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I bought a cheap AC panel meter from AliExpress: voltage, current, power, frequency, power factor and energy in one module, 20 A direct through an internal shunt. Because it runs from a separate 8–18 V DC supply and measures from 0 V up to 500 V AC, it suits a variac in a lab. Some listings even call it a variac meter. When it arrived there was no manual in the box. The listing had a few photos and a table of settings, and a search turned up nothing from a manufacturer. That’s normal for this kind of part, and it’s a problem if you want to put one in a real project. How much does it dissipate? Is the supply isolated? What can the alarm output handle?   The settings are also printed on the rear panel, but they are hard to decipher, and the listing’s table gives more information. I haven’t seen this table in other places where the meter is listed, such as Amazon. And when you search online or ask AI, you get partly wrong information.     The only other documentation I ...

In the previous article , I covered the functionality and issues of the D400-007D 400V DC linear power supply by Oltronix.  In this second part, I'll describe the repairs and improvements carried out and also provide a link to the complete KiCad PCB project. Main high-voltage board  On the main high voltage board, the two main filter capacitors (C80, C81) were replaced with two 500 V, 100 µF capacitors. Although the original capacitors were still within specification, they were replaced to ensure reliable operation under modern mains conditions. Equivalent axial capacitors are expensive, so two more cost-effective radial capacitors were used (high-quality 105 °C types).  Two mounting holes were drilled in the PCB, and the capacitors were secured using cable ties. Jumper wires were then used to connect them to the original capacitor pads. The increased capacitance is not an issue due to the high resistance of the transformer secondary. It does not introduce significan...

Update, there is now also a part 2 .  This is another 1960s-era laboratory power supply from Oltronix. This article provides an overview of the design and highlights key considerations when repairing or restoring the unit. To ensure reliable operation on modern mains voltages, several components require upgrading or replacement. Below is a screenshot from the original LABPAC 30-series product presentation:           Front view.      Rear view with the EL34 series pass element visible.