![Front Panel](https://www.experimental-engineering.co.uk/wp-content/uploads/2021/12/IMG_20211211_174223-scaled.jpg)
![Front Panel](https://www.experimental-engineering.co.uk/wp-content/uploads/2021/12/IMG_20211211_174223-300x225.jpg)
Here we have an optical power meter, vintage 1991! This is a Gentec unit, with scaling to to handle up to 10W with a suitable powerhead. Powered either with 4 PP3 9v batteries, or from a 24v DC jack on the rear, this unit is quite versatile. I managed to get this for very little money on eBay – similar units cost over £1,500 new – without a powerhead.
![Meter Movement](https://www.experimental-engineering.co.uk/wp-content/uploads/2021/12/IMG_20211211_174228-300x225.jpg)
The unit is completely analogue, with no digital circuitry at all. The meter movement has a mirror on the scale for parallax correction. Under the movement are the main power switch & battery test switch, which uses the meter itself to show battery level.
![User Controls](https://www.experimental-engineering.co.uk/wp-content/uploads/2021/12/IMG_20211211_174237-300x225.jpg)
The right hand side of the unit has the Zero adjustment, the range switch, and the DB15 powerhead input connector.
![Board Left](https://www.experimental-engineering.co.uk/wp-content/uploads/2021/12/IMG_20211212_124936-300x225.jpg)
Taking the unit apart, with just 4 allen screws on the case reveals the mainboard. There’s very little back here! The active components are just Op-Amps – An OP07 ultralow offset in the bottom left corner is most likely the front-end amplifier, along with a few LF442ACN precision JFET input devices in the same area. The other amplifiers are LT1001 precision devices, with a 10mA output current capability. Most of the passives in this area are also high-stability & high precision parts.
![Board Right](https://www.experimental-engineering.co.uk/wp-content/uploads/2021/12/IMG_20211212_124940-300x225.jpg)
The other side of the board handles the meter movement, and the power input section. There is a small daughterboard with another LT1001 Op-Amp on board, along with some passives, and the battery inputs go into here, however I’m not exactly sure what this is doing – there is another connection to the rear panel 1v analogue output BNC jack, so it may be the driver for that section. The 24v input is a single DC rail, however the 4 PP3 battery holders are wired as parallel pairs of batteries back to back, so a split +9v/-9v supply is generated.
![Main PCB](https://www.experimental-engineering.co.uk/wp-content/uploads/2021/12/IMG_20211212_124943-300x225.jpg)
An overall view of the board shows the wiring back to the battery holder, 1v analogue output jack & DC input connector.
[…] thermopile sensors with TECs – but no way to verify any results until now. Since I have my Gentec meter to calibrate against, I can finally get on with the project. In the photo above, is the […]