Replacing pots

[mgazb]

I started looking into the electrics on my bass. There seem to be two issues.

1 - the tone control doesn't make any difference.
2 - the volume controls are silent for 3/4 of the turn and then go to full.

I checked the resistance and the tone pot doesn't show any change across the range. The volume pots only register a change on the last 1/4 turn.

Can I just replace pots using ones from an electonics supplier? What are the power requirements for these controls? Could the tone problem be caused by a bad capacitor?

thanks

 

[Andy D]

What kind of bass is it? Active or passive, etc.

 

[Crater]

Can I just replace pots using ones from an electronics supplier? YES, you can. But why? Proper guitar pots are available at your local GC (USA customers) and they aren't expensive.

What are the power requirements for these controls? None, pots of any power rating will work. (watts not the same as ohms)

Could the tone problem be caused by a bad capacitor? Could be, but more likely it's the moving parts - the pot - that is bad. Film and ceramic caps last practically forever.

 

[SGD Lutherie]

I started looking into the electrics on my bass. There seem to be two issues.

1 - the tone control doesn't make any difference.
2 - the volume controls are silent for 3/4 of the turn and then go to full.

I checked the resistance and the tone pot doesn't show any change across the range. The volume pots only register a change on the last 1/4 turn.

Can I just replace pots using ones from an electonics supplier? What are the power requirements for these controls? Could the tone problem be caused by a bad capacitor?

thanks

What kind of bass is it?

How did you check the resistance? With the pot wired int he circuit?

Audio taper pots don't do much until you get to about 8, so that's what you are hearing. When you have two volumes used for mixing together two pickups, linear taper works better.

What are you plugging the bass into to test everything?

Your tone pot may have a wiring problem. I'd check the wiring. If you want better taper on the volume controls, get two linear taper pots of the same value.

 

[line6man]

Perhaps the pots have bad tapers. What is the taper of each pot? You might try using linears instead of logs or vice versa.

There are really only two things can happen to a capacitor. It can become leaky or internally short, which would cause the tone pot to vary the impedance over the entire signal, resulting in the tone pot behaving as a volume control. Or, they can develop an infinite impedance, which would be quite rare. This is unlikely to be a problem with the capacitor itself. Perhaps you have a bad pot or cold solder joints.

As Crater said, the power handling capability of a pot is irrelevant in such an application as a volume or tone control in a bass. You are dissipating many orders of magnitude less power than any common rating for pots.

 

[SGD Lutherie]

The ground or hot connection might also be disconnected on the tone pot.

 

[mgazb]

Thanks for your replies.

It's a passive bass.

I found a site in the uk that sells guitar pots so I'll look into those instead.

I measured the resistance with the bass disconnected using an ohmeter across the terminals of the pots. I haven't removed the pots yet.

I don't think that it is a cold joint as each pickup works when set to full (8+) volume.

How do I check the taper? Is it marked - as far as I can remember they are marked something like B500K or D500K - I need to check the letters again.

 

[line6man]

Thanks for your replies.

It's a passive bass.

I found a site in the uk that sells guitar pots so I'll look into those instead.

I measured the resistance with the bass disconnected using an ohmeter across the terminals of the pots. I haven't removed the pots yet.

I don't think that it is a cold joint as each pickup works when set to full (8+) volume.

How do I check the taper? Is it marked - as far as I can remember they are marked something like B500K or D500K - I need to check the letters again.

B is linear.
A is log.

Sometimes the taper is marked, usually it is not. In which case, you have to measure it. A linear pot reaches 50% of it's resistance range halfway through the rotation. A log pot won't reach 50% of it's resistance range until about four-fifths or so.

FWIW, measuring the pots while they are still wired up won't do you any good. They are in parallel, and with the pickups feeding into them, which decreases the resistance.

 

[mgazb]

Checked last night. The two volume pots are marked B500K and the tone pot D500K.

I had a play after reassembling it and the controls seemed to work better than before. Maybe a case of a good looking at.

More worryingly maybe something got jiggled ...