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benfinoradin
February 20th, 2017, 10:36 AM
Hi all,

I am wondering what the general conventional wisdom is around powering on vintage personal computers that have not been powered on, or even plugged in at all in many years, even decades.

Are there any safety precautions that can and should be taken both for personal safety, but also the safety and integrity of the computer itself?

Are there any risks involved in powering on an old machine?

Thanks!

Malc
February 20th, 2017, 11:21 AM
I just power them up and run for cover :-)

Wear eye protection if you have the cover off when powering up, as capacitors can go snap crackle and pop, had that happen many times over the years.

Stone
February 20th, 2017, 11:24 AM
I just power them up and run for cover :-)I omit the 'and run for cover' part.

paul
February 20th, 2017, 11:30 AM
Use an RCD/GFI in your power lead to minimise damage from Y-cap failures, and for your safety.
You could load the power supply with a dummy load first and check the output voltages.
Look out for odd smells in the first hour (from the computer.)

glitch
February 20th, 2017, 11:30 AM
For regular PCs with an AT supply? I usually test the supply on a board I don't care about (I don't have a power supply tester for AT). They're switchmode so there's no bringing it up slowly. For most PCs I just flip the switch though.

Older pre-PC machines are often a different case. If a machine with a linear supply hasn't been powered on in a while, I typically pull the filter capacitors and reform them on a bench supply, if they're easy to get to. If not, I bring it up on a variac with a dummy load. Automotive tail lamps work great, 6V bulbs for the +5 rail, 12V bulbs (in series if necessary) for the rest. Then you've got weird stuff like some DEC supplies, which come up as linear and kick over to a ferroresonant switchmode after a certain current draw.

Chuck(G)
February 20th, 2017, 11:51 AM
If the system is in the moderate power range (e.g. 100W) and you're really unsure of things, try the first power up with a 250W-500W incandescent in series with the line. If the lamp glows brightly, look for problems. It may save you from a "kaboom" or two.

KC9UDX
February 20th, 2017, 01:20 PM
If it's a Commodore with a potted power '"brick,"' I shelf the power supply until I get time to rebuild it. For everything else, I usually just power it up and stay alert.

pearce_jj
February 20th, 2017, 01:24 PM
Another vote for the dim bulb as Chuck has described.

benfinoradin
February 21st, 2017, 02:43 AM
Wow thanks for all of the responses everyone!


If the system is in the moderate power range (e.g. 100W) and you're really unsure of things, try the first power up with a 250W-500W incandescent in series with the line. If the lamp glows brightly, look for problems. It may save you from a "kaboom" or two.

I've seen this trick before, but forget the facts about what this indicates / prevents can you elaborate a bit for me?

Malc
February 21st, 2017, 09:22 PM
Google ' Series light bulb trick ', I've used it many times and like chuck said it can save you from a kaboom or two, = time and money.

KC9UDX
February 21st, 2017, 11:02 PM
I've got a panel on my bench which has two different wattage bulbs, an isolation transformer, and four outlets. I use it for testing analogue electronics.

Twice I've accidentally plugged things with switching power supplies in and both times the power supplies went up in smoke. So I can't recommend anyone do this with a computer. But, it appears others have not had this happen.

pearce_jj
February 23rd, 2017, 11:06 AM
I've seen this trick before, but forget the facts about what this indicates / prevents can you elaborate a bit for me?

Good old incandescent bulbs have a very low filament resistance when cold, so if the appliance (to which it is connected in series) is working properly, i.e. not drawing much current, the filament won't get hot enough to light (much) and the voltage drop across it will be negligible, meaning the appliance works normally.

If the appliance has an issue most specifically a short to ground or neutral, the resulting current through the filament will light it brightly but there will also be a significant voltage drop across the bulb, the filament resistance increasing rapidly as the bulb lights. Hence the appliance is protected from big voltage and current and survives to be fixed.

Filament bulbs are used because of this property (increasing resistance with increasing current) in series with high-frequency speakers to protect their small and sensitive coils from amplifier distortion, usually referred to as 'optical protection'.

Chuck(G)
February 23rd, 2017, 11:46 AM
The bulbs do have to be large enough. For a PC, a 100W incandescent is insufficient and may result in PSU damage; I suggest at least 250W to 500W.

paul
February 23rd, 2017, 12:34 PM
Although the ancient light bulb technique suits many appliances when their condition is unknown, personally I think it's unsuited to switchers. A slow-blow fuse and RCD/GFI provides sufficient protection IMO.

Chuck(G)
February 23rd, 2017, 01:23 PM
If you want to get picky about it, you can also use a current-limited AC supply on an isolation transformer.

paul
February 23rd, 2017, 02:43 PM
...essentially what you have in the secondary part of the switcher... :)

Chuck(G)
February 23rd, 2017, 04:04 PM
...too bad that it's often the primary circuit in a switcher that results in the more spectacular smoke. :)