Archive for April, 2009

Refurbishing My Isolation Transformer

Thursday, April 30th, 2009

An isolation transformer, as used in electronics testing and repair, is a line-powered transformer with 1:1 windings (e.g. in the US, ~110VAC in, ~110VAC out). It’s used to isolate a circuit under test from line power for two main reasons (that I know of):

  • With line-powered equipment, if you were to touch a hot part of the AC side of the circuit while it was plugged straight into wall power, you could become a return path to ground and get electrocuted.

    The transformer isolates it from the rest of the world, so your risk is reduced to becoming a return path within the circuit, and you’re not at risk of conducting from any single point to ground. This doesn’t eliminate the need for care, but it reduces the hazard.

  • Some line-powered circuits use a local ground that is something other than earth. (The mixer power supply I repaired recently uses a local ground on its primary side that’s at about -85VDC with respect to earth.)

    If you want to use test equipment (oscilloscope, line-powered meter, etc.) to probe the circuit, you need to connect the equipment’s ground to the circuit’s ground. If they’re both line-powered and the circuit uses something other than earth as ground and you connect the equipment’s real-earth-ground to the circuit’s specifically-non-earth-”ground,” you’ll create a short circuit fatal to one or both pieces of equipment.

    The isolation transformer “floats” the circuit so its local ground is safe to connect to your test equipment’s ground. Of course, this potentially reintroduces the shock-to-earth hazard mentioned above, during the time you have the grounds connected for testing.

Viz WP-31A isolation transformer with carrying handles removed

While repairing the aforementioned power supply, I dug my isolation transformer out of the basement. I had acquired it grungy and broken (and this is odd, but I don’t remember whether I got it from Slim or purchased it at Lloyd’s) and it had been through a basement flood, so it needed rework before I could use it. I’ve just polished off the last of the repairs.


Two Birds

Monday, April 27th, 2009

I want to replace the abraded power cord on my brother’s sump pump in exchange for his letting me borrow it. Cort needs four panel-mount BNC connectors for an amateur radio handheld direction finder project. Convergence.

VideoCipher II television descrambler, front

Don’t panic. If for some weird reason you need one of these, I have more where this came from.

VideoCipher II television descrambler, rear

It’s a little hard to see from these shots, but the case profile is a weird trapezoidal shape.

VideoCipher II television descrambler, interior

Physically large linear power supply; two main PCBs.

Lithium 1/2 AA cell

Lithium 1/2 AA cell from April of 1989. Rated for 3.6V and still holding 3.69V after twenty years.

VideoCipher II television descrambler, PCB sliding out

Not sure why they used two PCBs (don’t tell me they really needed the extra 20 square inches), but it’s cute the way this one slides out.

Four PCB- / panel-mount BNC connectors

Every one of these had its shield pins broken free of both solder joints. Looked like cold solder, but I assume it was just mechanical stress.

Four PCB- / panel-mount BNC connectors

Voila! Four connectors for Cort. And a power cord for my brother, that I’m out of time to swap onto his pump tonight. Tomorrow, then.

Reducing Kickback Noise in Soundcraft Spirit E6 Mixer Switching Power Supply

Sunday, April 26th, 2009

Last weekend, I had repaired a Soundcraft mixer switching power supply, but still had switching noise spikes from the transformer primary showing up on the circuit’s ground, and asked whether anyone could suggest how to reduce them or whether they were an artifact of the way I was using the oscilloscope.

Soundcraft Spirit E6 Audio Mixer

Many thanks to everyone who wrote in with ideas! Because of your suggestions, the mixer is now fixed (enough), back together, and ready to go back to the radio lab.

Here were the ideas and their results:


Repairing a Soundcraft Spirit E6 Mixer Switching Power Supply

Sunday, April 19th, 2009

A few weeks ago, John mentioned to me that one of the audio mixers in the art and technology / Internet radio station lab had stopped powering on. I said I’d have a look at the power supply and see if I could fix it, and he sent it with me. Turns out we both got a little more than we bargained for — I in terms of effort required and he in terms of time without the mixer.

Soundcraft Spirit E6 mixer power supply board

I didn’t know whether it’d be a linear or switching power supply, and it turned out to be switching. I figured it’d just have some baked electrolytic capacitors I could replace, and it turned out that was just the beginning.

I’ve got it mostly fixed now, and it’s been a long and interesting road.