REALLY WEIRD pickup rewinding question (Tele content)

[Steve Conner]

With 50 years worth of beer, sweat and drool spilled onto it, anything can become conductive. Dipping it in hot nitro lacquer might not have helped either.

The eyelet boards in some old Fender amps were notorious for going conductive and causing really weird faults.

Was Forbon really used for insulation in high voltage transformers? Maybe it started out as a coil bobbin material and Leo adapted it to pickups.

The microwave test is kind of harsh. There are materials that will heat up, but are still perfectly good insulators at DC and audio frequencies.

[David Schwab]

With 50 years worth of beer, sweat and drool spilled onto it, anything can become conductive.

He said the magnets were clean, so it doesn't sound like things seeped into the pickup. That's when you get the "Inner Coil Pole Corrosion" going on.

[ken]

I'm sorry about that, I shoulda clarified.

When I said the nitro lacquer was 'hot', I meant it had a lot of fresh thinner mixed in it so it was probably more chemically active than normal. In other words, when I opened the can, I saw the lacquer was getting viscous so I added some thinner to it, stirred it up, and immediately dipped the bobbin in it. I think maybe I should have waited a couple of hours or so after I mixed it up to dunk my bobbin. Also, I was amazed at how clean the magnets still were, after all those years they almost looked like they were chromed.

Once a board becomes conductive, I'm told the only way to stop it is to find out exactly where the leakage path is and cut or drill right through the board across the path to break it. Once you cut the board, insulate both edges of the cut. You can imagine what this would do to the value of a 1950's Fender amp with this problem. My ham buddy calls this problem 'amp cancer', and he says that there is no real way to know what causes this, except that it seems to be more common near heat generating parts like large power resistors. When I told him about the pickup, he couldn't figure it out.

I'm told that the light gray Forbon has no carbon black in it, so this would be the best kind to use for high voltage circuits. I wonder why Leo used the black Forbon, since the light gray version is usually the cheapest anyway. It seems Forbon is the same chemically as MDF, only the Forbon uses sodium silicate? glue and the MDF is dried under low heat and only enough pressure to make it flat.

Anyways, I'm going to make my customer a new bottom plate to salvage the pickup, and he's going to let me keep the old plate for my collection.

ken

[David Schwab]

It seems Forbon is the same chemically as MDF, only the Forbon uses sodium silicate? glue and the MDF is dried under low heat and only enough pressure to make it flat.

Not at all. Not the same ingredients or manufacturing process.

From Wikipedia:

Medium-density fibreboard
Medium-density fibreboard (MDF) is an engineered wood product formed by breaking down hardwood or softwood residuals into wood fibres, often in a defibrator, combining it with wax and a resin binder, and forming panels by applying high temperature and pressure.

Formaldehyde resins are commonly used to bind MDF together, and testing has consistently revealed that MDF products emit urea-formaldehyde and other volatile organic compounds that pose health risks at sufficient concentrations, for at least several months after manufacture.

Vulcanized fibre
Vulcanized fibre is a laminated plastic composed of only cellulose. The material is a tough, resilient, hornlike material that is lighter than aluminium, tougher than leather, stiffer than most thermoplastics.

The process started with paper made from cotton rags. Before the processing of wood pulp and chemical wood pulps in the mid-19th century, the dominant fibre source for paper making was cotton and linen rags. The cotton rag sheet produced for conversion to vulcanized fibre is made like a sheet suitable for saturating. A paper is made for saturating by omitting any sizing additive, either beater added or surface applied. Today most paper sheets made for writing, printing, and coating have internal (beater added) sizing provided by rosin, alkyl succinic anhydride (ASA), or alkyl ketene dimer (AKD) and surface sizing provided by starch. A sheet made for saturating would have none of those chemical ingredients. The unsized saturating cotton fibre paper prepared for vulcanized fibre would be passed through a vat containing a zinc chloride solution.

The final product is a homogeneous nearly 100%-cellulose mass free from any artificial glues, resins, or binders.

Here's the spec sheets for Forbon

http://www.nvf.com/forbon/menu.htm

[ken]

So, what you're telling me is that MDF and Forbon are both cellulosic products, with the only nonchemical difference being the origin of the raw materials?

This sounds a lot like US 'paper money' actually being made from cotton rags...

ken

[David Schwab]

So, what you're telling me is that MDF and Forbon are both cellulosic products, with the only nonchemical difference being the origin of the raw materials?

You are over simplifying it. MDF is particleboard, or sawdust and glue. Forbon is cellulose. It contains no resin or glue at all.

They both contain cellulose, but so do other things, like Nitrocellulose lacquer and all living green plants. Is Nitrocellulose lacquer chemically the same as forbon?

You said; "It seems Forbon is the same chemically as MDF". It isn't anymore than wood "is the same chemically", and it's not. It has cellulous in it but a lot of other stuff too. Wood contains a composite of cellulose and lignin.

Try making a piece of MDF as thin as Forbon and see how strong it is. It's not even all that strong when its thicker. MDF is made from small particles of wood and glue. Forbon is not sawdust and glue.

Electrical grade vulcanized fibreboard, AKA fishpaper is made from cotton.

And yes, paper money is made from cotton paper. Cotton paper is made from cotton linters and used cotton rags. Is there a reason you don't believe that?

Cotton paper - Wikipedia, the free encyclopedia

[big_teee]

You are over simplifying it. MDF is particleboard, or sawdust and glue. Forbon is cellulose. It contains no resin or glue at all.

They both contain cellulose, but so do other things, like Nitrocellulose lacquer and all living green plants. Is Nitrocellulose lacquer chemically the same as forbon?

You said; "It seems Forbon is the same chemically as MDF". It isn't anymore than wood "is the same chemically", and it's not. It has cellulous in it but a lot of other stuff too. Wood contains a composite of cellulose and lignin.

Try making a piece of MDF as thin as Forbon and see how strong it is. It's not even all that strong when its thicker. MDF is made from small particles of wood and glue. Forbon is not sawdust and glue.

Electrical grade vulcanized fibreboard, AKA fishpaper is made from cotton.

And yes, paper money is made from cotton paper. Cotton paper is made from cotton linters and used cotton rags. Is there a reason you don't believe that?

Cotton paper - Wikipedia, the free encyclopedia

I wonder why Pickup makers in the 50s and 60s didn't use more thin fiber glass for bobbin material.
Maybe cost? I think fiberglass was a hot new item in late 50s and 60s.
G10 or Garolite is still as cheap as buying fiber board?
T

[Spence]

I suspect that what's really happening here is that your coil is exposed somewhere and making contact with the cover when you put it on. This is very common with tele pickups. Desoldering the cover will cure the problem in a way but it's not really a cure is it. I have seen many examples where people have saved the situatation by taping over the outside of the coil and reinstating the cover.
Personally, I would start again.

[ken]

Cotton fibers can be used for lots of different things. In fact, nitro lacquer is actually very well digested cotton fibers too, and gunpowder (guncotton) is also made of nitrocellulose simply processed differently than lacquer.

I was born in Appleton, WI, the original home of the Institute of Paper Chemistry. Every year in school I had to go there for a science class field trip and watch paper being made from various materials, including cotton rags. Yes... I know paper money is made from cotton rags. I wish it was made of gold though because I could buy more.

All I was actually trying to say is that Forbon is a green (recyclable) product made of cellulose, so I couldn't figure out why it wasn't used more than it is.

I wonder why Pickup makers in the 50s and 60s didn't use more thin fiber glass for bobbin material.
Maybe cost? I think fiberglass was a hot new item in late 50s and 60s.
G10 or Garolite is still as cheap as buying fiber board?

Leo was incredibly cheap, and in the 1950's fiberglass was still relatively expensive compared to Forbon. The earliest fiberglass circuit board construction that I ever saw myself was the behind the dashboard gauge wiring on my 1964 Olds Cutlass.

ken

[David Schwab]

I wonder why Pickup makers in the 50s and 60s didn't use more thin fiber glass for bobbin material.
Maybe cost? I think fiberglass was a hot new item in late 50s and 60s.
G10 or Garolite is still as cheap as buying fiber board?
T

How many pickup makers were there in the 50s and 60s? What materials did they use? You had Fender using forbon. Carvin using plastic (like on the Mosrite pickups). Gibson using plastic. DeArmond/Rowe using various things.

Fiberglass was probably too much trouble. Garolite and Bakelite were more common.

[David Schwab]

Cotton fibers can be used for lots of different things. In fact, nitro lacquer is actually very well digested cotton fibers too, and gunpowder (guncotton) is also made of nitrocellulose simply processed differently than lacquer.

Nitro lacquer was developed using nitrocellulose guncotton left over from WWI! Good use of the material.

As an aside, I tested some gray and black forbon from Mojo, and the black is not conductive, even when pressing the tip of the probe into it.

[ken]

I read a book once on how to make gunpowder, and it said how nitro lacquer was made from guncotton. Geez, I thought my job was bad... at least I don't have to worry about blowing myself up.

Anyway, I think I might have a possible answer for why Forbon becomes conductive...

According to my research, compacted and baked carbon dust is what a carbon comp resistor is made of. You take carbon dust, add a liquid binder, press it into sticks, and how hard you press it and how much you bake it dry determines how much actual resistance you have in your resistor. Carbon film resistors simply spray the liquid carbon onto a ceramic core before they are baked.

Now, Forbon is colored black with carbon dust. Let's say that the carbon in the Forbon wasn't mixed correctly, leaving localized concentrations of carbon ("lumps")
in your mixture. Now, you extrude the Forbon into a sheet, press it, and bake it into our flatwork. These processes would concentrate the carbon black lumps under pressure at random wherever in the finished sheet they would occur. Now, this is fine if the sheet has no terminals in it anywhere near the 'lump'. But let's say you accidentally drill into one of these lumps while inserting a terminal eyelet, and place another eyelet close enough to it to make the connection. Voila... instant high impedance short. Or, the board is heated a bit, say with a preamp voltage dropping resistor, and/or there is enough voltage difference between the two eyelets, you still have a conductive board. Remember that these carbon concentrations would not have to appear on the surface of the board at all to be conductive, they can be totally surrounded by properly mixed Forbon and still make a resistance. This is why you could even use needle probes to test the boards and still have no errors. Also, since light gray Forbon has no carbon in it at all, there would be no issues with conductivity.

I wonder how many stuffed amp boards Fender ended up throwing away because they were conductive at the factory and didn't pass the voltage tests... or why Leo didn't use light gray Forbon to make his amps?

ken

[David Schwab]

Now, Forbon is colored black with carbon dust. Let's say that the carbon in the Forbon wasn't mixed correctly, leaving localized concentrations of carbon ("lumps")
in your mixture. Now, you extrude the Forbon into a sheet, press it, and bake it into our flatwork. These processes would concentrate the carbon black lumps under pressure at random wherever in the finished sheet they would occur. Now, this is fine if the sheet has no terminals in it anywhere near the 'lump'. But let's say you accidentally drill into one of these lumps while inserting a terminal eyelet, and place another eyelet close enough to it to make the connection. Voila... instant high impedance short. Or, the board is heated a bit, say with a preamp voltage dropping resistor, and/or there is enough voltage difference between the two eyelets, you still have a conductive board. Remember that these carbon concentrations would not have to appear on the surface of the board at all to be conductive, they can be totally surrounded by properly mixed Forbon and still make a resistance. This is why you could even use needle probes to test the boards and still have no errors. Also, since light gray Forbon has no carbon in it at all, there would be no issues with conductivity.

I agree. Assuming it is colored black with carbon, or lampblack (once used as caramel coloring in Coke!), I think the combination of lacquer thinner and hear made some leach to the surface.

[ken]

After all these posts on what Forbon is made of, I wonder what it would be like if it was made of hemp instead of cotton. Tone Tubby pickups, anyone?

ken

[David Schwab]

After all these posts on what Forbon is made of, I wonder what it would be like if it was made of hemp instead of cotton. Tone Tubby pickups, anyone?

ken

Irie mon!