Need help to diagnose power supply making funky noise...

[Ego Tripper]

So my studio recently picked up a Soundcraft Ghost. We got a killer deal because it came without a power supply. It would have been an old switch-mode PSU anyway, so I was happy.

Anyway, here I am, combining three Power One power supplies in a single chassis for + and - 17.5V and +48V. I was following Family Hoof's instructions posted here at the forum:

I would do it like this if possible...

power supplies:
If they both have their own chassis then make sure that it's not already tied to the 0V rail. Put them both in the master chassis with the IEC inlet. AC ground lug gets screwed to a bolt through master chassis. If the individual chassis are not already tied to master chassis via mounting hardware (chassis connections must be electrically conductive, test for continuity) then you can run a thick wiere from each to the same AC ground bolt star point. AC hi and lo go to both units. Tie the 0V connections at their DC outputs and then run this off to the AC/chassis ground bolt star point (you can use passive device such as 10ohm resistor to do this to help prevent loops if you want).

Now we will assume that everything inside your audio circuits chassis is grounded correctly with 0V NOT tied to chassis at any point. Now, if you 1) use a 4 conductor power cable with a shield, 2) make sure the shield is tied to the outer shell/casing of each 4-pin connector, and 3) make sure that the mating connector shell/casing is tied to chassis on both the power supplies and audio circuit then you should be set. The shield in the cable plus connectors/receptacles act as the fifth conductor to carry chassis ground creating an extension of the chassis between the audio and power boxes. The chassis is tied to actual (AC) ground. The shield surrounds the power wires like a chassis should. This is ideal.

In the course of wiring up the supplies, I apparently skipped the step where the DC 0V is tied to AC/chassis ground. So, when I plugged the whole arrangement into the mains outlet, I immediately heard a violent bang from within the enclosure and I unplugged it real quick.

So I took the unit home to my bench and disconnected the power supplies (the phantom supply was not wired initially anyway). The PSU that was functioning as the negative rail seems fine, but the positive supply, while still outputting the spec'd +17.5V, is now making a weird crackling noise. My guess is that:
A) This noise is coming from the power transformer OR the big filter caps.
AND
B) This noise is bad.

So, even though the power supply's outputting the proper voltage, I'm guessing it's not stable in it's current condition. And since I don't know how unstable it is, I'm loath to leave it plugged in for the purposes of troubleshooting & probing. Has anyone ever done this before and can offer any insight or suggestions as to what might component(s) might be fried??

- edit -
Well, I disconnected the secondary leads (which I was initially reluctant to do without solder on hand to reattach, but I found some solder), and apparently, it is the power transformer. I have another one sitting around in another identical model power supply with roasted filter caps. 

I'm gonna throw that transformer in this power supply and hope the filter caps are still good-- these screw terminal electrolytics aren't likely to stocked at Fry's or RadioShack...

 

[sodderboy]

Not making a "technical earth" should not have caused a "bang".  You would expect a hum across any output, but no bang.

The prudent way to introduce a new PSU to a console is with as many modules and sections disconnected as possible.  Best world is testing an empty frame.

Was there any way to verify that the console was in working condition before you got it without a supply? 

Mike

 

[CJ]

you need to plug in the power supply with no load and see if it works for a long time,

if it works for a long time, then the stuff it it is powering is probably suspect,