Faulty Bartolini on board bass preamp - how to un-pott?

[markusw]

After some years of use my Bartolini NTMB on board bass preamp suddenly got faulty. It's kind of spooky: suddenly it stops working. Often I don't even have to touch it to make it happen. Usually, after removing the cover and fumbling a little with the wires it starts working again. Sometimes it starts working again if I just wait for the next day. I' ve checked all the connections that are not potted, resoldered all joints accessible, however, nothing helped. Actually, I could'nt even find a broken wire or a dead solder joint. (the battery is OK, the pots are not scratchy, jack is fine, cable too). Since it was a little too unreliable for my taste (at least one drop out when we practice) I ripped it out and replaced it with the old Carvin stock preamp. I am no really happy with the sound, but it's working and a few cap changes should do the trick.
Anyway, since I won't use the Bartolini electronics anymore I thought of cannibalising it and to see what's under the black cover in those two little black boxes. Any idea how to get rid of that black stuff without destroying the electronics completely or at least whithout removing all the labelling from the components?  :twisted: At least I'd have a little fun for my 140 Euros.

Markus

[Paul Perry (Frostwave)]

I can't guarantee it, but some of the synth-diy guys have successfully 'decoded' epoxied modules with a pain-stripper hot air gun and a dental pick.
It sounds VERY MUCH like a broken wire, though.

[loscha]

I can't guarantee it, but some of the synth-diy guys have successfully 'decoded' epoxied modules with a pain-stripper hot air gun and a dental pick.
It sounds VERY MUCH like a broken wire, though.

 I've read similar accounts. Maybe google for guys working with Arp 2600 synth modules, notorious for the potted packaging.

[Rob Strand]

It could be the pots, most likely the bass pot if it is.

I haven't pulled a Bartolini appart but from what I can work out it's a simple ckt - highly likely a Baxandall/feedback type tone control.

Perhaps you could measure the DC voltages on the pot pins in the working and faulty state.  It might be possible to deduce something from that..

[markusw]

Thanks for your answers!

I also thought it has to be a broken wire but I could not find it. I was really careful when fumbling with the wires trying to move just one wire at a time with some tweezers but I never found a crackling one. (By the way, is "fumbling" the right expression in this context??) Also thought of sticking some needles into the wires to get as close as possible to the surface of the black epoxy and then check for continuity. Do you think this is a valid option for finding the bug?  

I've taken the voltages some time ago and it was quite interesting: around 4.5V at the three tone control pots in the working state, and values from about 1 to 8 depending on the setting of the mid range and IIRC the bass pot. However, I don't think it's one of the pots since they are not scratchy. Also, fumbling with them never turned it back to working.

It's pretty anoying compared to stompbox building where you can easily trace the signal  :?

This is how that pain in the a.. looks




Markus

[Paul Perry (Frostwave)]

If it works by sending DC voltages (from the control pots) to a buried block, then it must be using one of those consumer electronics DC controlled EQ chips, so it should be pretty easy to replace the thing.

BTW 'fumbling' is close enough, but 'fiddling' is probably better.

[markusw]

Sorry for the confusion: I meant 4.5 V at the 3 tone control pots in the working state (irrespective of the pot settings). In the non-working state I measured voltages varying from 1-8V at the three pots depending on the settings of the mid range and IIRC also the bass pot. Next time I will take more care when posting   :oops:

Markus

[Rob Strand]

I measured voltages varying from 1-8V

OK that's starting to sound like maybe cap is shorted or failed.

depending on the settings of the mid range

If you are certain the midpot has an effect I would strongly suggest checking (or replacing) the caps on the mid frequency switch.  Possibly check for shorts on the switch as well.

[markusw]

Checked the mid frequency switch: there are two resistors soldered to it that have ~45k which fits to the colour code. The switch itself seems to work. It's interesting that the off resistance of the switch is determined by the two resistors. Regarding the cap it should be 0.1 µF (104K) but with my dmm I get ~10 µF, but maybe this is because the cap is still soldered to the circuit   :?:

BTW, Paul thanks for the english lession  

Markus

[Rob Strand]

The things that look like resistors could be caps.  In fact they *should* be caps.  If not then you may have a situation where the unit works with the switch in one position then stops working in the other.

Check out the Bartolini wiring diagrams, you can see the caps:
http://www.bartolini.net/instructions/electronics/ntmb/ntmb_family.htm

You should confirm:
- when the unit works, it works with the mid switch in both positions
- when the unit fails, it fails with the mid switch in both positions

I still think that area of the circuit is worth checking.

maybe this is because the cap is still soldered to the circuit

Yes, definitely.  Caps can rarely be measured properly in circuit.

[markusw]

I'm pretty shure that the resistors are resistors because the resistance measured with my DMM fits perfectly to the colour code. Also there is a variant of the NTMB with just one cap (0.1 µF: res freq 917 Hz; no cap: 419 Hz). However, I will reassemble the electronics with some pickups and a jack an see if I can figure out whether the this area is involved. Any ideas how to generate a signal that I might send into the electronics without having to put it back into my bass?

Thanks for your advices,

Markus