1955 P90 muddy - defect insulation?

[orangedrop]

I recently bought a pair of P90:ies from 1955. They both measure just above 7 Kohm. One sounds just great and the other one muddy (but nice). The muddy P90 also makes the vol pot act differently (more in steps rather than a log curve). The bobbin is intact and the magnet wire tape/paper looks untouched, the PU looks dead stock.
When I measure resistance, the muddy P90 seems OK. When I check for a short, the multimeter does not indicate a short but ads a number on the display – like 1.958. When doing the same maneuver (checking for a short) with the stock 2007 P90:ies the display goes 1. See pic



Does this indicate that the winding (insulation) is defect? I had a similar problem with another PU. That time the problem disappeared when I changed the wire (PU->pot).

[big_teee]

If they both read similar DCR, I would check the magnet gauss between the two?
T

[David Schwab]

Sounds like a short to me, especially the way it works with the volume pot.

How are you checking for a short? With the continuity setting? That wont work for a pickup, it checks for a low resistance between the probes. You already know what the resistance is. You would need to check the AC resistance to see if there was a short.

But the dull sound seems to indicate a short. Are the magnets similar strength on both pickups?

[orangedrop]

Sounds like a short to me, especially the way it works with the volume pot.

How are you checking for a short? With the continuity setting? That wont work for a pickup, it checks for a low resistance between the probes. You already know what the resistance is. You would need to check the AC resistance to see if there was a short.

But the dull sound seems to indicate a short. Are the magnets similar strength on both pickups?

Thank you for replying,
I took it apart and desoldered the wire to the PU. Still the same reading. So it's the magnet wire. I tested three other PU:s that reads OK, like on the pic.
How do I test AC resistance?

Bjorn

[Fluoroscope 5000]

Probably insulation and/or wire corrosion. Either would kill highs. Insulation break down would increase capacitance, while wire surface corrosion would increase resistance on highs which travel closer to the surface without really changing the Rdc reading. Check the capacitance: + to - tied with the poles/chassis/ground wire. There could be micro insulation cracks that would cause eddy currents -- all kinds of cancellations. Sounds like it needs a rewind.

[orangedrop]

Probably insulation and/or wire corrosion. Either would kill highs. Insulation break down would increase capacitance, while wire surface corrosion would increase resistance on highs which travel closer to the surface without really changing the Rdc reading. Check the capacitance: + to - tied with the poles/chassis/ground wire. There could be micro insulation cracks that would cause eddy currents -- all kinds of cancellations. Sounds like it needs a rewind.

OK. Thanks for the reply. What would a normal capacitance be?

Bjorn

[David Schwab]

OK. Thanks for the reply. What would a normal capacitance be?

You really can't measure the capacitance on pickups. Clearly this pickup needs to be rewound. There's no other way around it. Regardless of how it measures, it doesn't sound good.

[JGundry]

Before you slice the wire off the bobbin weigh it without screws, unwind to determine the turn per layer and also read the outer dimeter of the wire with a micrometer, then take 10 feet of the wire by wrapping around a one foot ruler 5 times and take an ohms reading and divide by 10 to get an ohms per foot. If you want to keep the tone as true to original as possible use the above info for the rewind.

[Mike Sulzer]

Probably insulation and/or wire corrosion. Either would kill highs. Insulation break down would increase capacitance, while wire surface corrosion would increase resistance on highs which travel closer to the surface without really changing the Rdc reading. Check the capacitance: + to - tied with the poles/chassis/ground wire. There could be micro insulation cracks that would cause eddy currents -- all kinds of cancellations. Sounds like it needs a rewind.

The skin depth of copper at 10 KHz is a bit under a mm. #42 wire is something like .05 mm in dia. I do not think skin effect is an issue, period.

How is he going to measure the capacitance? Not going to get an accurate reading with a normal meter.

Micro cracks causing eddy currents? Stupidest thing I have ever heard.

You are offering advice on things you know nothing about. Your advice is best ignored.

[orangedrop]

You really can't measure the capacitance on pickups. Clearly this pickup needs to be rewound. There's no other way around it. Regardless of how it measures, it doesn't sound good.

True

[orangedrop]

Before you slice the wire off the bobbin weigh it without screws, unwind to determine the turn per layer and also read the outer dimeter of the wire with a micrometer, then take 10 feet of the wire by wrapping around a one foot ruler 5 times and take an ohms reading and divide by 10 to get an ohms per foot. If you want to keep the tone as true to original as possible use the above info for the rewind.

Very good advice! Thank you

[JGundry]

Very good advice! Thank you

Let us know the info if you can. Particularly the ohms per foot and wire diameter.

[Mike Sulzer]

You really can't measure the capacitance on pickups. Clearly this pickup needs to be rewound. There's no other way around it. Regardless of how it measures, it doesn't sound good.

So you are thinking that a few turns might be shorted out? That is, not enough to change the dc resistance by a noticeable amount, but enough so that you couple in enough energy at high frequencies to increase the damping.

[David Schwab]

So you are thinking that a few turns might be shorted out? That is, not enough to change the dc resistance by a noticeable amount, but enough so that you couple in enough energy at high frequencies to increase the damping.

Yes. I've had that happen on pickups I've wound, and they are very dull sounding. They don't read all that different than they should either.

[JGundry]

Vintage bridge P-90's in particular seem to be prone to shorting out windings but still working. Gibson often put 1/16' spacers between the bridge magnets and the baseplates but still screwed the baseplate flush to the keeper bar. This mashes the magnets against the lower half of the bobbin and sometimes cracks the bottom of the basplates. Over time I think they can end up with quite a few shorted out windings when this happens. I have a P-90 here like that. The ohms per foot and wire diameter and resistance of the pickup does not match up. The bobbin weighs 3 grams more than it should and I have to believe some turns are shorted out.