Chasing wild ghosts: Page 5 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.
and would make a good test substitute for the DUT T1 center-tapped unloaded primary to verify my test setup. I removed the original T1 from the circuit and jerry-rigged this choke to the DUT using the same alligator clip wires.

The results were an eye-opener. See Figures 7 and 8.

Emitter current through 1Ω; 1V = 1A

(Iavg on bench supply meter was 220mA)

Q2 collector voltage

Scope ground –ve side of 1 ohm
 

Figure 7  With the common mode choke in place of T1 there is no change when Q2 is turned on, but the current is nicely ramped when Q1 is turned on.

Emitter current through 1Ω; 1V = 1A (Iavg on bench supply meter was 220mA)

Q1 collector voltage. Spike hits 200 volts offscale. Scope ground –ve side of 1 ohm

Scope ground –ve side of 1 ohm

Figure 8  Cannot explain above. Why does only Q1 have the desired current ramp?

The common mode choke load showed very different switching characteristics between the two push-pull drive transistors, unlike the original transformer. Swapping the choke windings made no difference, the problem stayed with Q2. Very obviously something was wrong with the Q2 portion of the circuit.

Finally with a solid clue to work with, I poked around again with an ohmeter. Both Q1 and Q2 collectors measured 21.5 ohms to the chassis. I now had the culprit – must be that 1µF capacitor between T1 center tap and chassis that had previously checked good. But nope – pulling that cap off the pcb made no difference. And there was no other connection to the chassis in that circuit, in spite of what the ohmeter was telling me.

Then a nasty suspicion hit, and with the ohmeter still connected I backed off the mounting screw of Q2 about a quarter turn. The meter jumped to about 8K ohms.

I backed off the mounting screw of Q1 –

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