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

​

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

​

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

​​​

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

​

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

​

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 Aluminium oxide layer, which is the defining characteristic of Kanthal A-1. Nichrome would rapidly oxidise and fail under the same conditions. The gray color is consistent with a well-oxidised Kanthal A-1 element.

 

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

However there are different chemical compositions of Kanthal, aka FeCrAl, see [T18].

Kanthal A-1 wire alloy composition: Cr 22% + Al 5.8% + Fe.

Kanthal D   wire alloy composition: Cr 22% + Al 4.8% + Fe.

Kanthal D   is normally 0.30mm (AWG 29 [0.28mm is also expressed as AWG 29]): 20.5Ω/m

Kanthal A-1 is normally 0.32mm (AWG 28): 18.0Ω/m

I will therefore assume mine is also 0.32mm, as my calipers cannot differentiate that finely.  

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Why the element is bare​​​

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The element is exposed to air because the instrument needs an IR source covering wavenumber range
4000cm⁻¹ to 650cm⁻¹ otherwise expressed as wavelength range 2.5µm to 16.7µm, i.e. MWIR-LWIR. 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 beneficial as it causes the external surface of the wire to become insulated, otherwise the coil would short circuit.

​​​

Lamp resistance increases over time

​

After 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. Kanthal being an alloy of FeCrAl, it is due to the aluminium within it oxidising, and there is no way of reversing this.

​

Deepseek calculated the likely original resistance:

​​

Resistance of Kanthal A-1 = 18Ω/m

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

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

Calculated Resistance: 0.0353m x 18Ω/m = ~0.6354Ω

​​

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.

​

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

​​

​​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

​​​​​​

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.

​​​​

Brazing

​

​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

​

​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 1.2mm which corresponds to the barrel size for a nominal cross-sectional area of 1.5 mm², aka 16AWG bare ferrule, which is typically rated for a wire diameter of 1.29mm to 1.31mm.

 

A naked ferrule is typically 8-9mm long, which is easily accommodated 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.

​

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.

​​​​​​​

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​

​

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: assume 6.

​

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.

​

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).

​​​​​

insulating the coil

​

​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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Total Wire Length for 6 Turns

Length per Turn = π x 2.00mm = 6.283mm

Total Length = 6 turns x 6.283mm/turn = 37.7mm (0.0377m)

 

Power Needed for Oxidation at ~1200°C

Surface Area: First, find the surface area of the 37.7mm length of wire.

Wire Diameter = 0.32mm = 0.032cm

Length = 37.7 mm = 3.77 cm

Surface Area = π x diameter x length = π x 0.032cm x 3.77cm ≈ 0.379cm²

Power Density: ~17W/cm² (required to reach ~1200°C for Kanthal oxidation)

Power Needed (P) = Power Density x Surface Area

P = 17 W/cm² x 0.379cm² ≈ 6.44W

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Voltage and Current for Pre-Oxidation

Resistance of Straight Wire: R = (18.0Ω/m) x 0.0377m ≈ 0.679Ω

Ohm's Law Calculations:

Voltage (V) = √(P x R) = √(6.44W x 0.679Ω) = √4.37 ≈ 2.09V

Current (I) = P / V = 6.44W / 2.09V ≈ 3.08A

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Summary:

For 0.32mm Kanthal A-1 wire on a 2mm Former

Target Turns = 6

Total Wire Length = 37.7mm

Pre-Oxidation Power = 6.44W

Pre-Oxidation Voltage ~2.1V

Pre-Oxidation Current ~3.1A

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

​​

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.

I think the best solution in terms of minimum Kanthal length is bonding, and crimping is second.​

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​DIY bonder

​​

I decided to build my own bonder, see Projects: Capacitor Discharge Bonder].

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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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Recalculated for 1.2mm former:

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Total Wire Length Needed for ~0.7Ω

Resistance per mm = 0.018Ω/mm (from 18Ω/m for 0.32mm wire)

Total Length = Target R / Resistance per mm = 0.7Ω / 0.018Ω/mm ≈ 38.9mm

 

Number of Turns on 1.2mm Former

Length per Turn = π x 1.2mm ≈ 3.77mm/turn

Number of Turns = Total Length / Length per Turn = 38.9mm / 3.77mm/turn ≈ 10.3 turns

 

Conclusion: Wind approximately 10 turns of the 0.32mm Kanthal A-1 wire around the 1.2 mm former. This will give a resistance of ~0.7Ω.

 

viability of 0.7Ω load

​

​​Before building the replacement I needed to determine if DeepSeek's assumptions were correct, and 0.7Ω would be an acceptable IR load. I had already observed the analyser flags an error when the IR source is absent. Since it was now broken again, I soldered a 2Ω aluminium clad power resistor across its internal connector pins. The analyser produced a steady (if noisy) ~2.4Vdc across the source and reported it present but with no energy.

 

I replaced the 2Ω with 0.75Ω (4-wire HP3478A), the closest I had to Deepseek's suggested 0.7Ω replacement and again the analyser was happy, still outputting a steady ~2.4Vdc.

 

This equates to (Ohm's Law: P = V²/R = 2.4V²/0.75Ω = 7.68W; 3.2A) ~7.7W.

 

I now have reasonable confidence to build a replacement 0.7Ω IR source (2.4V²/0.7Ω = 8.23W; ~3.4A).

The original may be rated 10W (R = V²/P = 5.76/10 = 0.576Ω; ~4.2A @ 2.4A) [0.57Ω ~ 10.1W; ~4.2A].​​​​

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Kanthal power handling

​​

I asked DeepSeek if a 35mm length of Kanthal with 2.4V across it could handle the wattage:

​

Source Voltage, V = 2.4V

Coil Resistance, R = 0.7Ω

Kanthal A-1 Diameter, d = 0.3mm = 0.03cm

Total Wire Length, L = 35.3 mm = 3.53cm

Kanthal A-1 Resistivity, ρ ≈ 18 Ω/m (1.45Ω·mm²/m)

​

Calculate Operating Current and Power:

I = V/R = 2.4V/0.7Ω ≈3.43A
P = I² × R = (3.43A)² × 0.7Ω ≈8.23 W

 

Calculate Wire Surface Area:

Asurface = π × d × L = π × 0.03 cm × 3.53 cm ≈ 0.332 cm²

 

Calculate Surface Load (Power Density):

Surface Load = P/Asurface = 8.23W/0.332 cm² ≈ 24.8 W/cm²

 

Conclusion:
The calculated surface load of 24.8W/cm² is well within the standard operating range for Kanthal A-1 in open-air elements (typically ~40-60W/cm²). This confirms the 35mm length of 0.3mm Kanthal A-1 wire, configured as a 0.7Ω coil, can safely and reliably withstand the 8.23W power dissipation from the analyzer's 2.4V supply.

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Kanthal max power

​

Lastly, I asked DeepSeek to determine the maximum power the 0.3mm Kanthal A-1 can withstand at 2.4V:

​

To determine the minimum safe load resistance, we must find the maximum power (and thus current) the Kanthal A-1 wire can handle, and then calculate the resistance that would draw that power at 2.4V.

 

Establish the Maximum Power Limit:
The key limiting factor is the surface load. Using the conservative upper limit of 40W/cm² for Kanthal A-1 in air:

Wire Surface Area (from previous calc): Asurface ≈ 0.332 cm² 

Pmax = Surface Loadmax × Asurface = 40 W/cm² × 0.332 cm² ≈ 13.28W

 

Conclusion:The Kanthal A-1 coil can withstand a maximum of approximately 13.3W.

 

The 0.7Ω design coil 8.23W provides a 38% safety margin below this maximum limit.

​

​

Next, calculate the lowest permissible value in ohms with 2.4V across it:

​

Calculate Maximum Current and Minimum Resistance:

Pmax = Imax² × R

Imax = Pmax / R = 13.28W/0.7Ω ≈4.36A

​

Using Ohm's Law with the fixed 2.4V source:

Rmin = V/Imax = 2.4 V/4.36A ≈0.55Ω

 

At the analyzer's fixed voltage of 2.4V, the lowest safe load resistance is 0.55Ω.

​

The 0.7Ω design coil (drawing 8.23W) provides a 21% safety margin below this maximum limit.

 

We shall therefore not attempt to improve upon this. 
 

Kanthal oxidisation

​​​

The Kanthal outer surface must be oxidised in order to insulate it from other turns when it is wound as a coil. Kanthal oxidises when exposed to a high temperature, and the easiest way of achieving that is to pass a current through it:

​

Power Needed for Pre-Oxidation

Surface Area: of the 38.9mm straight wire.

Wire Diameter = 0.32mm = 0.032cm

Length = 35.3mm = 3.53cm

Surface Area = π x 0.032cm x 3.53cm ≈ 0.332cm²

Power (P) = Power Density x Surface Area

P = 17W/cm² x 0.332cm² ≈5.64W

 

Final Pre-Oxidation Parameters:

Power: 5.64W

Voltage: √(5.64W x0.7Ω) ≈ 1.99V

Current: 5.64W / 1.99V ≈ 2.83A

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The coil wire would ideally be oxidised by clamping its 35mm length between two metal vices and 2.0V at 2.8A applied to them (Amrel PPS-1202 programmable PSU). The insulated coil would then be wound on the ceramic former and the bare ends from within the vices wrapped around the lead wires and bonded.

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Mechanics

​

The IR source leads are soldered into a 17mm diameter round pcb and lead to large copper slugs on the reverse that mate with spring-loaded test clips on a small pcb near the base of the analyser.

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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 solder the lead wire into the top end, or perhaps for more strength, a solderable female connector contact with the lead wire in the contact end [no solder to melt] CHECK IF I HAVE ANY SUITABLE

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eBay china sells flat head copper rivets that could mimic the original copper slugs.
I need to measure the original slug diameter, 6mm below is a guess:

18/11/25 M2 x 3mm x5 = ~6mm dia x 1.5mm thk head on M2 x 3mm rod: £1.31 + £1.79 post = £3.10/5 = £0.62ea

These look a little too perfect (plated brass?)

https://www.ebay.co.uk/itm/176053045710
 

18/11/25 M2 x 3mm x10 ~6mm dia x 1.5mm thk head on M2 x 3mm rod: £1.19 + £1.79 post = £2.89/10 = £0.29ea
These look more like copper:

https://www.ebay.co.uk/itm/274442806036

 

IR reflective surface

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Deepseek suggested the brittle white reflective surface coating the cavity that houses the IR source is very likely Barium Sulphate BaSo₄ (eBay UK 100g 99% £6.69), which it said I could replicate by mixing with Potsil. The source is on a similarly coated pedestal that slots into the cavity. It is this pedestal that I need to replicate.

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The function of the coating is to reflect the IR energy into the analyser. However where it lies behind the source, it will reflect emitted heat back towards the source, increasing its temperature and degrading power safety margins, another reason not to deviate from 0.7Ω:

​

Reasoning based on established material properties:

BaSO₄ as an IR Reflector: Barium sulphate is a classic, high-performance diffuse reflector for infrared radiation, especially in the mid-IR range. The cavity lining opposite the source will directly reflect a significant portion of the radiated IR energy back onto the coil.

 

Thermal Impact:

This reflected energy is re-absorbed by the Kanthal wire. To maintain radiative equilibrium (where power radiated equals power input), the wire's temperature must increase.

A hotter wire radiates more power according to the Stefan-Boltzmann law (P ∝ T⁴).

Conclusion:

The BaSO₄ lining creates a more efficient 'oven' around the source. For the same electrical input power (e.g., 8.23W), the coiled filament will reach a significantly higher operating temperature than it would in open air. This is beneficial for the instrument's output intensity but must be considered for the coil's longevity.

​

Improvement Note:

the previous power calculations (8.23W operational, 5.64W for pre-oxidation) based on open-air surface loads are now a conservative starting point. The coil will likely reach its target temperature and IR output at a slightly lower electrical power due to the reflected energy.

​​​​

WHAT IS THE COIL HEAT % INCREASE DUE TO INTERNAL REFLECTION?

I MUST MEASURE THE CAVITY DIMENSIONS.

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Detector

​

I found a hand written note inside the analyser saying 'CPV 3.0mV'.

 

Deepseek offered an explanation and expanded the context:

​

'CPV' in the context of an FTIR analyzer almost certainly stands for Cell Photo Voltage. The 'cell' refers to the detector. In many FTIRs, especially older models, the detector is a pyroelectric device, usually a DTGS [Deuterated L-Alanine doped Triglycine] detector, which generates a small voltage signal in response to changes in infrared light intensity. It is Doped with Deuterium to raise its Curie temperature to ~59°C.

​

The CPV is the dc bias voltage measured at the detector when the source is on and the instrument is in a stable state, but before the moving mirror starts scanning for an interferogram. It's a measure of the total average IR power falling on the detector and is directly proportional to the intensity and temperature of the IR source. 'CPV = 3.0mV' was likely the healthy, baseline reading of the Cell Photo Voltage taken by a technician when the instrument and its source were known to be working correctly.

​

In fact, I suspect it may be the measured output of the detector with a brand new full power IR source,

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Detector board

​

It wasn't long before I found the detector board complete with helpful labels on its silkscreen.

​​

The detector pcb is labelled 'tmp ctrl DTGS' and along the pcb edge I found test points together with two pots located side by side, although no labels on the pots. However there are two labelled test points DET- DET+ to the left of one pot, suggesting it is associated with them.

 

To the right of the other pot are test points with the following labels: INT, INV, and power test points V+, V-, GND. Further into the pcb and very close to the detector are 3 more test points labelled: SP1, SP2, DIFF. The detector appears to be in a TO-66 package mounted on a simple aluminium C shaped heatsink. It looks like there may be 8 pins on the package which is gold in colour, but has no visible markings. Date codes on other devices suggest it was manufactured ~2005.

​​

Deepseek commented:

​

This is a typical configuration for the preamplifier and bias electronics for a pyroelectric detector like a DTGS. Here is a logical breakdown of the test points and pots.

​

The Primary Detector Signal:

DET+ and DET- are almost certainly the points where the CPV (Cell Photo Voltage) is measured in mV.

​

The pot immediately associated with these test points is very likely the CPV adjust or gain adjust pot. Tweaking this will directly change the measured dc bias level.

​​

MEASURE DET+/DET- OUT OF CURIOSITY [DOUBTLESs NEEDS A WORKING SOURCE]

​

The Second Pot & INT, INV Test Points is likely related to the interferogram signal processing:

INT: Likely stands for Integrator Output: the raw signal from a DTGS detector needs to be integrated to form the interferogram.

INV: Likely stands for Inverter Output as the signal is often inverted at some stage in the chain.

​

The pot between these points is likely the balance or offset adjust for this stage of the amplifier.
If so, its purpose is to minimize dc offset in the ac signal chain to prevent saturation.

​​

I think it's more likely associated with the TEC, and adjusts its temperature.

​

A DTGS detector is a pyroelectric device. It often has two identical, back-to-back elements to compensate for common-mode noise and temperature drift.

​

SP1 and SP2 almost certainly stand for Signal Plate 1 and Signal Plate 2. These are the individual outputs from each of the two detector elements.

​

DIFF is the Differential Output, the final signal difference between SP1 and SP2 (DIFF = SP1 - SP2).

This cancels out noise that affects both elements equally.

 

The Gold-Color, 8-Pin Detector is likely a hermetically sealed package to protect the hygroscopic DTGS crystal. The 8 pins are usually for:

Case/Ground

Signal Plate 1

Signal Plate 2

Possibly a separate guard connection

Connections for a temperature sensor (as indicated by 'tmp ctrl' on the PCB)

Heater connection (many DTGS detectors have a small internal heater to stabilize them just above ambient temperature for reduced drift).

​

A brief summary in the analyser hardware manual confirmed DeepSeek's description:

'Detector: 1.3mm diameter, thermoelectrically cooled dTGS'.

 

TEC function

​

The detector is mounted on top of the TEC inside the TO-66 and the heatsink dissipates the TEC heat.

 

The TEC primarily stabilises the detector temperature near its Curie point. The DTGS detector output voltage is highly sensitive to temperature changes caused by absorbed infrared radiation. The material has a Curie point (the temperature above which it loses its pyroelectric properties) close to room temperature. The TEC maintains the detector at a stable operating temperature just below this point to prevent performance fluctuations and signal drift caused by ambient temperature changes.The TEC ensures the DTGS detector operates at peak sensitivity and maintains a linear response over a wide range of IR radiation.
 

I think it is likely the pot on the right sets the TEC temperature. There is a TO-220 power transistor near the detector and the pot & TEC may be associated with that. 

​​​

Detector is unknown

​

DeepSeek could not find a matching DTGS detector from 2005 and in 2025, parts and datasheets seem to be virtually non-existent: I could not find any dual element DTGS detectors on the web. I did find a range of single element DLATGS detectors from Leonardo, with a TEC cooled one in an 8-pin TO-38 (a smaller version of the TO-66) and a similar pinout to DeepSeek's description, and their 99 series has a 1.3mm detector admitting 0.3µm to 40µm.

​

Leonardo's brochure explains the DLATGS is a modern variant of the original DTGS detector, differing by 'being Doped with L-alanine to prevent permanent depoling after excursions above Curie temperature'.

​

Leonardo has been around a long time under different names so potentially they could have made the original in 2005. Alternatively it may have been a custom part, for which no records would publicly exist. They do degrade over time, fingers crossed mine still works.

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Repair

​

I think there is enough here to work with, All I need do is fit the replacement IR source, see what it does, and compare the CPV. If the analyser reports sufficient energy I'm quids in. If it says it's too high I think it's less risky increasing the Kanthal length than twiddling an unknown pot, no matter how convincing DeepSeek sounds: the CPV value will remain no more than a curiosity point.

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