Bad Preamp? - STC-3M4 Question

[JPSC]

I have a STC-3M4 that came installed from Warwick on my single humbucking FNA 5-string. The bass is around 13 years old.

A few days ago I plugged it into my amp and there was no sound. I have checked all the usual suspects (cable, battery, output jack), and I have verified the preamp board is receiving power from the battery. I noticed that if I unplug and the reconnect either of the two molex connectors on the board I hear a popping sound through the amp. Since that happens with either molex connector, I am thinking that the board is passing signal through it.

That led me to think it was the volume pot, but it tests as functioning too.

Just to be sure I did touch up the solder connections on the volume and tone pots to rule out any cold solder joints.

Any ideas? Could the preamp be bad event though it appears to pass a signal (based on the popping). I was unsure where to put the probes from my multimeter to check the board that way.

 

[Funkfingers]

Re: Bad Preamp? - STC-3M4 Question

Welcome to the forum.

The tests that you have performed so far indicate that the PCB is functioning.

The next test is to touch something metallic on the input terminal of the volume pot. Turn the EQ knobs. See whether this affects the noise from touching the pot.

Next, try tapping something metallic lightly on the top surface of your pickup. Try the EQ again.

The next test will be direct connection of your pickup to a jack socket. Do you get a signal?

One issue that I see on some older Warwick basses is the condition of the battery clip and holder.

 

[JPSC]

Re: Bad Preamp? - STC-3M4 Question

Thanks for the info - I will try those steps tonight.

If the EQ does not affect the noise from touching the pot, then I am guessing I have something wrong with the pot that has no effect.

If there is no noise from touching the input terminal of the volume pot, then I am guessing the volume pot is bad.

Finally, if there is no sound from directly connecting the pickup to a jack socket, then I may need to replace the pickup.

Am I on the right track? I appreciate the help - I haven't messed with an electronic circuit in about 20 years.

 

[Funkfingers]

Re: Bad Preamp? - STC-3M4 Question

Am I on the right track?

Sort of.

The tests that I described are a systematic check to find where the signal path is interrupted.

Logically, it would make sense to begin with the signal source, the pickup. I mentioned the pickup-to-socket test last deliberately because it involves the desoldering of components. All of the tapping, buzzing and control knob twiddling tests can be performed swiftly and without disassembling anything.

 

[JPSC]

Re: Bad Preamp? - STC-3M4 Question

Tried all but running the pickup straight to a 1/4" plug. I will have to do that next as there was no sound when I tapped an allen wrench against the pickup magnets. The volume pot/push-pull switch and eq controls all had a noticeable affect on the hum while touching an allen wrench to the volume pots input terminal.

That leaves testing the pickup. At most I will be out the cost of a replacement pickup, but I am hoping its a bad/broken connection I can fix. Thanks again for your help. You saved me several hours of frustration.

 

[Funkfingers]

Re: Bad Preamp? - STC-3M4 Question

You saved me several hours of frustration.

Possibly.

Is your pickup the stock MEC offering or a replacement? The former tend to be sealed in resin, leaving just the output and battery cables emerging.

 

[JPSC]

Re: Bad Preamp? - STC-3M4 Question

It has the original MEC.

 

[ginormous]

Re: Bad Preamp? - STC-3M4 Question

I would suspect bad Molex connectors... depending on the plastic used, these tend to be troublesome.

My dad was a sound engineer, and he told of how one brand of tape recorder used Molex connectors for their internal PCBs, that literally crumbled into dust after years of steady-on power.
The old-school analog studios left their units on 24/7, rather than powering down at night, and risking a power-on surge or spike damaging equipment.
Crapped-out Molex can create all kinds of issues, including grounding and voltage arcs across circuit traces and components.

Another possible issue -- some pup manufacturers used resin to eliminate microphonics from loose windings.
They don't always count on the exothermic heat from curing resin damaging the varnish used as magnetic wire insulation. This may render the pickup useless.

Anyhow, Funkfingers has it nailed. Start from the source and slowly work your way out.

 

[ehdwuld]

Re: Bad Preamp? - STC-3M4 Question

I would guess the volume pot has a bad connection

Or less maybe a bad ground at the jack

*(Sent from my durned phone!)*

 

[Funkfingers]

Re: Bad Preamp? - STC-3M4 Question

Since all of the SD/Basslines components have been proven operational, the fault is either where the various pickup conductors are soldered or the pickup, itself, is faulty.

Diagnosing this depends on the number of conductors leaving the MEC pickup and on how the coil combinations are switched (or balanced).

 

[Funkfingers]

Re: Bad Preamp? - STC-3M4 Question

The cursory background reading that I have done suggests that the STC-3M4 and SMB-5A pickup are standard equipment on the FNA bass.

Something that is common on Warwick bass guitars but not on the STC-3M4 kit are push-pull pots for series/parallel coil mode switching and to bypass the EQ. Thus, your controls could be a mixture of Warwick and SD/Basslines components.

Examine the pots in the control cavity. If your pickup has four conductor + shield cable and all four coil conductors connect to the switching terminals of a push-pull pot, check that all five solder joints are good and fresh.

 

[JPSC]

Re: Bad Preamp? - STC-3M4 Question

The stock MEC has a single hot lead that runs from it to back of the volume pot. A short jumper connects it (the single lead from the pickup) to the grey wire that leads into the STC-3M4. The MEC design is sort of new to me as I have never seen a pickup that did not have at least two lines (a hot and ground) running from it. [Edit - The ground of the MEC appears to be a braided outer cable and the hot line runs under it. I checked with my meter (black probe on the outer braid, red probe on the center), and the pickup appears to be working. With the meter set to 20k, I'm getting a reading of 1.2]

I looked over the wiring diagram for the SMB-5A and STC-3M4, and everything else in the control cavity appears just as it does in the diagram.

What really struck me was the difference in quality between the wiring of the STC-3M4 part of the circuit and the line from the pickup to the back of volume pot and the jumper to the line from the preamp. It looks like someone at the Warwick factory dropped in a prewired preamp and control harness from Seymour Duncan and then did a rather sloppy job with soldering the pickup connection. I did desolder and redo that connection, but it didn't fix the no sound issue.

I'm sort of thinking about just dropping in a new pickup (SMB-5A) and new STC-3M4, and I'm sort of thinking about just selling it as is and going with a Steve Harris Precision. I've been playing this bass steadily since I purchased it in 2001, so it might be time for a change. I did play a Precision for 15 years before I moved to the Warwick.

 

[Funkfingers]

Re: Bad Preamp? - STC-3M4 Question

Judging by everything that you have posted so far, the pickup is the likeliest cause of your problem.

Two further questions.
1) Have you tried testing the pickup by connecting it directly to a jack socket?
2) Does your Warwick have either a push/pull pot or a DPDT mini switch to bypass the EQ in the event of battery failure?



In my opinion, you are correct to assert that an American pickup would bring some sonic improvement. The SD/Basslines SMB-5A is one option. Delano and Bartolini are two others.

 

[JPSC]

Re: Bad Preamp? - STC-3M4 Question

No DPDT mini switch, but it does have a push/pull pot. However, it does not bypass the EQ in the event of battery failure. In the past when the battery has died, the sound would cut out regardless of the position the push/pull was in.

Any other time I would have two or three quarter inch jacks lying around, but of course now I don't. I am going to pick one up today and try that. If there is no sound, then I am going to start by purchasing a new SMB-5A.

 

[JPSC]

Re: Bad Preamp? - STC-3M4 Question

Definitely the pickup - connected it straight to a jack and no sound. Took it out, wired it up again and had no sound when tapping on magnets.

SBM-5a ordered.