Design Article
Bitten by debug
Bill Schweber
2/27/2010 12:00 PM EST
But I didn't do that, since I view a broken appliance as yet another hands-on training session for the most difficult interdisciplinary engineering skill: debug. Certainly, finding out what is wrong with a prototype or new item which has never worked–or which has a short track record–is very different than finding out what has gone wrong with something that has worked well for many years, but it's still a good mind and discipline exercise.
One thing I always keep in mind when debugging is to never assume the problem is due to a single fault. Further, the "double-fault" can take on three guises:
- There's the serial fault, where Problem A leads to Problem B, which in turn leads to the observed problem
- There is also the parallel fault, where two independent problems combine to cause the observed problem, where either one by itself would not be sufficient.
- And finally, there is the unrelated cause double fault, where two completely independent problems separately cause the same observed problem.
So what about my clock radio? My initial assumption was that one of its capacitors had dried out after all these years. But when I paid closer attention, I could hear that the audio would shift from being OK to being very distorted, which leads me to believe that, instead, there is an intermittent connection that is perhaps thermally sensitive.
Some work with a heat gun and freeze spray to induce consistent failure, plus careful visual inspection under the magnifying glass and poking with a non-conductive stick, is my next step. ♦
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RFPowerMaster
3/3/2010 8:28 AM EST
Bill,
An excellent summary that is probably not already known by many of your readers. I wait eagerly to hear your final point, after you apply the heat-gun and freeze-spray to find the probable intermittent connection.
Nat Sokal, nathansokal@gmail.com
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Guru of Grounding
3/11/2010 7:21 PM EST
You didn't mention whether this radio was old enough to use vacuum tubes, as in the "All-American Five" designs. I repaired thousands of these when I worked part-time in a repair shop while going to high-school. A very frequent cause of the symptom you describe was leakage from primary to secondary in one or both IF transformers. This caused the AVC voltage to be biased positively, increasing the gain of the IF stage until it overloaded and distorted the audio. If you remove the square aluminum can from the transformer, you'll often find a pair of capacitors built into the plastic base. They saved money by using a single sheet of mica and two sets of clips to contact two silvered areas on each side. The problem occured because silver oxide "tendrils" would grow around the silvered areas and eventually create the leakage path between primary and secondary. The cure was to break off the clips, remove the mica sheet, and install two discrete mica caps at primary and secondary. If I remember correctly, they were about 100 pF each (and, of course, retune the IF transformer). This phenomenon was probably aggravated by the high humidity of Florida, where I grew up.
Bill Whitlock
president/chief engineer
Jensen Transformers
www.jensen-transformers.com
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RFPowerMaster
8/28/2010 8:35 AM EDT
"Bitten by debug
Bill Schweber
2/27/2010 12:00 PM EST " ...
"Some work with a heat gun and freeze spray to induce consistent failure, plus careful visual inspection under the magnifying glass and poking with a non-conductive stick, is my next step. ♦"
What did the "next step" reveal?
NathanSokal@gmail.com
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