Chasing wild ghosts: Page 2 of 7

January 10, 2016 //By Glen Chenier
Chasing wild ghosts
Glen Chenier describes how he sets out to track down some wild ghosts in a system.
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 20 volts instead of the normal 48 volts – 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.

Q1 base
 
Ground is Q1 emitter

Q1 collector

 

 


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

That power supply led me on a wild ghost chase. To continue testing at the reduced 20 volts 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 18 volts, sometimes not until 30 volts. 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

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