm? Linear? Audio? What size/type capacitor are you using? Need more info. According to the plan, the tone should work no matter what position your switch is in because the pot is right at the end of the chain before the jack. Are any of the lugs on the tone pot grounded DIRECTLY (like one lug on the volume)? If so, that will negate your tone pot. Lastly (and most simply), is the pot good? Is the cap good? Disconnect everything and test it by themselves.

Cheers

[curtisa]

The sudden step change in level on the volume pot sounds like you've installed a linear taper pot in place of what should be a logarithmic taper (or audio taper) pot.

The tone pot not working at all sounds more like an open circuit, not a short. The continuity you're measuring from the RH lug on the tone pot to ground sounds sorta OK, but I'd say you're measuring in parallel with everything else in the circuit, hence the changing readings as you move the controls around.

I'd say the key continuity points to check on the tone circuit would be the input side of the tone pot (ie LH lug on the volume pot to the middle lug on the tone), RH lug to cap, other side of cap to ground - all should read zero ohms.

Also would be worth verifying if the tone pot is any good. You'll have to disconnect it from the circuit to check it though. Once you've got it out measure with your ohm meter the outer two lugs to make sure the pot's value is what it says it is, and also between the middle lug and either outer lug while twisting the shaft to make sure you can see its resistance change.

The cap size is important too - it's not mentioned in your diagram what value it is. If you've installed something too small of value it won't work. Or at least technically it'll be working, but the frequencies it affects will be so far up the audio band its effects will be inaudible

[mingus]

The sudden step change in level on the volume pot sounds like you've installed a linear taper pot in place of what should be a logarithmic taper (or audio taper) pot.

The tone pot not working at all sounds more like an open circuit, not a short. The continuity you're measuring from the RH lug on the tone pot to ground sounds sorta OK, but I'd say you're measuring in parallel with everything else in the circuit, hence the changing readings as you move the controls around.

I'd say the key continuity points to check on the tone circuit would be the input side of the tone pot (ie LH lug on the volume pot to the middle lug on the tone), RH lug to cap, other side of cap to ground - all should read zero ohms.

Also would be worth verifying if the tone pot is any good. You'll have to disconnect it from the circuit to check it though. Once you've got it out measure with your ohm meter the outer two lugs to make sure the pot's value is what it says it is, and also between the middle lug and either outer lug while twisting the shaft to make sure you can see its resistance change.

The cap size is important too - it's not mentioned in your diagram what value it is. If you've installed something too small of value it won't work. Or at least technically it'll be working, but the frequencies it affects will be so far up the audio band its effects will be inaudible

I should have mentioned that they're definately audio taper pots (the long shaft version from stu mac). I tested them as you describe before installing them so i know they are working. The step change in the vol pot is really sudden, i only have to touch the knob to make it jump up in volume, then i keeps increasing at the same rate as before over the last bit of the pot

I have just tested the continuity of the tone circuit and everything reads zero as you say (well 0.03ohms anyway, a assume this is close enough?). The cap is a 0.047micro-f, marked with 47 on the side which i assume means 47pf. I guess they could have been in the wrong drawer at the shop so i ended up with the wrong size, is there any way to test the capacitance?

[digi2t]

Insofar as the volume pot is concerned my research is telling me that you may have installed a 1MOhm (or higher) pot. Check out this link;

http://www.wdmusic.com/selecting_pots.htm

and read the section "The Effect Of Potentiometer Values In Relation To Output And Tone:" Pay particular attention to what's written at the end of the first paragraph of this section. The higher the resistance to ground, the brighter (fuller) the sound. But on the flip side, this will also make for a much narrower taper at some point. Check out this link;

http://www.guitarelectronics.com/category/...asicwiringfaqs/

and read the part "How to check the taper with an ohm meter:". This might sort out the problem with you volume.

Did you do the second test on the tone pot as per Curtisa recommendation? You didn't mention if you did.

As for testing caps, the only method I know of requires an analog (dial) type multimeter. It won't tell you the capacity of the cap, but it will let you know if the cap is short or good. Link is;

http://www.applianceaid.com/micro4.html

Even has a groovy little graphic that shows you what your meter should do when you test.

Hope this helps,

Cheers

[curtisa]

I should have mentioned that they're definately audio taper pots (the long shaft version from stu mac). I tested them as you describe before installing them so i know they are working. The step change in the vol pot is really sudden, i only have to touch the knob to make it jump up in volume, then i keeps increasing at the same rate as before over the last bit of the pot

Not really familiar with Stewmac's parts system. Do they use a long shaft to identify the pot as being log taper? Could they have accidentally supplied you with the wrong taper? The pot should be stamped (or printed) with its value in ohms (or K ohms) plus a single letter code describing it's taper, eg "250KA". The last letter is the taper type - "A" is log, "B" is linear, "C" is antilog. The linear and antilog types could give the behaviour you describe, particularly antilog.

Digi2t has good info there too.

I have just tested the continuity of the tone circuit and everything reads zero as you say (well 0.03ohms anyway, a assume this is close enough?).

Yep Near enough is close enough

The cap is a 0.047micro-f, marked with 47 on the side which i assume means 47pf. I guess they could have been in the wrong drawer at the shop so i ended up with the wrong size, is there any way to test the capacitance?

Not easily, unless your multimeter has a capacitance function. Although if the cap has got just "47" written on the side it's more than likely just 47pF as you say, which is far too small to be of any use to you. You definitely want something that says "0.047" or "0.047u" or "47n" or "473" - they're all the same value, just shown in different numbering formats.

Edited December 19, 2007 by curtisa

[mingus]

Not easily, unless your multimeter has a capacitance function. Although if the cap has got just "47" written on the side it's more than likely just 47pF as you say, which is far too small to be of any use to you. You definitely want something that says "0.047" or "0.047u" or "47n" or "473" - they're all the same value, just shown in different numbering formats.

Well, now i feel stupid. Hands up who can remember the names for the various orders of magnitude (definately NOT me). You would think i'd know this stuff after 5 years of engineering at uni Oh well, at least now it should work, i'll get some a new cap tomorrow and see.

So that just leaves the Volume pot. It is a 500k log taper pot (part 3482 on this page) and is marked 500xL on the side which doesn't match with curtisa's scheme. Doesn't matter though as i tested that is was a log taper before installing it (by centereing the knob and checking that the resistances betwen the center and each outside lug was different). Besides, if it was a linear pot, there would be a large increase in volume at the start with less change at the end, whreas this sudden change is happening at the end, something like this.

OK, i think i've answered all the important questions, will chek back later.

[curtisa]

Ack! 10 bucks for a pot?!!

I digress...

As an example this pdf is from Alps' online catalog for one of their pot styles. If you scroll down to page 6 it shows the various types of taper you can order from their catalog.

by centereing the knob and checking that the resistances betwen the center and each outside lug was different

How much different? The difference at 50% rotation will change depending type of pot you have. On the graphs in that pdf, "B" taper is 50% resistance from either lug to middle at 50% rotation (as you would expect for a linear pot). "A" taper is about 25% from middle-right lugs, and 75% from middle-left lugs. "C" taper is similar at 50% rotation but the the way it changes in the first half-turn is completely different.

Can you sub in a cheap Radio Shack 500K log pot and see if it works? That way at least you can eliminate the pot as the culprit.

[mingus]

Ack! 10 bucks for a pot?!!

I digress...

As an example this pdf is from Alps' online catalog for one of their pot styles. If you scroll down to page 6 it shows the various types of taper you can order from their catalog.

by centereing the knob and checking that the resistances betwen the center and each outside lug was different

How much different? The difference at 50% rotation will change depending type of pot you have. On the graphs in that pdf, "B" taper is 50% resistance from either lug to middle at 50% rotation (as you would expect for a linear pot). "A" taper is about 25% from middle-right lugs, and 75% from middle-left lugs. "C" taper is similar at 50% rotation but the the way it changes in the first half-turn is completely different.

Can you sub in a cheap Radio Shack 500K log pot and see if it works? That way at least you can eliminate the pot as the culprit.

Well, i tought i replied to this a couple of days ago, but my net connection has been crappy so i guess it got lost somehow. Anyway, i am now very suspicious of the pots (both Vol and Tone) as i have switched the Cap to a 47nf one and can hear the same evvect on the tone as on the volume, where there is very little change for most of the rotation, then a sudden change right near the end. The thing is, the sudden change occurs at the opposite end of rotation for the tone pot as compared with the volume. i.e., i'm turning the volume pot clockwise and when it's nearly at the end it changes suddenly, but i'm turning the tone pot anticlockwise when the same thing happens.

A couple of things come to mind:

- The pots are definitely 500k audio taper (480k ohms measured between left and right lugs, 120k between middle and right, 460k between middle and left at 50% rotation)

- I changed the cap for an even larger one (220nf) to see if it made any difference, and it didn't. the sudden change still occured in the same place and virtually no change around the rest of the rotation. (i have now replaced the 47nf in the circuit)

- I bought these about 2 years ago (when i first started this project.....it's been a VERY long process), so they have been exposed to dust etc. for that time, altough i have tried to keep them in a clean environment

- I will definitely take at least one of the pots out and test is seperately to se if i can measure a corresponding change in resistance, though i may not get around to it till after christmas

- I can't sub in a cheap radio shack pot as i don't have access to radio shack (i'm in australia) and the local versions (dick smith and jaycar) leave something to be desired

- It seems $10 for a pot is quite high, obviously it's higher than normal but i couldn't find long shaft pots ( which i need as it's a carve top) much cheaper. They are $8 at guitarelectronics.com but their postage prices are much higher. Is there anywhere else they're available cheaper if i do need to replace them (or for future builds)?

Thanks for you help so far everone.

[curtisa]

- I can't sub in a cheap radio shack pot as i don't have access to radio shack (i'm in australia) and the local versions (dick smith and jaycar) leave something to be desired

Jaycar or Dick Smith will do. I was only talking about getting another cheap one to test with, not to permanently install.

- It seems $10 for a pot is quite high, obviously it's higher than normal but i couldn't find long shaft pots ( which i need as it's a carve top) much cheaper. They are $8 at guitarelectronics.com but their postage prices are much higher. Is there anywhere else they're available cheaper if i do need to replace them (or for future builds)?

Locally? Try Pro Audio in Canberra or Venue Music in Sydney. Pro Audio's online catalog isn't much, but their physical inventory is massive. They've got a freecall number, so drop them a line. I believe Venue Music have a 10 day, no questions asked, money-back guarantee on any sale. So if you go that way, at least you can return it.

[mingus]

Jaycar or Dick Smith will do. I was only talking about getting another cheap one to test with, not to permanently install.

Yeah, but they don't seem to carry much stock!

Locally? Try Pro Audio in Canberra or Venue Music in Sydney. Pro Audio's online catalog isn't much, but their physical inventory is massive. They've got a freecall number, so drop them a line. I believe Venue Music have a 10 day, no questions asked, money-back guarantee on any sale. So if you go that way, at least you can return it.

Thanks, i'll check those out.