CurtZHP
Well-known member
EmRR said:That's PSU and chassis related. The lid is redirecting the PT flux.
What else changed in the last 24? Besides moving spaces? Distrust the outlet you are using now. Or a light dimmer. Etc.
Take the PT outside and at a distance, run the AC to the PSU through 3 feet of wire and see what happens. Or skip that and use an outboard PSU.
What's the chassis? Should you trust it? I've seen chassis that proved to be the problem, no good continuity between any panels.
One site is my workbench at home. Been building stuff like this for 15 years on that bench. The most obvious sources of noise are generally acoustic in nature, but just for troubleshooting I've started unplugging things whether they could be the cause or not. No dimmers to be found anywhere in the house.
The other site is my workbench at work. Again, been working there for years. Never had this much trouble. On days when I know it'll be slow, I bring this stuff with me.
As for any changes, I had reoriented the power transformers. They were originally laying flat on the bottom of the chassis. I moved them to being vertically attached to one side, opposite from the rest of the circuit, in an attempt to reduce noise. Fat lot of good that did!
The chassis is a 2RU aluminum body with a steel front panel. It used to be some sort of SCA monitoring equipment that a radio station had tossed. I stripped the guts out and replaced the front panel with a steel blank. Your comment about it brought to my recall an interesting situation. One of the "features" I was hoping to incorporate into this beast was an instrument input, using a switching 1/4" jack just south of the input transformer. I had done this exact thing in my last preamp with positive results. Of course, it hummed like crazy (worse than it does now). In the process of troubleshooting that, I discovered that, as long as the jack was NOT touching the front panel, the hum was significantly reduced. Sometimes, just turning the jack or bumping it a certain way got rid of the hum. Another clue?
Anyway, I decided I wasn't married to that feature and did away with it. Again, didn't gain much for my trouble.
I suspect that part of the continuing problem is the introduction of the "series/parallel" switch. That was another very recent addition. I should eliminate that entirely to see if that eliminates the problem.
Oh, by the way....
This thing's phantom supply appears to eat toggle switches. I'm trying to put a toggle switch on the phantom supply at the output of the actual supply rail, for obvious reasons. I've gone through two switches already. They only lasted for a short while before the voltage passes unabated regardless of switch position. Granted they were used switches, but both were rated for plenty more than this rail should be putting out. And it does put out the correct voltage. I guess I'll try again with a really beefy switch. You know the old saying: "Once is happenstance, twice is coincidence, three times is enemy action!"