Author Topic: Help with bass circuitry identification (Read 357 times)

irishstu · « **on:** April 09, 2015, 10:12:04 PM »

Hi all, Boy am I glad to find this forum. I've been asked if I can help to bring a dead Alembic back to life. By "dead" I mean that no sound is coming out of it at all. I'm handy with electronics, but I have no experience with these particular circuits at all, so any information I can get here at all would be gratefully received. To make a long story short, here's the bass in question:

And here's the circuitry:

I have more photos if anyone needs them. Does the bass or circuitry look familiar to anyone and can anyone tell me more about it? Even the model name of the bass or if the circuitry has an official name would be helpful. Thank you, Stu

elwoodblue · « **Reply #1 on:** April 09, 2015, 10:22:38 PM »

Welcome Stu,
The model is a Spoiler. Have you connected a battery?(had to ask )

There is a troubleshooting section, a good place to start a thread with needed pics. Looks like everything is there.

irishstu · « **Reply #2 on:** April 09, 2015, 10:54:35 PM »

elwoodblue! Thanks for the welcome.

And thanks for replying so fast. Would I be wise to start another thread in the troubleshooting section, or can a mod move this?

Thanks for the model name, and yes, I did try a battery, ha. Actually the guy that asked me to fix this (a guitar tech) said that when he tried, the battery ran hot, though I wonder if that was because something was touching the shielded cavity sides when he tried it (everything was already unscrewed by the time he showed it to me). I tried a multimeter on the battery clips (with no battery attached) and there's plenty of resistance.

I just noticed that the trim pot may be turned down to zero. Boy would that ever be an easy fix.

On the subject of everything being there, I've been assured that nothing has been tampered with, although, you know, I'm taking that with a pinch of salt.

Thanks again,
Stu

irishstu · « **Reply #3 on:** April 10, 2015, 05:09:18 AM »

Are you sure it isn't a Persuader, elwoodblue? I can't seem to find any Spoilers with the same pickup configuration.

cozmik_cowboy · « **Reply #4 on:** April 10, 2015, 05:20:23 AM »

It is, indeed, a Persuader; same wood as a Spoiler, different p/ups & circuit.

Peter

FC Bass · « **Reply #5 on:** April 10, 2015, 05:46:25 AM »

Yep, Persuader: Volume, Volume and Filter.

If you don't have them, you can get the knobs here: http://alembic.stores.yahoo.net/hatknobs.html

Looks like the ferrite bead is missing, look here: http://alembic.com/club/messages/16271/16909.html

Make sure everything is connected right, its hard to see but it looks like there's something mixed up there...

Mica will correct me if I'm wrong (I hope) :-)
It looks like one of the pickupleads is connected to the filter unit (the board with the trimpot)
I think that pickuplead should be connected to the upper volume control board.
The connector that is now in the upper board (coming from the lower volume control) should go to the filter unit.

Also I would switch the filter unit with the lower volume control, so you have both volume controls on top.

If the electronics still aren't working, best would be to send it in for repair to Alembic.

(Message edited by fc_spoiler on April 10, 2015)

irishstu · « **Reply #6 on:** April 10, 2015, 07:05:22 AM »

Thanks, everyone. I have the knobs.

Ferrite bead--check. I know where I can get one easily enough.

Very interesting about the leads. It makes sense to me.

Sending it to Alembic isn't an easy option, since I'm in Taiwan, though I will mention it to the owner if I can't fix it. I'd really like to try a repair, as I'm fairy confident with electronics and the boards are pretty repair-friendly (thanks, Alembic!) It would just be handy if I knew the general layout of the circuit (a schematic would be even better!)

You've already given me some very useful information, kind folks.

Thank you.

elwoodblue · « **Reply #7 on:** April 10, 2015, 09:11:28 AM »

I'm glad others chimed in with the correct info,
sorry about that Stu. I guess it was late for me
last night.

elwood

mica · « **Reply #8 on:** April 10, 2015, 10:34:44 AM »

Make sure the solder side of the circuit boards is not touching the side of the cavity. It looks like it might be. This will shirt out the circuit and result in no output.

mica · « **Reply #9 on:** April 10, 2015, 10:50:00 AM »

And it can make that battery hot as well. There's a chance that nothing else burned out with the hot battery, but if it doesn't function after the shorts are resolved, we can talk about that later.

What happens is the wood cells compress over time under the pressure of the nuts holding the bushing of the pots in place. When the cells compress enough, there is a little play, and if the user reaches the end of the pots rotation, the board inside can still spin and go along for the ride, until it hits a wall that is!

Just holding the board inside while you tighten up the nut to keep it from spinning will seat it again for another 20 years or so.

irishstu · « **Reply #10 on:** April 10, 2015, 07:32:49 PM »

Thanks again, all. You're so generous with your knowledge and I truly appreciate it.

elwood, no worries. your information got me on the right track to finding the Persuader.

mica, unfortunately when I received this, the pots had already been loosened, so I don't now if they would've been touching the cavity when the bass stopped working. However, with any testing I've done since, I've made sure that everything was insulated.

I tried moving the leads so that each pickup is attached to one of the volume boards, as per fc spoiler's advice, and then the second (slightly larger) volume board is fed to the filter unit.

My thinking is that it's unlikely for both volume boards to be non-working, and that therefore the filter unit is the most likely culprit, so I'll concentrate on that for now. I notice that after the filter unit, the signal is fed back to the larger volume board before going to the output jack, so I'll need to try to trace the path there too.

Again, thank you for your help, everyone. Any other advice will be very gratefully received, no matter how unimportant you might think it is.

irishstu · « **Reply #11 on:** April 11, 2015, 12:15:40 AM »

Hey, good news everyone! It's working.

After changing where the leads were plugged in (thanks again, fc_spoiler) and then trying to reverse engineer the circuit a little bit, I could see that the chip was the wrong way around (9v was going to the negative of the chip and 0v was going to where 9v should be [and these two pins are on diagonally opposite sides of this particular chip]).

I think that what may have happened was that someone disconnected the pickup leads for some reason (maybe the boards were shorting with the cavity, or maybe someone just got curious, ha), then when they reconnected them, they put them in the wrong place. They then thought something had broken, took the chip out to... I don't know, check the number or something, and then put it back in the wrong way around. I doubt I'll ever be told the honest truth.

A snowball effect of errors basically.

Anyway, all working now. Thanks again for everyone's help. I really appreciate it.

elwoodblue · « **Reply #12 on:** April 11, 2015, 12:26:26 AM »

Great news!
...and good eye catching the chip kerfuffle.That could be easy to miss.
We are lucky to have FC around

David Houck · « **Reply #13 on:** April 11, 2015, 07:49:00 AM »

Congrats!