exxos's Atari, Amiga & retro forum

Having just recapped my Mega STe power supply successfully (I wrote a "how to" here) I noticed some electrical arcing spots right underneath where the AC household current enters the PSU circuit board. What might be the cause of this? Loose wires, causing sparks (and unstable power for the computer) or something else? The PCB traces look OK, but I made sure to solder it well back again with new solder of course.
One thing worth mentioning is that the AC on/off switch at the back of the PSU has been extremely hard to push at times. Almost like getting stuck and needing brute force to turn on or off. I would think this could cause a lot of sparks when pushed (causing rapid on-off-on-off-on-off.. connections), but I opened it up, cleaned it with some electronic contact cleaner, reassembled it and now it works a whole lot better (well, now and then anyway).

Shouldn't there be a plastic sheet between the PCB and metalwork ?

I don't know. There wasn't one on mine (I bought the computer 2nd hand so I can't say if there ever was one to begin with).
Are you saying the arcing is a common thing and not a fault?

I have some transparent plastic overhead sheets -perhaps I could use one or two of those layered, or are we talking thick plastic?

I recapped an MSTE PSU not long ago, went back and looked at some photos I took. Sorry to say, I don't remember and can't see from the photos anything about a protective sheet between PSU and the metal casing.

/Troed

Pacman wrote: ↑Thu Oct 19, 2017 10:34 pm I don't know. There wasn't one on mine (I bought the computer 2nd hand so I can't say if there ever was one to begin with).
Are you saying the arcing is a common thing and not a fault?

I have some transparent plastic overhead sheets -perhaps I could use one or two of those layered, or are we talking thick plastic?

I only had one MSTE PSU in bits, can't remember. But mains voltages next to huge metal areas which are basically grounded isn't a good idea. All the STF/E PSU's had the insulation sheet. I will try and open on up tomorrow and see.

Okay so the power supplies I have for a different model number I think...

PSM-8012-2

I guess in a way this is good as I have a new power supply to fiddle around with some point


In any case, you can see the insulation sheet on this one anyway. The PCB does seem to fit high up away from it anyway. Though of course with wires sticking out from the bottom of the PCB it is possible some could come very close to the metalwork.

Usually a manual switch produces sparks and voltage spikes when opening or closing the current. My guess would be, that this spike uses the lower solder point as an electrode and arcs to the chassis.

So your idea to cut some overhead sheets in shape and place them there should be sufficient. In addition, swapping the switch might be a good idea, if I remember correctly, Exxos has these in his store.

Yes I have the switches.

I think it is switches start getting stiff to switch or sound like they are arcing internally, then they should be changed. I the rest of the power supply they are 30 years old. And I am not convinced they were that good from new

IngoQ wrote: ↑Fri Oct 20, 2017 12:37 pm Usually a manual switch produces sparks and voltage spikes when opening or closing the current. My guess would be, that this spike uses the lower solder point as an electrode and arcs to the chassis.

Really? So AC household voltages actually go through the metal shielding of the PSU (and also the GND plane of the computer's motherboard)? That sounds both dangerous to me and harmful to the computer.

So your idea to cut some overhead sheets in shape and place them there should be sufficient. In addition, swapping the switch might be a good idea, if I remember correctly, Exxos has these in his store.

Most definitely something I will be doing (both, I just ordered a switch from him).
I suppose if 230V AC tries to go through to the chassis I would need some very thick insulation -are you sure one or two overhead transparencies would do, or would the sparks/arcing go right through? Will changing the switch (so that I get a definite on/off instead of "on-off-on-off-on-off..." every time I switch it to the different position) help prevent this arcing/sparking from trying to go through the wire and towards the chassis in the first place?

Hello Pacman,

Please don't overdo.
Yes, AC-Live voltage can be dangerous.
That's why you need to have the ground wire connected. (Yellow/Green)
And than it's no problem.

Overhead sheet for insulation is just fine.
Arcing or sparks from the switch normall cannot get out of the switch.
Some incident may have caused the marks. But usually not one of the standard components.
And the insulation sheet should prevent the metal from being blackned.
The original sheet is Polyesther, quite flame retardant and sturdy. (Will not schrink when moderately heated)

And if still in doubt, buy a power supply from EXXOS.
The transformer prevents (almost all) dangerous events, as there's no "live mains" on the bottom side of the PCB.
BR/
Guus