100K pot reads lower, is it dying?

[TheFantod]

The Big Muff I'm working on seems to have a bit of a volume drop. I removed the volume pot and measured with my multimeter. I measured both the ends and the wiper, and got the same reading, 85.5K.

Does this mean the pot is weak, and should be replaced?

[John Lyons]

Pots are some of the worst toleranced parts we use.
Usually 20% tolerance carbon composition.
So for a 100K pot you could get 80K up to 110K
roughly. I don't think the 15K will make much difference
in the Big Muff output.
I like a linear pot in the volume position in a BM.

The emitter resistor At Q4 is usually 2k7 or so.
Try a 2k2 there and see if that helps.

[TheFantod]

Many thanks for your reply!

Previous owner had, for some mysterious reason, replaced some caps with those cheap ass Radio Shack ceramic dealies. I replaced them with metal film, and put in a 2.2 resistor at R23. The volume is MUCH better now, with no drop.

Thanks again!

BTW, what is the deal with those Russian resistors? They aren't banded, but colored instead? Those wacky Rooskies!

[azrael]

There's things written on them, usually.

Those are awful when it comes to tolerance, too! I've modded a few Russians, found parts ranging from 30-10% off the written value.

[Mark Hammer]

Pot wipers scrape against the surface of the resistive strip.  Depending on the quality of the wiper mechanism, they can provide various degrees of "scrapage".  One of the sources of pot noise over time is the buildup of stuff that can be scraped off the surface of the resistive strip.

So here's what I wonder.  If you scrape the resistive strip a little bit each day, does the value of the pot get higher or lower over time?  My money is on higher, but I'm open to other better informed views.

[edvard]

I'm betting lower.
More scrapes = less resistive material = less resistance.

I've squirted WD-40 inside a pot to clean it up before.
Sometimes it works well, sometimes not so much.

[newfish]

WD-40 is not the best things for pot.

Gives it a funny flavour, y'know?

In all seriousness, WD-40 leaves behind a lubricating film, which, as far as I've tried, isn;t always the best thing for controlled conductivity.

Switch cleaner does exactly what ot says on the tin, but will not do much for rusted bolts.

Horses for courses.

WD-40 if you must, but proper cleaner if you can.

[Mark Hammer]

I'm betting lower.
More scrapes = less resistive material = less resistance.

I'm betting higher because that is not resistive material in there, but rather conductive material.  And theoretically, the less of it you have, the higher the resistance should be.  Or is the resistive strip particles embedded in something that has a very low resistance?

There are some specific opinions we need to settle this.  I will not name any of you since it is Monday morning and you're allowed to sleep in a little.

[zombiwoof]

I'm betting lower.
More scrapes = less resistive material = less resistance.

I'm betting higher because that is not resistive material in there, but rather conductive material.  And theoretically, the less of it you have, the higher the resistance should be.  Or is the resistive strip particles embedded in something that has a very low resistance?

There are some specific opinions we need to settle this.  I will not name any of you since it is Monday morning and you're allowed to sleep in a little.

I've been told that if you scrape the SIDES of the resistive track, the resistance of the pot actually goes up.  I thought this was weird, but  a lot of guys are doing this with new low-reading guitar pots to bring the resistance up to the stated value.   The reason for this was explained by someone on a forum but I have forgotten exactly why it works (it has something to do with a narrower resistive element having more resistance than a wider one, like the analogy of a smaller diameter pipe constricting the flow of water more than a larger diameter pipe).  Many of the new  CTS guitar pots that are distributed by WD and Allparts have a 20% tolerance, so you can get a 500k pot that reads around 425k, so guys open them up and scrape the sides of the track to get them closer to 500k.  I would think if you scrape the TOP of the track (making it THINNER), the resistance would go down, but I haven't verified this.  Scraping the side of the track to raise the resistance has been verified by a bunch of people, like the guys on the LPF, many of whom are anal about using high-resistance pots in their LP's.  If you don't believe it, open up a spare pot and try it!  Also try making the track thinner and see what the effect is.  I haven't actually tried it yet.

Al

[DiscoVlad]

The resistance of a conductor (R) depends on three factors:

The Length of the conductor (L, in metres).
Its cross sectional area (A in sq. metres).
The resistivity of the material it's made from (p [Greek rho] in Ohm-metres).

which has the formula:

R= p * (L / A)

If you do the maths, it's pretty obvious that decreasing the cross-sectional area of the conductor, i.e.  narrowing the width, or shaving material off the top will increase the resistance.

This is just opinion, but I'd expect that pots would be made lower than their nominal value so that over their lifetime, and since the resistance increases as they wear out, they don't go too far over the upper limit of their tolerance.

[TheFantod]

Funny Follow Up:

I was getting the "volume drop" again yesterday and started to get frustrated, but then I realized something.

I had switched the positions of the "Volume" Pot and the "Distortion" pot when I reassembled it....

OOPS! 

Now everything is copacetic. Thanks all, again!   

[edvard]

The resistance of a conductor (R) depends on three factors:

The Length of the conductor (L, in metres).
Its cross sectional area (A in sq. metres).
The resistivity of the material it's made from (p [Greek rho] in Ohm-metres).

which has the formula:

R= p * (L / A)

If you do the maths, it's pretty obvious that decreasing the cross-sectional area of the conductor, i.e.  narrowing the width, or shaving material off the top will increase the resistance.

This is just opinion, but I'd expect that pots would be made lower than their nominal value so that over their lifetime, and since the resistance increases as they wear out, they don't go too far over the upper limit of their tolerance.

OK, I may stand corrected, but darn it seemed so logical.

...And this isn't the first time electronics has completely weirded me out...