How to clean and service pots on guitar amplifiers

[Music] hi and welcome to another video from berkshire guitar amplifier repairs i'm stuart smith and today we're going to be looking at potentiometers that constant source of noise and irritation with guitar amplifiers you've probably experienced it yourself dead spots on the pot a scratchiness as you turn it so we're going to be looking today about how you clean a pot properly and also how you free it up if it's become completely jammed but before that we're going to have a look at a pot and take it apart so that you can see inside and see exactly why they can get so troublesome well let's now just have a quick look at the circuit symbol for a potentiometer in old-fashioned circuits you'll see it drawn a little bit like this and more in in more modern circuits it's a straight rectangle like that these are fixed resistors and they'd have a certain value this might have a value of let's say 10k and let's say this might have a value of say 47k doesn't really matter the symbol for a potentiometer is made by simply adding a little arrow to the side here and what that's meant to symbolize is the wiper of the pot moving up and down the resistance so we have our three terminals on our potentiometer one two three one two three one two three the middle one is the wiper middle one is the wiper here and here i think you can see that um if this was moved all the way down to here let's do that with dotted lines then the resistance between here and here is is very low and in fact zero if it's right at the one end of its travel whereas if this was moved all the way up here let's do it on this one to here then the resistance between here and here is the full 47k and that's quite simply how a pot works well why don't we go ahead and take one apart and have a peek inside here's a pot i'm sure you've seen it but that basically it has the three terminals that we mentioned the middle one is the wiper and as we turn the pot rotate it around this wiper rotates around a resistor which is actually in a horse shoe shape to pick up the different resistances i expect you've never had a peek inside one of these so although this one is brand new why don't we just go ahead and take it apart and have a look inside i think it gives you a much better idea of why they can become troublesome and noisy to do it to do that it's not actually too difficult it's held the back of the pot is held on by these four bent tabs here and i just get a pair of cutters and get in there and just bend them up straight like that done this sometimes if i've had the right resistance value but the wrong shaft and i can swap the back out onto a onto another shaft if i've been desperate for a pot value that i haven't had in the workshop so there we go all of those are now bent vertical and the pot just comes loose from its housing this is just a tin tin housing on the back so now we can turn it around and have a look and as we turn the pot you can see it rotating quite nicely now i wonder if you can see um let me get here let me get this pencil to point with um i wonder if you can see in there that there's a kind of a wiper it's a little bent piece of metal which rests against this black track that's printed around on the on the underside of this um this mica so what they've done there is actually uh screen printed a carbon track onto the back of the uh back of the pot before we go any further let's just have a quick look at um what's going on inside the pot so if um this is the pot body here and here are our terminals one two three on the underside here they've printed a carbon track using special ink which has um a certain amount of resistance per per centimeter they obviously chews the ink depending on the value of the pot that they want and so they print this thick carbon track on the inside and they connect one side of it to this terminal here and connect the other side of it to this terminal here and in the center they have a kind of a wiper arm which is a cheap little bit of bent metal which wipes around the carbon track as you turn the pot and of course the center is connected to the the center pin there and uh that's quite simply how it works if this wiper arm is very close to here of course you have a very low resistance between the center and here and if the wiper arm is all the way around here to there you have a high resistance between this center all the track and that tag there simple cheap and effective and the remedy is quite simply to spray some cleaner in but i'm going to talk to you about which cleaner to use in a moment the symptoms of a bad pot are usually dead spots so you'll be turning up the volume and it'll be okay for a certain length of time and then it'll be a dead spot there and then it'll come and go a bit then it's all right there suddenly it's gone again there you know the sort of thing and that's just dirt on the tracks but i want to make a very important point about dirty pots because there's a another problem with pots you can get which is nothing to do with the with dirt on the tracks so if you get and i'm going to try to imitate it if you get this kind of symptom with a pot let me see if i can imitate it that kind of noise very even noise as you rotate no dead spots just that constant noisiness that's almost certainly not a dirty pot and what that is is some dc some dc voltage is getting onto the wiper of the pot i'll just briefly explain that because that might be of interest to you so okay so here is our pot and what you will normally find is this is being fed from something for example um the output of a valve which has quite a high dc voltage on it so let's just say somewhere up here we have 100 volts dc we don't want 100 volts onto our potentiometer so they will normally put a capacitor in the way there and feed the signal in here with 100 volts dc but of course we're only interested in the ac component there the capacitor simply blocks the dc from this side of the circuit on older valve amplifiers these capacitors can get leaky and a certain amount of dc leaks through the capacitor to end up on the pot it doesn't have to be very much it can only be well even 10 volts would would be a lot one or two volts you would probably hear and um because this wiper here is just a simple bent piece of metal like this going on to the carbon track as you turn the pot this piece of metal bumps up and down on the carbon track of course of course we're talking on a microscopic level here and this makes and breaks the dc to a certain extent and that ends up as that kind of noise you hear so if you turn your pot and you get an even noise as you rotate it it's almost certainly a leaky capacitor feeding it and the remedy of course is simply to change the capacitor well i'm sure you've guessed by now that the way to clean a pot is to spray some kind of solvent cleaner in in into the insides of it i hope to get rid of some of that gunk but what cleaner should you use well i want to tell you first of all don't use wd-40 you need to use a contact cleaning solvent of some sort and there are quite a few on the market the one i recommend is is this uh cake i believe it's made by caig uh deoxit and this is uh d5 delta five i use this all the time for cleaning contacts but i want to have if you'll forgive me a moan about this product and if anyone's watching who who who creates this product please take this on board it's to do with it with the delivery system i've got some wd-40 here just to show you the the uh the spout and the and the delivery mechanism which you'll recognize from having used the sort of thing yourself this is a very useful flexible delivery system you can't get anything out of one of these spray cans um if you hold it like this i mean you just won't get anything and if you hold it upside down you'll get nothing it has to be pretty much depending on how full your can is has to be pretty much vertical well often times you can't hold the can vertically you need to get into a difficult awkward corner this delivery system allows you to do things like bending down bending down the spout you see i can get that all with 90 degrees and that would still deliver or i can i can bend it forwards and backwards and that gets you out of a lot of problems when you're trying to get this this nozzle end into a potentiometer they can be very fiddly to get to sometimes so this is fantastic so what did they do deoxide always came with this kind of top so what did they do they came up with this travesty first of all 10 to 20 times the plastic thick polypropylene plastic to not biodegradable will be around a thousand years they've got this delivery system here which has got twice at least twice the diameter of an ordinary tube which means you deliver way too much product which wastes product and it's not very cheap this either so waste product floods the thing with product which which you just don't want to do secondly you cannot bend this spout it's rigid and it's got this nice fold away mechanism which isn't all that's cute however if you fold this down even a tiny little bit that's cut off the supply you can't get nothing works if you press this so you can't bend that down and say okay i'd left against that that that awkward corner now know that the uh the flow's completely cut off in that point so so um this is this is very very poor they've ruined the product in my view and i'm looking around for another product to use by the way wd-40 have done exactly the same thing so you can't now buy wd-40 at least in the uk with this nice flexible sprout they come with this great big chunk of plastic on top and and the same kind of problem complete madness anyway that's my moan over it's it's um far from ideal but i have put a piece of um heat shrink sleeving over the end of one of these and i've got some fish tank uh clear plastic tube which we'll go over over the end of there and now i can bend this around and i've got a long length as well a longer length which i can put on and i can guide this into into the pots it's very far from satisfactory um just because the diameter of this is very large but it's got me out of a few holes this absolutely drives me mad okay well if i find a better one or if you know the better one then uh then let me know so let's now have a look at how to um how to clean a pot and also how to free up a jammed pot here's our pot and um one of the things that you know you'll notice immediately is how open to the elements it is if we tip it up like this this is completely open in here and that's good news for us because that's where we're going to spray our uh cleaner so we take our deoxit and we point it at the gap and we give it one very quick squirt just like that and then then we uh work the the pot after it from this side we work the pot back and forwards to work the cleaner in and you'll find that that completely solves your dead spot problem now before i go on to show you how to get to some awkward parts let me talk about how to free up a stock shaft i often get this you go to turn the knob and it's really stiff needs a lot of force to do it now for that i use a bit of wd-14 you can you can do this from top side so you only need to take the knobs off if you don't want to take the chassis out of the amplifier and i take a little bit of wd-40 and i just spray it on the where the shaft goes in to the uh to the body there just spray it around there you might not think that would actually do very much um but actually it does and now you can work the shaft backwards and forwards backwards and forwards like this and it takes some time by the way i usually put a knob on when i do this because it can get quite hard on the fingers turning this backwards forwards but with a knob it's much easier and it can take usually sometimes a couple of minutes before that wd-40 will work itself down the shaft but eventually you'll feel it comes a little bit freer and a little bit free and a little bit freer and after a couple of minutes you'll find that frees up quite nicely that won't do anything to cure dead spots of course to do that you need to spray into the pot body um here's another little tip sometimes if you can't get to this this underside bit you'll see one of these little notch outs here you can see that quite clearly on that one and again you can that that's an entry into the pot body again you can get the nozzle into that notch and spray in there and the product will get inside there as well that's about it except to say sometimes the pots are very difficult to get to let me show you an example i've got a board i've taken out of an amplifier here um i'm just yeah there it is there and i want you to imagine that this board is actually in the chassis and so you open it up and you see uh you see your pots that need cleaning let's just take this pot here and you will notice you can't get to the to the underside of it the underside of it is it's kind of there and you can't get the the spray in there so when you open up it looks like that to you and there's no way unless you can find one of those handy notches which this one doesn't have you can't get the product in i've had amplifiers where it's been exceptionally difficult to clean the pots i've only got a couple of tips for you on that one is if you remember what we did with the wd-40 we sprayed it onto the shaft and um and allowed it to to to run down to run down the shaft you can sometimes do that with deoxit too in other words you soak the shaft with the oxide and turn it and some some of the product works its way down and uh and gets inside it's a little bit hit and miss that now the other thing i've done many times is i turn so imagine now this is in the chassis it's not not a bare board like this i turn the whole chassis upside down and you will see that i can just about and point inside there i think that's where we want to get the product and with the with the product with these with the chassis on the bench like this i can just about get i'm kneeling down in front of it and just about get this in there spray and allow it to run down and that works as well but it can be devil's own job sometimes just to get the product in there well i hope you found that useful little um tutorial on pots how to clean them how they work what they are and how to be aware of that dc problem that you can sometimes get on a pot i hope it'll save you a bit of money taking your amp to a service technician just to have the pots done i'm going to do another similar video on how to clean jack sockets as well because that's another area which um you know just just requires maintenance from time to time well that's it from me i'll catch you on the next video you

Channel: Stuart ukguitarampguy

Views: 49,945

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Id: MR3m2vPH2ys

Length: 19min 55sec (1195 seconds)

Published: Fri Jan 15 2021