Had a "wide-ranging" discussion with another DIY'er last night and a topic came up that may be of interest - as we had both "fallen victim" to it.
A lot of commercial metal cabinetry comes flat or "knocked down" and must be assembled into it's final form - usually with nuts/bolts or perhaps self-tapping sheet metal screws. When done, the unit looks certainly strong enough AND one would assume that all the joined metal parts would be (how do I put this) - electrically bonded together. BAD ASSUMPTION. I had a preamp with both a hum and oscillation problem, which I eventually traced to the front panel not making a good ohmic connection to the rest of the chassis - nor were any of the metal control cases (pots/switches) making good ohmic connections to the panel. The powder coating on the panel was so "tough" that the supplied hardware didn't bite through it to the other panel.
His problem was more (potentially) severe - a power transformer mounted to a powder coated chassis developed an internal HV short to the laminations. Turns out that the transformer even though screwed down to the chassis wasn't "electrically bonded" and was therefore "floating" at a pretty high voltage above the rest of the chassis. He did detect the problem without further incident.
In any case, If you use a painted cabinet/chassis for a project, I suggest you think about this grounding/bonding situation and where necessary, scrape away paint between adjoining surfaces down to the bear metal . Could save a LOT of troubleshooting headaches later.
Cheers
Charles
A lot of commercial metal cabinetry comes flat or "knocked down" and must be assembled into it's final form - usually with nuts/bolts or perhaps self-tapping sheet metal screws. When done, the unit looks certainly strong enough AND one would assume that all the joined metal parts would be (how do I put this) - electrically bonded together. BAD ASSUMPTION. I had a preamp with both a hum and oscillation problem, which I eventually traced to the front panel not making a good ohmic connection to the rest of the chassis - nor were any of the metal control cases (pots/switches) making good ohmic connections to the panel. The powder coating on the panel was so "tough" that the supplied hardware didn't bite through it to the other panel.
His problem was more (potentially) severe - a power transformer mounted to a powder coated chassis developed an internal HV short to the laminations. Turns out that the transformer even though screwed down to the chassis wasn't "electrically bonded" and was therefore "floating" at a pretty high voltage above the rest of the chassis. He did detect the problem without further incident.
In any case, If you use a painted cabinet/chassis for a project, I suggest you think about this grounding/bonding situation and where necessary, scrape away paint between adjoining surfaces down to the bear metal . Could save a LOT of troubleshooting headaches later.
Cheers
Charles
That's a really good tip and something that is often overlooked.
Its a technique I have used, look at the orange wire on the front panel (post #286)
http://www.diyaudio.com/forums/solid-state/96192-post-your-solid-state-pics-here-15.html#post1417418
Its a technique I have used, look at the orange wire on the front panel (post #286)
http://www.diyaudio.com/forums/solid-state/96192-post-your-solid-state-pics-here-15.html#post1417418
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