Pot or Pickup??? What do you think?

PilbaraBass · #1 11-19-2006, 04:36 PM

Something strange has happened to my Yamaha RBX170 P/J...
I noticed at practice the other night that the volume from my P-pickup seemed to be quite weak..???...

and when I dialed in the roll-off on the tone switch, the sound all but disappeared...

no issue with my J-pickup...it still works fine...

I know my issue is genuine and not just perceived, because I have a push/pull pot for series (switch is on the J-volume pot), and when I pull the switch (series), the sound gets softer...normally, it is quite the opposite.

it is either a problem with the p-VOLUME potentiometer, or the pickup itsself...obviously, i'm going to try changing the pot, first, and checking the resistance of pot...I haven't had a chance to do more than have a look for loose wires and possible shorts, yet.

the question I have is, has anyone ever experienced something similar to this? and what was the correct diagnosis?

can potentiometers fail in this manner?

If it's the pickup...what could cause it to go like that (again, not complete failure, just severe drop in volume and it is even across the 4-strings (strange for a split P-pickup)....

GlennW · #2 11-19-2006, 09:12 PM

If it isn't the pot I was going to guess that a magnet came loose, but with a P-pickup that would only affect two strings unless they both fell off. If you have alnico rods that would scrap that idea. IME pickups are all or nothing unless a dislodged magnet is the culprit.

You likely have a loose connection or something messing with the pot.

PilbaraBass · #3 11-19-2006, 11:53 PM

Quote:

Originally Posted by GlennW

If it isn't the pot I was going to guess that a magnet came loose, but with a P-pickup that would only affect two strings unless they both fell off. If you have alnico rods that would scrap that idea. IME pickups are all or nothing unless a dislodged magnet is the culprit.

You likely have a loose connection or something messing with the pot.

thanks, glenn.

I think it is either the pot or the wire going to the DPDT switch...all the wires on the pot are good...I'll have to take the electronics out of the cavity so that I can have closer inspection and do a little snooping with my multimeter.

but because it is a p-pickup and the problem is not limited to 2 strings I don't suspect a dislodged magnet...and since there still is a bit of signal, I'm not suspecting the pickup so much as the pot or a partial short...

when I fix this thing, I'll report what I found.

PilbaraBass · #4 11-20-2006, 03:26 PM

I had a look at the bass last night.
I changed the p-pickup volume pot….no change…the pot checked out OK, but I changed it anyway

I noticed that the volume on the p-pickup drops SIGNIFICANTLY when the tone knob is dialled in (treble roll off). This is strange, as the tone knob does not have this dramatic effect on the J-pickup (bridge pickup). I did an ohm’s check on the tone pot and it checks out fine…perhaps something strange has happened to the tone capacitor, but why does this just affect the one pickup and not both?

At any-rate, I’m going to change the tone capacitor out…I don’t think this is the answer, it’s an inexpensive part..

For now, I clipped the tone pot out of the circuit and the bass works much better, but it still isn’t right…

I may even reconnect the tone pot to the bridge pickup only (by connecting it to the wiper of the J-pickup volume, rather than the signal lug). That way I can at least dial some harshness from the J-pickup without having to take the treble completely from my amp.

I’m thinking that it has to be the pickup…

I’m going to order a new pickup, and while I’m at it, I’m getting a concentric tone pot to wire the bass V/T/V/T and keep the series switch…I’ve been thinking about doing this anyway, and already know how I’m going to do it.

GlennW · #5 11-20-2006, 03:57 PM

That's weird that the J pickup is unaffected - it was sounding like the problem was the tone pot or cap, but it looks like both pickups would lose volume if it is. Good luck.

PilbaraBass · #6 11-20-2006, 04:41 PM

Quote:

Originally Posted by GlennW

That's weird that the J pickup is unaffected - it was sounding like the problem was the tone pot or cap, but it looks like both pickups would lose volume if it is. Good luck.

exactly...it is a strange problem...I thought it would be simple...

the problem still exists (but to a much lesser degree) with the tone circuit disconnected...so it could very well be the pickup...anyway, its a good excuse for a new pickup and to implement the V/T/V/T mod

PilbaraBass · #7 12-11-2006, 05:26 PM

Well I replaced the P-pickup last night...

I put in a chinese-made Wilkinson pickup that I got on the 'bay for $20 AU ($31 AU including shipping). Alnico V bar mags with 2-pole pieces / string...I measured the DCR at 8.8k

The pickup WAS the problem...
The sound of the new pickup compared to the old one (when it worked) is quite similar, this pickup may be a bit "tamer"...I dig it's mellow tone and think it will work well (we'll see when I use it again on Sunday).

Interestingly, when I tested the old pickup with my ohm-meter, I got open-circuit...yet the pickup DID produce a sound, but it was just a very FAINT, sound...I'm not sure as to the nature of the fault...I'll have to do some more checking, maybe.

GlennW · #8 12-11-2006, 05:49 PM

Glad you solved the mystery.

Your old pickup might have been on the verge of shorting out due to failing insulation. Was the old one potted?

PilbaraBass · #9 12-11-2006, 06:33 PM

Quote:

Originally Posted by GlennW

Glad you solved the mystery.

Your old pickup might have been on the verge of shorting out due to failing insulation. Was the old one potted?

The old pickup has a plastic plate between the coils and the magnet, so I cannot really see if it is potted. It is quite possible that it is not.

Even with a short, I should still see a "white wire to black wire" resistance...but I'm reading open circuit...
open circuit should result in a completely DEAD pickup...

perhaps, the pickup is acting a bit like a transformer and the induced voltage in some of the pickup is transmitting to the signal line somehow. This could explain why the pickup is so succeptible to being loaded up by the tone pot.

If it is not potted, I may take the pickup apart and rewind it, as a fun project. I've always wanted to make my own pickup.

GlennW · #10 12-11-2006, 08:50 PM

I was thinking that the coil could be shorted out and not giving you a reading; but, I was also thinking that while the pickup was still in the bass and being played the vibrations shook the wire enough to make it almost work, and thereby produce the low volume.

Just guessing. I'd like to know what you find.