The original source is a bare coil of wire on what looks like a ceramic tube former (photos above left).

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Quick fix

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I managed to temporarily restore the source by wrapping a 0.15mm thick strand of wire around its broken contact and measured 2.0Ω across it, but it didn't last long. The energy was also much lower than shown in the auction, and now too low to work. The manufacturer quoted me over $2k for a new one, which is ludicrous given it is essentially a bare light bulb filament. I then wondered how I could replace it.

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Alternative source

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By chance I found a couple of much cheaper new FT IR sources on eBay (photos top right) that were clearly intended for a different instrument and bought them, reasoning that if I was unable to come up with my own solution, I could instead use this type with my own custom power supply. They were described as 7.4Ω and I measured one at 7.1Ω. ​

 

Original dimensions
 

Measured with my calipers, the original ceramic tube is 4mm long and has an OD of 1.25mm. A 0.6mm diameter lead wire feeds into it at each end. There are 9 full turns across 2.6mm, making each wire 0.288mm diameter. It was difficult to measure the element wire for fear of breaking it, but I confirmed it is ~0.3mm diameter. The wire colour is grey. The lead wire to which it was joined is 0.6mm diameter.

 

Why the quick fix didn't last

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Deepseek suggested: the wire is likely Kanthal A-1 resistance wire, which goes a grey/green colour when its outer surface oxidises, which creates the insulator that stops the coil windings shorting together. Oxidation occurs when Kanthal A-1 is heated to ~1200°C.

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This would explain why my quick fix no longer worked: the heat of operation likely oxidised the break.

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Although grey and not green, Deepseek doubled down on its assertion it is Kanthal A-1:

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The classic 'gray-green' tint is often a fresh or specific visual description. A mature, fully developed, and stable aluminum oxide (Al₂O₃) layer on FeCrAl is most accurately described as a matte gray color. The key identifier is the bare, unenclosed coil that operates in air. This design is only possible with a self-forming oxide layer, which is the defining characteristic of Kanthal A-1. Nichrome would rapidly oxidize and fail under the same conditions. The gray color is consistent with a well-oxidized Kanthal A-1 element.

 

This ties up perfectly with the new eBay IR sources, which are indeed green.

Kanthal A-1 wire is an alloy of iron (base) + chromium (20-23%) + aluminum (5-6%), aka FeCrAl.

 

Why the element is bare​​​

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The element is exposed to air because the instrument needs an IR source covering a wavenumber range of
4000cm⁻¹ to 650cm⁻¹, otherwise expressed as wavelength range 2.5µm to 16.7µm, i.e. MWIR. Glass stops transmitting and starts absorbing above 2.5µm; quartz above 3.5µm. The element is therefore bare to transmit the maximum radiation, but exposed to moisture in the air. However oxidisation is a benefit as it causes the external surface of the wire to become insulated, otherwise the coil would short circuit.

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Lamp resistance increases over time

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When I first repaired it I measured the source resistance at 2.0 ohms (4-wire HP3478A).

However the analyser reported the IR energy at around 1/3 the maximum expected value.

When an element like this ages from use over time, its resistance increases.

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Deepseek calculated the likely original resistance:

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Resistance of Kanthal A-1 = 19.8Ω/m

Length per Turn: π * 1.25mm = ~3.93mm/turn

Total Wire Length: 9 turns * 3.93mm/turn = ~35.3mm (0.0353m)

Calculated Resistance: 0.0353m * 19.8Ω/m = ~0.70Ω

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The Kanthal is bonded to its lead wire just below the point at which it enters the tube on each side. Clearly it doesn't go all the way through, so this is purely mechanical to support the IR element.

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To replicate the original source I need the former and 35mm of 0.3mm diameter Kanthal A-1 wire.

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Connecting the resistance wire

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​​The Kanthal ends leading away from the coil need to be as short as possible, and the distance between the element and its pcb base is ~18mm. The next stage is to find a way to electrically connect the Kanthal to its normal tin/copper support wires. The original is bonded but I don't have a bonder. 

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Bonding

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Deepseek cautioned any bonding would need only 1-4 joules (see below). ​EBay is flooded with cheap capacitor-discharge bonders from China. Many are adjustable, but none has real specifications, instead typically boasting power wattage levels higher than their fellow sellers but it's highly unlikely they are anywhere close to this. Real bonders cost money, and the only practical solution is DIY.

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Brazing

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​Deepseek suggested using a blow torch to braze the two wires together but I'd have to buy a torch, high temperature flux and silver brazing alloy. A pen torch is cheap and ideal but the flux and alloy harder to find. It's also difficult to create a perfect bond without prior training.

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Crimping

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​I thought of crimping the wires together, of which Deepseek approved. Cheap ratchet crimping tools down to 0.08mm² (wire diameter 0.319 mm) and bootlace ferrules down to 0.5mm² (wire diameter ~0.8mm) are available on Temu. Wrapping the Kanthal A-1 around the lead wire would double it to 0.6mm, making a total of 1.5mm. DeepSeek said a naked ferrule is typically 8-9mm long, which is easily accomodated in the 18mm distance between the element and its pcb base. A second ferrule could potentially be crimped to the emerging lead wire and the new ferrule inserted and soldered into the pcb below.

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Compression fit

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I then thought of using the hollow ceramic tube as the former, and feeding both the Kanthal A-1 Wire and the tin/copper lead wire into the hole, the latter forming the element support. The wires are a push fit at room temperature. At the operating temperature they will expand and force themselves together, effectively creating a compression fit: no need for brazing or crimping and the length of the Kanthal A-1 element wire is as short as possible. Deepseek liked this idea too, and compared the Coefficients of Thermal Expansion:

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Alumina Ceramic: CTE is very low, typically ~6-8 x 10⁻⁶/°C.

Kanthal A-1: CTE is ~14-15 x 10⁻⁶/°C

Copper Wire: CTE is ~17 x 10⁻⁶/°C.

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The metal wires will expand more than twice as much as the ceramic tube.

The 'push fit' at room temperature becomes an extremely tight, compressed fit at operating temperature.

This immense mechanical pressure forces the metal surfaces together, creating a stable, low-resistance electrical contact. It also creates a seal that minimizes oxidation at the joint.
 

​​Deepseek said the ceramic tube should have a minimum wall thickness of 0.2mm to protect against thermal-mechanical shock.

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The smallest sizes I could find were on eBay China:

ID 0.4mm OD 1.0mm​

ID 0.7mm OD 1.5mm

ID 1.0mm OD 2.0mm <<<<

ID 1.5mm OD 3.0mm <<<<

ID 2.0mm OD 3.0mm​

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A 0.6mm lead wire and 0.3mm Kanthal A-1 comes to 0.9mm. The ceramic tube having ID 1.0mm OD 2mm is the closest fit. The Kanthal A-1 would be pushed in first, then the lead wire. Deepseek calculated 5.6 turns would be needed with a 2mm OD tube.

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I would have preferred wrapping the Kanthal around the lead wire but that would be 1.5mm and the nearest ID is also 1.5mm, leaving no tolerance. The OD also jumps to 3mm, significantly reducing the number of turns: DeepSeek calculated 3.75 turns would be needed with a 3mm OD tube.

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The smaller the number of turns, the more critical the position of the element.

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Sealing cement

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Deepseek recommended adding high temperature ceramic cement to secure the wires in place, since they would be loose at room temperature. Readily available cement is sold in bulk, typically to seal furnaces,  and is expensive. DeepSeek recommended making my own by combining Alumina Oxide Al²O³ powder (Temu is cheapest) with Potassium Silicate K²Si³ solution (Deepseek says eBay 'Potsil' is fine).

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insulating the coil

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​However before winding the Kanthal onto the ceramic tube, I would first have to oxidise its surface to insulate its turns, and this has to be done when the wire is straight. Deepseek calculated the necessary electrical parameters:

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With a 0.3 mm diameter wire and a target length of ~35mm (0.035 m):

Resistance (R): ~0.7Ω (our target resistance for the final coil)

Target Temperature: ~1200°C requires a surface load of ~17W/cm²

 

For a 35mm length of 0.3mm wire:

Surface Area ≈ π × 0.03cm × 3.5cm = ~0.33cm²

Power Needed (P): 17 W/cm² × 0.33cm² ≈ 5.6W

 

Therefore, to pre-oxidize it:

Voltage (V): √(P × R) = √(5.6 × 0.7) = √3.92 ≈ 2.0 V

Current (I): P / V = 5.6W / 2.0V ≈ 2.8A

The wire should glow bright orange/yellow at ~1200°C.​

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I was reasonably happy with this, but Deepseek is not always right and I was wary the wire might still oxidise inside the tube, inaccessible once the cement set.​

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​DIY bonder

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The best solution is to build my own capacitive discharge bonder.

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Deepseek:
Estimate Weld Nugget Size:
Thin wire diameter: 0.3mm (Kanthal)
Target Nugget Diameter: 0.3mm * 1.5 = 0.45mm
Nugget Cross-sectional Area: p * (0.45/2)² ˜ 0.16mm²

Apply Energy Density (500-1500 J/mm²):
Low Estimate: 0.16mm² * 500J/mm² = 0.8J
High Estimate: 0.16mm² * 1500J/mm² = 2.4J
This gives a theoretical range of ~0.8 to 2.5

 

Ideal Circuit Parameters:
Voltage: 20V - 50V (Lower voltage promotes current flow over arcing).
Capacitance: 5,000µF - 20,000µF (Large capacitance for a longer pulse).
Current Limiting Resistor: Very low, just to control the peak, e.g., 10mΩ - 50mΩ.

Electrodes: Use a pointed electrode on the Kanthal side and a flat electrode on the copper side.

The higher resistance Kanthal will heat up first.

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Step-by-Step Experimental Procedure: Start low at 1 Joule.

Example: With a 10,000µF cap, V = sqrt(2*1 / 0.01) = ~14V.

 

Test: Make the bond using a cross-wire configuration.

No Bond? Increase energy by 0.25J - 0.5J and repeat.
Weak Bond? Increase energy slightly.
Sputtering/Splash? Energy is too high. Decrease immediately.

 

Summary:
Start Very Low at 1J.
The sweet spot will almost certainly be between 1.5J and 3.5J.
 

Schematic to follow

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Full circle

 

I can now return to using a smaller ceramic former. eBay China offers solid rods with a finer range of OD and could now select 1.2mm, close to the original 1.25mm. Deepseek calculated a 1.2mm ceramic former would need 9 to 10 turns (the original was 9).

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OD 1.0mm

OD 1.2mm <<<<

OD 1.5mm

OD 2.0mm

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The coil wire would now best be oxidised by clamping its 35mm length between two metal vices and 2V at 2.8A applied to them. The insulated coil would then be wound on the ceramic former and the bare ends from within the vices wrapped around the lead wires, enclosed in nickel plate and bonded much the same as a crimp, except the bonder's free wires allow a closer fit to the coil.

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Mechanics

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The IR source leads are soldered into a 17mm diameter round pcb and lead to large copper slugs that mate with spring-loaded test clips in the main unit. If watch strap spring holders can be soldered, I could potentially use two to create my own mechanical mount into a pcb, but I no longer have a means of pcb fabrication and I've long forgotten how to use pcb CAD.

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Even though it's broken, I don't want to remove the existing element from its mount because I want to keep it as a reference. I therefore need to build my own assembly to replace it. I can't make a pcb but I can cut a 17mm fibreglass disc out of a fibreglass pcb blank, drill it and hopefully solder the bottom end of the watch support into it, then either solder the lead wire into top end, or else bond it.
I might even be able to bond together the Kanthal wire, lead wire and top of the watch holder as one.

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IR reflective surface

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Finally, Deepseek suggested the brittle white reflective surface within which the IR source is located, is very likely Barium Sulphate BaSo⁴ (eBay) which it said I could replicate by mixing with Potsil for my own mechanical design of the coated base section that slots into the rest of the BaSo⁴ coated IR cavity.

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Electrics

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​Before building the replacement however, I needed to determine if DeepSeek's assumptions were correct, and measure the actual drive voltage to the IR source. Since it was now broken again, I replaced it with a 2Ω resistor. 

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