Interesting fault and fix on a Dixell XW70K refrigeration controller

This started as a very ordinary fault with an easy fix, but then I discovered that a section of the circuitry was getting extremely hot and had to explore further. It turns out that a classic fault was causing a weird knock-on effect that was so unusual that I felt the need to reverse engineer that section to see why it was happening.

The main culprit was a classic bulging capacitor, but it's much less common to find that fault in equipment with traditional low frequency power supplies. It's much more common with modern high frequency supplies. The use of the Glan branded capacitor may be the issue here. The circuitry shouldn't really be stressing it with a guesstimated current demand of around 300mA with active relays.

The unit has the facility to connect a remote display and button interface with just two wires, and it appears to achieve that by using a current regulated supply to that module, and communicating in both directions by shunting its power wires in pulses.

The main capacitor failure caused an unexpected situation where the processor was unable to initialise, and by default the display output data shunt is active, meaning that the current regulation circuit was dissipating high power in its main transistor continuously.

As the capacitor progressively failed the unit would potentially have started glitching and resetting intermittently, getting progressively worse until it just didn't start up.

One huge advantage of doing an on-site repair on these units would be that it keeps their existing settings intact, which saves reprogramming the unit for a specific system from scratch. That can be a real time saver if you have to work out what everything is doing and how it's connected.

Without a working reference unit for voltage measurements, and perhaps a remote display module too, it's hard to say if other damage has occurred. But the configuration of the circuitry and now cool transistor suggests that the fault is fixed and these units have been saved from becoming expensive landfill.

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