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Power supply conundrum: A wild ghost chase

Posted: 20 Jan 2016     Print Version  Bookmark and Share

Keywords:oscilloscope  trigger  motors  commutators  ammeter 

It was pretty easy, at first. Circa 1985 equipment still in use by telcos, the DUT was a remote-operated twisted-pair telephone cable analyser. We had extensively reverse-engineered and repaired several of these a couple years ago, the OEM was defunct. The customer was so happy with our previous work that another one of these antiques recently came in for repair.

As the SME (subject matter expert) who did the initial reverse engineering I was handed the project. Pulled out my notes and tried to refresh my memory with what I had studied 2 years ago. Easy fix – power supply problem. This was a simple DC-DC converter with no regulation feedback: -48V in to a push-pull centre-tapped step down transformer, linearly regulated +-15Vdc and +5 Vdc out. Several resistors were burned to a crisp, a collector-to-emitter clamp diode was shorted, the 7815 and 7915 linear voltage regulators had failed leaky to ground (fortunate since this hopefully spared the load circuits from damage), five electrolytic capacitors were dried out and measured open on the scope. Non-electrolytic capacitors all tested good. As a precaution, I replaced all power supply silicon, even those that still tested good since they had likely been stressed and were now prone to imminent failure.

The obsolete TIP51 NPN BJT push-pull driver transistors were a bit difficult to find replacements for, but finally found the NTE394 which looked electrically suitable. It was a larger TO-249 package instead of the original TO-218, but it was the same bolt-to-a-heatsink style and all mechanical dimensions looked workable. In this case the metal chassis enclosure itself was used as the heatsink.

The original TIP51 transistors used rubbery conforming thermal insulators and plastic shoulder washers to isolate their collector flanges electrically from the chassis, but these insulators were too small to be re-used with the new transistors. Each NTE394 came with a large mica insulator, requiring a thin layer of thermal compound to be spread on both sides of the insulator for adequate heat conduction. A certain type of thermal grease – Arctic Silver 5 – was stocked in our parts inventory, so I used it. At first I was bit concerned since this grease consisted of microscopic silver particles suspended in an "advanced polysynthetic oil", but its data sheet stated it was not electrically conductive. The mica insulators supplied with the new NTE394s looked a bit beat-up, creased where they had been bent in shipping, but still in one piece so I used them.

Out of caution I did the first test-fire at 20V instead of the normal 48V – the smoke test is only successful if the DUT does not smoke. (Maybe it should be called an anti-smoke test). The scope on the collector of one of the switching transistors showed an oscillation for a few seconds (yeehaw!), then suddenly the oscillation stopped. Subsequent power-ups at higher voltages would result in 'squegging' – a short burst of a few cycles of oscillation as shown in figure 1, then a pause for several milliseconds before repeating.

Figure 1: The squegging repeated every few milliseconds. Vin is -48V.

Q1 base
Ground is Q1 emitter
Q1 collector

That power supply led me on a wild ghost chase. To continue testing at the reduced 20V I temporarily reduced the 46K4 base bias resistor to enable the squegging again, hopefully without blowing anything up. Then followed the most frustrating, head-scratching, excruciatingly painful troubleshooting experience I've ever encountered.

The squegging was not consistent with supply voltage, even with the base bias resistor reduced. Sometimes it would start squegging at 18V, sometimes not until 30V. When not squegging it would draw only about 10 mA, both push-pull transistors biased barely into conduction, so obviously no shorted components.

Figure 2: The circuit is deceptively simple. What could go wrong with this?

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