In this episode Shahriar explores the capability of some frequency counters to measure DC voltage. While this may seem non-intuitive initially, the video shows how the architecture of the HP 5245L Nixie tube frequency counter can be used to realize an integrating ADC. A particular plug-in module, the 5265A, leverages the Start/Stop counter control to implement an ADC and display the equivalent DC voltage at the plug-in input. The theory of operation as well as the teardown of the plug-in in shown.

The plug-in module unit is zeroed & calibrated initially and then using a Fluke 5720A the performance of the entire system is evaluated. Despite being in storage for decades, the unit is incredibly accurate and precise.

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In this episode Shahriar attempts the repair of a Keithley 2510 TEC Source-Meter. This instrument is capable of delivering up to 60W of power to any TEC in both polarities. Furthermore, the instrument integrates a flexible temperature read-out capability with 4-wire interface. Lastly, combining both capabilities, alongside a fully customizable PID controller, a precise temperature controller can be realized.

This particular unit returns erroneous readouts in all modes: voltage, current, resistance & temperature. The unit is also generating "squeaky" noises which is indicative of a failing DC-DC converter. The block diagram of the unit is examined and the fault is ultimately traced to a damaged opto-isolator in the isolated switching power supply. Several other components are also faulty which are located via thermal camera. The output Class-D / buck converter circuit & block diagram are also examined in details.

Unfortunately, the custom IC at the heart of the multi-slope ADC is also damaged. The IC is de-capped, examined under a microscope, and compared with its predecessor CPLD counterpart. This Keithley component is sourced from an older high-speed power supply. The correct readout is then verified. Several experiments are demonstrated using the PID controller and a custom TEC setup.

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In this episodes Shahriar repairs a Keithley 2500 Dual Photodiode Meter which does not power up. Teardown of the unit reveals two transformers coupled with two fully independent channels. The unit is able to source voltage in two quadrants as well as measure current down to 1-fA resolution. There is also a separate 5V switching power supply responsible for all digital circuits. This power supply is the reason why the unit does not power up. The fault is quickly traced to a bad switching power supply startup capacitor which is easily corrected.

The unit's voltage source & current measurement performance and operation is verified using a LakeShore M81 SSM system coupled with an SMU-10 module:

https://www.lakeshore.com/products/categories/overview/material-characterization-products/measureready-m81-synchronous-source-measure-system/measureready-m81-synchronous-source-measure-system

The Keithley 2500 is also calibrated using the same LakeShore system. An example of active fiber alignment (primary application for the Keithley 2500) is also demonstrated using a pair of infrared LEDs & detectors.

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In this episode Shahriar experiments with a SunPower CryoTel Mini Cryocooler:

https://www.sunpowerinc.com/products/stirling-cryocoolers/cryotel-cryocoolers/ds-2_1

A three-stage turbomolecular vacuum system is used to reach very low pressures of 10E-6 Torr to provide a suitable environment for the cryocooler. Unfortunately, due to the age of this unit, the maximum applied power is limited to 22W. Despite this, reaching temperatures of -210C is possible with no thermal load. A custom heater element is also used to inject thermal energy into the cold tip and measure the available lift from the engine. A new GUI is also developed to control the cryocooler, measure the cold tip temperature, rejection temperature and applied power as a function of time. This allows several cryocooler modes to be explored.

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In this episode Shahriar explores the mechanism and reasons for fiber optic polarization scramblers. The video discusses the many fascinating methods for embedding data onto fiber optic links. Amplitude modulation, phase modulation, dual polarization (MIMO), wavelength division multiplexing, & spatial division multiplexing are all discussed. The overall architecture of typical coherent optical communication systems is also discussed.

With the theory of coherent communication in place, we can dive into the importance & application of such a polarization scrambler instrument. We further dive into how the unit works followed by a complete optical test system which characterizes the polarization scrambling capabilities. Lastly, a complete teardown shows the ultrasonic, multi-segment transducers which induce birefringence on the fiber segment and cause polarization scrambling.

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