L-1505 pickup problem

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L-1505 pickup problem

Postby MarkusBass » Thu Aug 30, 2018 2:22 pm

I fix guitars and basses and I've got G&L L-1505 with increased hum, both in passive and active mode (also when switched to humbucker). First I found out that the bridge is not properly grounded. I fixed it but it didn't help. Then I used a copper foil for the cavity but this also didn't help. Now, I found that the pole pieces in the pickup are not grounded. Should they be grounded? In several other basses I checked the pole pieces are always grounded. But looking at the photo of the pickup I don't see how the pole pieces could be grounded. Can anyone check it with Ohmmeter whether the pole pieces are grounded? Or, maybe Paul can tell me.

Mark

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Re: L-1505 pickup problem

Postby Paul Gagon » Thu Aug 30, 2018 5:45 pm

MarkusBass wrote:I fix guitars and basses and I've got G&L L-1505 with increased hum, both in passive and active mode (also when switched to humbucker). First I found out that the bridge is not properly grounded. I fixed it but it didn't help. Then I used a copper foil for the cavity but this also didn't help. Now, I found that the pole pieces in the pickup are not grounded. Should they be grounded? In several other basses I checked the pole pieces are always grounded. But looking at the photo of the pickup I don't see how the pole pieces could be grounded. Can anyone check it with Ohmmeter whether the pole pieces are grounded? Or, maybe Paul can tell me.


The pickup poles are not grounded. You are correct. So.....let’s see if we can figure out this hum thing. You fixed the grounding on the bridge as well as copper shielding the cavity and it made no change to the hum. OK, please understand I’m not trying to be a wise ass but let’s get our ohm meter and do a few tests. Put one of your ohm meter probes on the ground connection of your output jack. It sometimes is easier to plug in a guitar cord and clip onto the metal plug body.

Using the other probe, check the resistance of your bridge to ground by pushing it hard on to the bridge plate. It should be around 1 ohm or less. Now check the resistance from the copper shield tape to ground. It also should be around 1 ohm or less. Now, check the resistance of the back of all your pots to ground. Again, it should be around 1 ohm or less.

Basically you are trying to make sure that all your grounds have excellent contact with your output jack ground. I know this seems obvious but this is where I always begin. If all your grounds are connected up and you have good cavity shielding the only way I can see your getting bad hum is if your pickup never really goes into humbucking mode.

Anyway, check your grounding connections and let us know what you find.
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Re: L-1505 pickup problem

Postby Ken Baker » Thu Aug 30, 2018 6:01 pm

One more thing to check: If the strings are coated, the coating may be acting as an insulator at the bridge. Sounds silly, but there is precedent for it.

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Re: L-1505 pickup problem

Postby deltafred » Fri Aug 31, 2018 12:36 am

I had a really oddball grounding fault that took a lot of head scratching and probing with a meter to figure out.

The pot back that all the grounds are soldered to was not making good contact with the pot body. Things that should have been grounded were not even though they looked like they were.

It's a long time ago and I cannot remember the fault symptoms, or even which bass it was on, but it had me doubting my own sanity/multimeter/ability for a while.
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Re: L-1505 pickup problem

Postby MarkusBass » Fri Aug 31, 2018 11:13 am

Thanks for suggestions but in all cases (output jack to bridge, to copper foil, to pots, to strings) I have less then 0.1 Ohm (using Extech LCR200 meter). Also strings are not coated. On the other hand, the resistance between pickup poles and the ground is in a range 12-18 Mega Ohms. Can you really confirm that the pickup poles are not grounded when the pickups are manufactured?
I'm looking at the preamp now (I have preamp rev. 1.0). I noticed two issues:
- a huge gain at 6 kHz (over 16dB). It looks like the issue was noticed and some users changed the R7 resistor from 2.2k to 27k. This changes the gain to +5dB at 1.4kHz. I checked it with LTSpice simulation. I think this is much better. For the moment I have 22k resistor there.
- a huge time constant of the bias circuit (two 1 Mega Ohm resistors and 10uF capacitor). The time constant is more than 10 seconds. It means that it takes more than 10 seconds to set the bias voltage to 4.5V. During this time the preamp does not work correctly. I see that the problem was noticed and in the preamp version 1.3 the resistors were changed to 22k. I think I will change the resistor to at least 100k or less.
Looking for other ideas. The circuit does not have the 1nF capacitor from pickup to the ground as I see in the other basses. Is this capacitor important?

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Re: L-1505 pickup problem

Postby Paul Gagon » Fri Aug 31, 2018 12:24 pm

MarkusBass wrote:Thanks for suggestions but in all cases (output jack to bridge, to copper foil, to pots, to strings) I have less then 0.1 Ohm (using Extech LCR200 meter). Also strings are not coated. On the other hand, the resistance between pickup poles and the ground is in a range 12-18 Mega Ohms. Can you really confirm that the pickup poles are not grounded when the pickups are manufactured?
I'm looking at the preamp now (I have preamp rev. 1.0). I noticed two issues:
- a huge gain at 6 kHz (over 16dB). It looks like the issue was noticed and some users changed the R7 resistor from 2.2k to 27k. This changes the gain to +5dB at 1.4kHz. I checked it with LTSpice simulation. I think this is much better. For the moment I have 22k resistor there.
- a huge time constant of the bias circuit (two 1 Mega Ohm resistors and 10uF capacitor). The time constant is more than 10 seconds. It means that it takes more than 10 seconds to set the bias voltage to 4.5V. During this time the preamp does not work correctly. I see that the problem was noticed and in the preamp version 1.3 the resistors were changed to 22k. I think I will change the resistor to at least 100k or less.
Looking for other ideas. The circuit does not have the 1nF capacitor from pickup to the ground as I see in the other basses. Is this capacitor important?

Mark

That’s great news that your grounds are all good. Since you mentioned that this hum was occurring in passive mode too, I didn’t really go into preamp issues. I think if you make the component changes you have mentioned, you’ll be in good shape. I can confirm that the pole pieces on the pickups are not manufactured in a way that grounds them.

The 1nf capacitor that typically solders from the bass control center lug to ground is for loading down the pickups which lowers their resonant frequency. The pickups will sound brighter without this cap. It’s only important if you find the pickups excessively bright and want to tame them down a bit.

It would be normal to have some hum from the bass when the pickups are in single coil mode but you should have a pretty quiet bass in humbucking mode. Is the hum level pretty much the same when switching from single coil mode to humbucking mode?
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Re: L-1505 pickup problem

Postby MarkusBass » Fri Aug 31, 2018 12:38 pm

Of course, in humbucking mode it is much lower level but I can still hear it. And the owner wants to make some recording with the bass. When I check the hum of Musicman Stingray (which has a humbucker and grounded pole pieces) I don't hear the hum. I have to try another pickup with the bass. Maybe it's a problem with the pickup.

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Re: L-1505 pickup problem

Postby Paul Gagon » Fri Aug 31, 2018 3:00 pm

MarkusBass wrote:Of course, in humbucking mode it is much lower level but I can still hear it. And the owner wants to make some recording with the bass. When I check the hum of Musicman Stingray (which has a humbucker and grounded pole pieces) I don't hear the hum. I have to try another pickup with the bass. Maybe it's a problem with the pickup.

Mark

That sounds reasonable. Try another pickup and keep us posted on what you find. I’m at least glad that the pickup coil configuration switch is wired correctly. If you weren’t getting humbucking mode, that would explain a lot.
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Re: L-1505 pickup problem

Postby Ken Baker » Fri Aug 31, 2018 4:29 pm

I know you said you repair basses & guitars. This may seem overly simplistic for you, but please understand that this isn't just for you.

If you suspect that the pickup is the problem, it may be an open coil. To find out, do a tap test.

    1. Plug the bass in to an amp and turn its volume(s) down to bedroom levels.

    2. Dime the L-1505's bass & treble controls.

    3. Set the L-1505's volume control to about 50%

    4. Set the coil select switch to parallel.

    5. With the tip of a small screwdriver, tap a pole piece of either coil.

    6. In a like fashion, tap a pole piece of the other coil.

You should hear a sharp, very present sound from the amp and both coils should sound the same. If the sound seems kind of distant, like someone banging on a pipe a long ways away, you may have an open coil. It could also be a poor connection of the coil to the switch, which is what I'd check first.

Also, please differentiate between a hum sound and single coil noise. Hum would be 60 Hz, about a B on the A string (B1). Single coil noise is higher frequency and kind of static-y. These problems are distinct and many times have slightly different fixes.

Moar!

The jacks in these basses are a known point of failure. Nature of the beast because of their design. Squirting contact cleaner in may help. Info on replacement can be found here. There are also instructions there to upgrade to a power switching stereo jack, if necessary.

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Re: L-1505 pickup problem

Postby MarkusBass » Sun Sep 02, 2018 10:21 am

Thanks for the detailed explanation but this was one of the first things I checked. I also checked that the DC resistance of the two coils are identical.
Today I check whether grounding the pole pieces helps. Since the strings are grounded correctly, I put screws between the strings and the pole pieces, check that in this case the pole pieces are grounded and tested the bass. It does not change the hum. I think I have to test the bass with another pickup.
I also noticed that when I stop touching the strings (so that the bass is not grounded by my body) I get a static, which is again much louder than with Stingray. Are there any guidelines to decrease the problem with static?
The output jack is OK.

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