75-4001 Pickup Issue

[edski]

I recently put a new wiring harness in my 75 4001, and it initially sounded like I remember, so all seemed OK. I had an occasion to do a rehearsal (my band flitters between a power trio with on bass, or if my son's available I might switch to the 2nd guitar), and I was pleased. I remembered that this was a very nice bass, and enjoyed the clank of having rounds on it for the first time in maybe 20 years.

My guitar buddy is a real tech-head, measures anything and everything on all his guitars and amps. He asks me if I want to measure the DC resistance of the PU's - I suspect he's mainly interested in the data for himself, but I'm interested. We both know that his method is approximate (just a cable out of the output to a volt-meter) since it includes all the pots, but it's relative to all the other guitars. We check out the neck pickup - 8.72 so called ohms (not sure, but I think it's a factor of 1000 or 10,000). I've seen his strats at 5.72 and such, seen the dimarzio fast-trac I's on my 66 Fender Mustang about the same. My 650 Dakota was odd in how it behaved (according to my buddy, and I can't recall what it did that was odd), but in the individual PU's I think were about 7-8 K-ohms. The 4004 Laredo's confirmed that the HB-1's were a solid humbucker.

We get to the 4001, and the neck PU registers 8.73. Looking good. The bridge PU? 90.9. Not 0.9, 90.9 K-ohms (presuming he had the volt meter on x1000 ohms). We repeated it several time to preclude us making some stupid mistake. We checked the guitar Joe was playing. The method was repeatable. The two PUP's together were 8.72, which mathematically checks with the physics about resistors in parallel.

The bridge PU on this bass has always sounded thin, but I have always thought that's expected from the bass-cut capacitor that is supposed to be in a February 1975 4001. But my friend did some research and found a discussion on the web (maybe from here?) where someone reported 138 K-ohms on the neck PU, and it turned out to be problem with the PU...

So, question - anyone know what is going on, or what the next steps might be? Am I looking at either a re-wind, replace, something else, or just live with it? Thanks!

[wim]

You are measuring the DC resistance with the capacitor inline, which gives you a false result for the pickup.
It should be around 8.5 Kilo Ohms

[edski]

Yeah, my friend's next thought was to measure the PU directly. So the cap can add that much resistance and make the normal idea of "more resistance = darker/warmer tone" flip?

Thanks!

[cjj]

A capacitor block DC (direct current). An ohm meter sends a DC voltage out the leads and measures the
"drop" and shows the equivalent resistance (yeah, that's super simplified, if you want the Techno-Nerd™ description, let me know). Notice that the ohm meter uses DC! This gets blocked by the in-line cap so you never even see the pickup!

So, you'll have to measure the pickup directly, or short across the leads of the cap.

On the subject of what the cap does, no, it doesn't really add resistance. Remember, I said a cap blocks DC. With AC (alternating current), which is what the sound signal from the pickup is, how much gets blocked depends on the frequency. The closer to DC you get (lower frequency), the more gets blocked. So, a cap works like a resistor whose value is dependent on frequency. Bass is low frequency and so, the cap blocks more bass than treble, which is why this in-line cap is often called a "bass cut" cap...

[edski]

CJ - I used to study atmospheric science a long time ago, so took a decent amount of physics and math, so gory explanations are welcome!

I'm looking at my old physics text (instead of paying attention to the business call I'm on!) about RC circuits and am really rusty, though!

[cjj]

Ah, well then. This should make perfect sense to you:

The impedance of the capacitor is:
Xc = 1/(2πfC)

Where f is the frequency in Hz and C is the capacitance in farads. As you can see, the impedance is inversely proportional to both frequency and capacitance and will be infinite for DC...

[BAD RONBO, KiLLeR DWaRfS]

ed...your 4001 is in the first 4 months of '75 and burgundy ???

[edski]

Yes, Ronbo - actually 1st 2 months, and Burgundy...

CJ - any idea about the frequency? To get the calc to work I have to assume somewhere around 2.5 Hz?

[cjj]

Yes, Ronbo - actually 1st 2 months, and Burgundy...

CJ - any idea about the frequency? To get the calc to work I have to assume somewhere around 2.5 Hz?

Well, I'm not really sure what you're asking here. I you are referring to what freq you need to get the resistance value shown on the ohm meter your friend was using, that's basically irrelevant since the meter almost certainly uses a DC voltage/current to measure the resistance. This DC will not go through the cap (for the cap, Xc = 1/0, which is infinite impedance). So, what you are seeing is the parallel resistance of the volume pot (or both pots if you aren't looking into the Rick-O-Sound jack or have the selector switch in the middle position).

The only way to get an accurate measurement of the pickup is to measure ONLY the pickup by disconnecting at least one lead from the rest of the circuit. Even then, an ohm meter tells you almost nothing. A pickup is a big coil of wire, which has inductance and inter-winding capacitance, along with the resistance of the wire. The "core" material on which the coil is wound (iron, magnets, whatever) will have a significant effect on the inductance too.

The only way to really measure a pickup is to use some sort of LCR meter (LCR means Inductance, Capacitance, Resistance. L is used for inductance because I was already taken for use as the symbol for current, and they used I because C was already taken for capacitance). These meters put an AC current of some frequency through the device and measure the various parameters. Even the series resistance (what you measure with an ohm meter) will change with the frequency.

For instance, here's the typical info for a modern RIC Hi-Gain bass neck pickup:
Freq Ls(H) Rs(Ω) Cp(F)
100 5.24 11.87k 34.50E-9
120 5.23 11.92k 33.20E-9
1k 4.38 18.12k 4.00E-9
10k 6.49 332.1k 23.50E-12

As you can see, the series resistance starts going up around 1kHz and is pretty large at 10kHz. But that's probably more info than you really wanted...

[edski]

I do remember some of that circuit stuff. I'll report back what we find, but I'm much more at ease now. Thanks for the physics lesson!

[johnallg]

Ed, if you really want the same type reading you got with the neck pickup (8.73k ohms), pull up the pickguard and measure the grey cable that comes from the bridge pickup. It should read about the same as the neck did. If you think the bridge pup sounds thin, take a small wire and wrap it around both leads of the smallest cap (.0047uF) and give a listen. If you like the increased bass response more than the thin response, solder the wire in place.

[iiipopes]

Ed, if you really want the same type reading you got with the neck pickup (8.73k ohms), pull up the pickguard and measure the grey cable that comes from the bridge pickup. It should read about the same as the neck did. If you think the bridge pup sounds thin, take a small wire and wrap it around both leads of the smallest cap (.0047uF) and give a listen. If you like the increased bass response more than the thin response, solder the wire in place.

Or add the push-pull circuit so you have both options.

[jdogric12]

Ed, you're back!!! Cool!!!

[jps]

Ed, you're back!!! Cool!!!

What about his back?

[cjj]

Ed, you're back!!! Cool!!!

What about his back?

It's cool...