How to check an active pre-amp to see if it is ok?

[John Meyers]

I have an old Bartolini 3 band bass pre. The last time I tried hooking it up it made all kinds of noice. Is there a way to test a unit before installing it? Using a meter to check? If so what settig do I use on the meter?

[Joshua Levin-Epstein]

I would mount it on a board with an old pick up and use a tuning fork (or a screwdriver) in place of a string. You should be able to judge if all the functions are OK.

Joshua

[Mark Swanson]

Unless you have a schematic, electronic testing equipment and the know-how to use it, hooking it up and giving it a try is your only other option.

[John Meyers]

I was hoping a volta meter would work. I know you can check pickups with it. I will give the mock up a try.

[Mark Swanson]

You can do some things with a voltmeter, like check for continuity on all the traces, measure resistances and current but to get the most out of it you'd need a schematic, and know what to look for.

[Greg Robinson]

Even with the equipment, knowledge and experience required to check a circuit as (relatively) simple as an in-instrument preamp, it is far simpler to hook it up and try it out.

[David King]

I have a pretty complete collection of Bartolini wiring diagrams going back 3 decades so if you tell us what the model is I can maybe tell you more about it. All the Bart preamps i've ever seen were encapsulated in epoxy so not much you can do fix them. The model is usually printed on a slip of paper and cast under the clear epoxy layer.

[John Meyers]

David thanks for letting me know because their web site does not offer the diagram for this unit. It is NSTMB-18. If it is on there the colors do not match the diagram. Hope you can help, thanks.

[John Meyers]

David i just did a search and and guess what your site came up and the pic you have of a Bart pre is the same one I have and the wires match the one in the picture. Also I do see some of the wires where they go into the epoxy, some bare wires are showing. Should I add more epoxy around them or some other kind of substance?

[David King]

Hi John,
That preamp came out in about 2000 or so. What you need to know about it is that it uses a bipolar supply has three battery connections instead of two. It requires two 9V batteries to operate. There is a -9V, a +9V and a neutral or center tap that goes from the the batteries's series link to the ring connection on a stereo 1/4" phono jack and turns the unit on and off when a cord is connected to the bass. Do not bother to install an active /passive switch with this circuit, it has serious DC offsets at the output due to the fact that it doesn't use coupling capacitors on the input and switching it will result in a serious "thunk!" through your amplifier that can damage speakers. Despite these limitations it's a very quiet and clean sounding preamp with effective EQ frequencies. The midrange control can be tuned with the addition of a cap in series with the brown wire going to the Mid pot. The standard implementation was a 3 way switch with 3 different caps for 250Hz, 500Hz and 800Hz center frequencies. The bass and treble controls are "shelving" with their frequencies fixed at about 40 and 4000Hz respectively. I've been told that the 118 dB signal to noise ratio is the result of an "expander" circuit that pumps the volume down when no signal is present. Some folks claim they can hear the pumping but I never noticed it myself.

The yellow wire is input.
The green wire is output.
Red is +9V.
The battery series link (Neutral) is white
Ground and -9V are black
Bass control connects directly to the main module.
Treble and Mid controls connect to the smaller module.

I'd just slip some 1/16" heatshrink tubing over the wires with exposed conductors but RTV or epoxy will also work.

I can scan the diagram for you if you PM me with your email address.

[John Meyers]

OK, I know it has been a while but I have finally gotten back to hooking this unit up to check it. Well it does work but I am getting a ground noise issue with it. It only happens when I am touching the strings. I have all the pots grounded together. I also have the control cavity copper tape shielded. Looking at the diagram it does not show all the pots grounded. It only shows the Volume and Blend pot grounded but only from the lug to ground and not showing it grounded to the back of the pot but straight to the ground only. When I got this bass built for me I had the builder add these electronics to it and he had all the pots grounded to one another, so I did the same and now I have this issue. I hope someone can help.

[Greg Robinson]

John, do you have the strings grounded?

[John Meyers]

John, do you have the strings grounded?

I do. I have the ground from the bridge running to the back of a pot, that connects to all the other grounds.

- Do you think the copper foil shielding could be causing this?
(I am not sure if the shafts of the pots running through the shielding tape would cause this?)

- Also should I run a ground wire to the shielding tape in the control cavity?

This is the one part I dislike the most when building and that is active electronics. All these wires running everywhere, makes me want to have an anxiety attack.

[Greg Robinson]

Ok, first make sure to follow the wiring diagram you got from David EXACTLY. If that means disconnecting the ground wire from the back of the pots, do it. Test again, and if you are still having noise issues, try wiring it up without the copper shielding tape.
Let us know how you go.

[John Meyers]

Sounds good and thanks for the info.

[John Meyers]

Ok, first make sure to follow the wiring diagram you got from David EXACTLY. If that means disconnecting the ground wire from the back of the pots, do it. Test again, and if you are still having noise issues, try wiring it up without the copper shielding tape.
Let us know how you go.

Don't you just hate it when the old saying is true "when all else fails, read the instructions". I did what you said and I did not need to remove any of the foil tape just the grounds from the tone pots. But I did see one thing I had missed and that was to ground the jack as well. So it is working like it should. So now time to clean everything up and get it ready to be added to my new build. Now that I know it works I really want to make sure all connections look better and try and get the wires to run in a smoother lay out then the way it is now. Not sure you can make this unit look good because of all the wires involved but I will give it a try. Thanks for your help

[Greg Robinson]

Glad you got it working John!