Author Topic: Intermittent loss of treble (take 2) (Read 189 times)

karl · « **on:** October 17, 2008, 12:00:58 PM »

Hello again, folks.

I've mentioned this one before, but I'm still no closer to a solution, and I was hoping one of the motherhip crew could give their considered opinion . . .

On plugging the jack into my SC deluxe, I'm occasionally finding that there appears to be no treble - the sound is at first very muffled - lots of bottom end but no top end definition. One thought is that maybe the bridge pickup isn't functioning. However, when I 'twang' the G string, the treble suddenly kicks in and all is generally fine from them on. It's almost as if there's some kind of 'gate' on the higher frequencies that only opens (and stays open) when enough force is used on a higher register string. It's very weird. Worst thing is that it only happens occasionally - I've taken the bass into the Bass Gallery in Camden and asked them to look into it. They kept it a week and it never once did this, so they couldn't do anything. Of course, once I get it home, it does it the second time I plug it in. Sod's law!

I'm kinda concerned that something in the electronics is about to give out, so I'm too afraid to use the bass for live work right now, just in case it decides to use a live show as the occasion to decide that it's not going to give me any treble at all, ever again . . .

When I posted this before, somebody suggested it could be a leaky capacitor - I would have no idea how to check if this were the case, and I mentioned this to the Bass Gallery but they didn't act on it - or might it be the bridge pickup dying? Or something different? I'd really appreciate it if anyone, especially Mica and co, could let me know if they've ever heard of this before and what the solution might be!

I'll add that it's not my instrument cable - it's happened with various different cables - and it's not my amp - again, I've had it happen with a variety of different amps. And it's not the battery - I had it happen once with a brand new battery.

Many thanks,

Karl

mica · « **Reply #1 on:** October 17, 2008, 12:48:21 PM »

Sounds like I would need to test the electronics. If you're not taking it out anyway, pull the guts and we'll see what we can find. There's no charge to test except for the shipping.

karl · « **Reply #2 on:** April 25, 2009, 08:03:54 AM »

Many belated thanks for the reply, Mica. I just haven't found a gap in my schedule when I thought I could do without my SC for long enough to ship the electrics to you for testing. Contrary to what I said in my first message, despite my worries about the electrics giving out, I find I can't bring myself to leave the Alembic at home when I gig - nothing else comes close to that sound! :-)

Anyway, she hasn't let me down yet. And I think I've narowed the problem down some. I had a look in the control cavity and found that the loss of treble occurs when the large magnetic (?) sleeve holding the wires from the pickups moves in the cavity. There's a piece of foam stuck to it, presumably to stop it rattling against the cover plate, but I don't thinks it's enough to stop it moving completely. Now I can't make out if the loss of treble happens simply as a result of the wires from the pickup moving (i.e. perhaps there's a loose solder on the pickup itself - I haven't taken the pickup out to have a look yet) or whether it's because the sleeve-thing occasionally touches one of the other components and maybe shorts something temporarily (it appears to come into contact with one of the connector posts on the bridge pickup Q switch occasionally).

For now, I've put a tiny piece of sticky tape in there to reduce the movement of that sleeve-thing, and that appears to have fixed the problem. But if that explanation gives you any more of an insight into where the problem may lie, I'd appreciate it if you could get back to me.

Many thanks!

Karl

keith_h · « **Reply #3 on:** April 25, 2009, 08:30:20 AM »

Karl,
That large magnetic sleeve as you called it is a ferrite bead. It is used to suppress RF interference (i.e. radio signals). Since it is made of iron could it be falling against the electronics and shorting something out. The way to resolve this is to use sticky tape to limit movement as you have done. You should also be able to wrap the bead in electrical tape to insulate it in the event it does fall onto a circuit board.

To see if it might be the pickup or pickup wires have you tried swapping the pickup connectors on the preamps? While the controls would be backwards this would tell you if the problem is with the pickup or the electronics.

Keith

karl · « **Reply #4 on:** August 19, 2009, 08:11:39 AM »

Problem finally resolved, so I thought I'd post up on here, just in case any of our community ever have anything similar happen and can't work out what the problem is.

After much frustration, I worked out that the 'lack of treble' sound was identical to having the bridge pickup filter rolled right off. So that narrowed down my search for the source of the problem. Some rummaging around in the control cavity later, wiggling various wires connected to the bridge pickup circuit board, I discovered that there's a tiny bit of movement in the brown plastic snap-connector on that board. And that bit of movement was just enough to break the connection, or part of it, which resulted in that loss of treble. Such a tiny bit of movement that even jiggling the bass a touch was usually enough to reconnect the break and restore the sound to what it should have been. And I guess when I was popping the G string, that was just enough vibration to bring back the treble. Hence my initial conclusion that there was some kind of gating going on.

Anyway, to cut a long story short, just a tiny bit of tape on the snap-connector to stop any movement and I'm now back in business. Much relief!

David Houck · « **Reply #5 on:** August 19, 2009, 08:18:54 AM »

Congrats Karl!

georgie_boy · « **Reply #6 on:** August 19, 2009, 08:32:39 AM »

Thanks for letting us know!!
No doubt, it'll stand many members in good stead

G

mica · « **Reply #7 on:** August 19, 2009, 08:39:07 AM »

Thanks for the update - but even with the friction lock you're noticing this wiggle? Was the wiggle from the board to the male connector or between the two conenctors?

karl · « **Reply #8 on:** August 19, 2009, 10:20:49 AM »

Hi Mica - yes, even with the snap-lock: the wiggle is between the two connectors - after you've snapped the two connectors together, there's still a bit of room for movement - the snap-lock doesn't quite hold it absolutely rigid.

If it would help for future reference, I can take the tape off again and try to photograph the range of movement for you.

K.