'97 Rogue hum

[Neilgb]

I am new to Alembic ownership with this 97 Rogue 5 string. I have a question about a hum. It occurs when I adjust or hold the tone control. It is as if I am acting as an antennae that is being picked up by the potentiometer. It is not a major problem as the bass is very quiet apart from when I touch the tone knob - I would like to know if this is normal or if it is symptomatic of something. This does not occur when I touch the volume or pickup selector knobs.

None of my other basses do this - they are susceptible to RF noise in other ways!

Thanks for any insights you can provide.

Neil

[StephenR]

Hey Neil, hope you are enjoying your new Rogue despite the hum issue. One thing that seems to help a myriad of issues related to the pots in Alembic instruments is rotating them 50 or more times. They are self cleaning and simply rotating them is the first step to try when having a problem or noise. If the pots are really dirty it can take 100 or more rotations to clean them properly. If that doesn't help I suggest you provide a photo of the electronics cavity so it can be visually checked for signs of a short in the wiring or anything funky going on.

Good luck!

[edwardofhuncote]

Sounds like the Filter pot isn't making good contact with the sheilding. Ease the knob off (does it have collet type or hat type? I see you have collet knobs... different removal instructions, see link) and cinch the nut just a twitch. You might even give the pot a degree or two of twist inside the electronics cavity before re-tightening.

I bet that solves it.

*instructions for avoiding sadness in collet knob removal:


https://club.alembic.com/index.php?topic=217.0

[mario_farufyno]

Are the PUs wire corectly winded through the magnectic beads?

[edwardofhuncote]

Are the PUs wire corectly winded through the magnectic beads?

Very good advice too... here's some help with that if not familiar: https://club.alembic.com/index.php?topic=219.0

[Neilgb]

Thanks everyone for the help.

I investigated further and found that:
- the pickup leads are routed through a ferrite as are some other preamp leads.
- turning the pots 50+ times cleared up some very faint scratchiness but didn't affect this problem.
- The tone pot appears to be firmly fixed in place - I didn't try tightening it yet as I noticed some additional issues as I was poking around.

Additional issues
- the output jack is very crackly indicating a poor ground so I cleaned it up with contact cleaner which improved the crackling but hasn't completely resolved it.
- touching the output jack (or cable jack) stops all the problems as does touching the shielding paint (suggesting the latter is in good shape).
- Touching the back of the tone pot causes a loud humming
- touching the strings does not affect any of the various noises suggesting they are not grounded - is this normal for an Alembic?

Next steps:
- Clean up or replace the output jack
- Investigate string ground
- ensure good contact between tone pot and shielding

Here is a picture of the control cavity.

[edwardofhuncote]

Those side jacks, sturdy as they were, did occasionally wear out. The shop can help you with that. I still think the connection between the Filter (tone) pot and the shielding is sketchy. It's much easier than it sounds. The Q-switch on my bass (connected to the Filter) was not contacted well to the shielding and produced noise like you're describing.

[adriaan]

You mention " the pickup leads are routed through a ferrite as are some other preamp leads." AFAIK, only the pickup leads should pass through the bead, and at least one should make a loop to pass through twice.

[lembic76450]

Mica did tell me that the jacks could be funky.  They are plug in connectors and very easy to replace.
I changed mine out on a similar age Rogue.

[Neilgb]

Thanks again for the input.

As you said, The problem is that the tone control (and switches) arent connected to the shielding. Connecting the tone potentiometer directly to the jack ground fixed that problem completely.
Tightening the tone pot nut didnt help nor did loosening/jiggling/retightening it. I will probably have to remove the electronics and repair the shielding paint to ensure good contact. This should fix the switches,too.

Adriaan, there are 2 ferrites. One has the pup wires looped around/through. The other has some other wires looped.

I need to source some shielding paint - any recommendations?

[rv_bass]

Stewmac has shielding paint.

[edwardofhuncote]

...I will probably have to remove the electronics and repair the shielding paint to ensure good contact. This should fix the switches,too.

Adriaan, there are 2 ferrites. One has the pup wires looped around/through. The other has some other wires looped.

I need to source some shielding paint - any recommendations?

Yes, but this is pretty easy, just remember what goes where, take a picture. A little brush-stroke or two of the Stew-Mac black where the pot and switches seat, AND the hole.


In the picture, it looks like the second ferrite bead has your battery leads looped through it. I don't know what that does for cancelling noise, but I doubt if it generates any. My Wire Dunce Hat is a 2XL though. 


Stay on it man, that Rogue is going to blow you away.

[Neilgb]

Quick update for the record.

I installed a new jack socket and shielding in the cavity, making sure every control was connected to ground. The hum has now gone and the tone control appears to work more effectively. There is some residual noise that is about the same as any well made instrument.

I took the bass to rehearsal for the first time and it made a completely different suite of sounds that I assume are caused by external RF signals (regular patterns to the sounds) from electrical equipment near the rehearsal room (which is in a late 40's era home).

i have emailed the helpdesk about the RF mod and will get it done (assuming it hasn't already been done - in which case I am stuck!).

Neil

[hammer]

It may that I’ve just been out of this loop but I’ve only heard of RF updates needing to be done on Series basses which have single coil PUs.

[gtrguy]

You might use a VOM meter to prod around in there to test that things are all grounded, using the barrel of the output jack for the negative. You might also mark where the internal trim pots are set and then exercise them back and forth a few times.