Weirdest Circuit Problem I've Ever Seen

Tweakers' Asylum

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I've just spent several days of spare time tracking down one of the oddest problems I've ever seen. It started when I decided to bench test the individual components from a Leslie speaker - audio amp, power supply and motor control - for the purpose of transplanting them into a different cabinet. When I hooked up the control unit to the motors, the fast speed motor would growl and crawl for a second or two, then stop. It would do this every time power was applied. The slow motor was fine. Both motors worked OK after the initial weirdness; grounding either control line would activate and run the respective motor perfectly.
In this controller, each motor is driven by a reed relay that activates a triac, a very straight forward circuit. While troubleshooting, I discovered that the bad driver would exhibit the problem even if I disconnected the coil of its relay. That particular relay had been replaced with a Radioshack version by a previous owner though, so I decided something was probably wrong with the contacts. I replaced it, but no go, same problem. Next, I replaced the triac. Again, nothing changed, same problem. Now grasping at straws, I thought it might be the motor itself, some sort of inductive kickback forcing the triac to conduct. So, I disassembled the Molex connector for the motors and swapped them. Now the slow motor had the problem. I swapped the motors back, then disconnected B+ to the controller board. That kills all the control circuitry, leaving only the AC going to the triacs. Still did it.
After several more hours of illogical troubleshooting, I finally found it. Almost unbelievable, it turned out to be the proximity of the power transformer to the relay/triac circuitry. The bad driver is right next to the transformer; the good channel several inches away. I finally proved it by inserting a piece of sheet metal (steel) between the transformer and the control board. Problem gone! All that time, I was fighting a factory anomaly. Apparently, they all do it! Good grief...

Edits: 08/20/16

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Topic - Weirdest Circuit Problem I've Ever Seen - Triode_Kingdom 17:19:03 08/20/16 (5)

Another option... - RadioWonder 11:04:59 08/21/16 (1)

RE: Another option... - Triode_Kingdom 12:37:30 08/21/16 (0)

I used this 3M Product - RadioWonder 10:49:05 08/21/16 (0)

You would think that the.... - Cougar 17:37:33 08/20/16 (1)

RE: You would think that the.... - Triode_Kingdom 22:32:26 08/20/16 (0)

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