Fender 5-String Jazz Bass with Sadowski 4-Knob Preamp--loud constant RFI

[nevetslab]

One of the staff from CenterStaging's Guitar Dept (Burbank, CA) brought over the Natural Finish Fender 5-String Jazz bass, which he had swapped out Fender's preamp assy for this Sadowski 4-Knob Preamp assy. His problem was the constant HF RFI when you had the gain all the way up, HF boosted. I handle all their backline gear maintenance & repair along with a lot of client work, having my shop in the main building (Rehersal Studio complex/Equipment Rental Depot).

He had the strings loosened up enough so you could get the pickups in and out of the cavities, pick guard removed. I looked at the wiring, and saw a pair of wires that had # 6 solder lugs attached, screwed into the wood below the pickups, very close to their discrete wires that weren't dressed in twisted fashion to mate up with the pickup and ground wire terminals. No idea why those two wires were screwed down, and removed them, after verifying no shielding was present. Though, I don't know for fact that the black coating inside the compartment isn't some high impedance shielding.

After I had restored the two pickups to twisted pair lead wires, and dressed the output jack wiring the same way, I connected the two pickups, the bridge ground wire and plugged in the output connector and battery connector to have a listen.

It was better, but still very strong RFI / HF noise when you cranked up the gain.

I removed the preamp, the battery and output jack wiring assy's to listen to the preamp without the pickups attached, both with 10k resistor input termination and without. I connected the preamp assy with a short guitar cable back up to the test amp, and all was dead quiet, with just nominal hiss. No RFI or HF noise as it had WITH the pickups attached (and back on the instrument).

At that point, I removed both pickups, replaced the twisted pair wires with equal length Mogami single-cond spiral shielded cable (26AWG core, ~3.5mm dia jacket. Don't recall the cable model). Reinstalled the pickups, battery, output jack assy and the preamp. Put the pick guard back on, tightened up the strings, tuned it up, and gave a listen again.

Now, no more RFI or HF noise...just the traditional single-coil field pickup of florescent lighting RFI, which by turning, it cancelled out. I assume that RFI I was getting before was from the twisted pair leads from the pickups to the preamp inputs. The 'black coating inside the body cavity for the pickups and controls.....is that some form of shielding, or just cosmetic. The two 'ground wires that were screwed into the wood connecting to that seemed wrong.

[MarkusBass]

The 'black coating inside the body cavity for the pickups and controls.....is that some form of shielding, or just cosmetic. The two 'ground wires that were screwed into the wood connecting to that seemed wrong.

If you had two wires screwed into the wood, I'd say that this is graphite carbon paint shielding. It's a shielding because it conducts a current. You can verify it with an ohmmeter. Touch the paint with meter leads in a distance of 1 cm and check whether you get some readings (it can be in a range of 1 kOhms). But if this is really shielding, it is extremely poor. When I service guitars, I always replace it with copper foil. With copper foil you get resistances below 1 Ohm and it's way better than the paint.

Mark

[Leo_Gnardo]

Yes as Mark says, conductive paint, graphite carbon is the conductive component. So much easier for the manufacturer to swab some into the control cavity compared to lining it with copper foil. But copper foil works a lot better, so you might consider doing that to this bass. Also it's questionable what kind of connection the paint makes with those grounding leads screwed into it. Without doubt copper will be a much better conductor, one you can count on to form a proper faraday box/cage around the preamp & controls. You might also line the pickup cavities with copper, and connect that with wire to the control cavity shield. Also make sure the copper shield has a wired connection to the master ground on the output jack.

[Mick Bailey]

Conduction and DC resistance in itself is not an indication of how well the screening behaves at RF. You have to look at the intended mechanism by which the screening is designed to operate and this can be reflection, absorption or conduction. Or a combination of these. This is why some coatings may appear to give a relatively high DC resistance reading - we're not looking at them from the perspective of RF.

You also need to know which spectrum that the coating is intended to screen; it may be a particular band of frequencies or type of interference to satisfy regulatory compliance. For example, to give immunity to mobile phone or WiFi frequencies, which may do nothing at 50 or 60 Hz. All of course, done as cheaply as possible to minimize production cost.

[nevetslab]

I didn't measure any DCR on the black painted surface, but had a thought it might be a shield coating. I had discussed cutting some brass foil I have on hand (10 mil thick) and lining all the insides with that. I left that as a possibility if the re-wiring efforts didn't solve the problem. And, it's easy enough to add a grounding wire to pick up that coated surface. The copper foil I have on hand is a bit narrow to be effective, only 1/2" wide, which would take a lot of runs to accomplish the task.

I'll re-attach a grounding wire with the solder lug when I get back to the shop Monday morning. Many thanks for all your input!