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Steps to take before powering up old kit?

SteveH

SteveH

Experienced Member
Joined
Apr 30, 2003
Messages
301
Location
Shropshire, ENGLAND
Hi,

I've just picked up an old NEC APC computer that is in very poor condition (well, the casing has LOTS more ventilation than originally designed :(). I've been told it works, but I'd like to know if there are any checks or tests I should perform before attempting to power it up. I've read that power related capacitors could fail but am not sure how I'd go about testing them. Are there any other likely problem areas for old kit? Now as far as test/repair tools go, all I've got is a digital multimeter and a soldering iron. :rolleyes:

Hints, tips or suggestions appreciated. When I get a chance, I'll post some pics.

Cheers,
Steve
 
As the NEC APC (ISTR) has a switching-mode power supply, putting it on a Variac and breaking it in isn't a possibility. Unplug the power supply from the main board and check for shorts with a meter. Beyond that, I'd probably power the thing up with a 100 watt incandescent lamp in series with the machine. That should lessen the possibility of very violent "magic smoke" Make sure than any fuses are the right ones. There's nothing wonderful about finding that some idiot has installed a 15A fuse where a 2A one belongs. You'd be surprised how often that happens.
 
aaah YOU got that! I'd been watching it, and mentally measuring up the remaining loft space (in cubic inches) and considering "do I need another project?"
I'm glad someone else from the board has that.

As chuck says with the incandescent bulb, plus give it a once-over inside, check for screws in "the works"

Problem with switchers and variacs, is that they draw constant power, i.e. more current at lower volts, so you're actually stressing it even on a low voltage source.

Personally, I tend to separate the PSU from the rest and power up the psu first time with a resistor (1 Ohm 50W metal one usually for bigger stuff) as the 5V load. that way you can check the psu is happy before subjecting the motherboard etc to it's tender mercies.

I also check the resistance across the power rails on the main board, as I have had so many shorted tantalum bead caps recently, and they have a habit of overheating power (+ edge) connectors.

Good luck!
 
Chuck(G) said:
...Make sure than any fuses are the right ones. There's nothing wonderful about finding that some idiot has installed a 15A fuse where a 2A one belongs. You'd be surprised how often that happens.
Click to expand...

And fuses are a slightly variable science. I've seen one cartridge fuse visibly char and bow, but actually hold instead of blowing. That was at the correct rating for current and voltage too.
 
IBMMuseum said:
And fuses are a slightly variable science. I've seen one cartridge fuse visibly char and bow, but actually hold instead of blowing. That was at the correct rating for current and voltage too.
Click to expand...

Reminds me of the grumblings of a DEC field engineer about "transistor-protected fuses" in DEC equipment... :)
 
Chuck(G) said:
Make sure than any fuses are the right ones. There's nothing wonderful about finding that some idiot has installed a 15A fuse where a 2A one belongs.
Click to expand...
Internal fuses appear to be originals, but I'll check properly later.

nige the hippy said:
aaah YOU got that! I'd been watching it, and mentally measuring up the remaining loft space (in cubic inches) and considering "do I need another project?"
I'm glad someone else from the board has that.
Click to expand...
Well, it was located less than 10 miles away... so I bid the minimum and was surprised when I won it. :D

Personally, I tend to separate the PSU from the rest and power up the psu first time with a resistor (1 Ohm 50W metal one usually for bigger stuff) as the 5V load. that way you can check the psu is happy before subjecting the motherboard etc to it's tender mercies.
Click to expand...
I've pulled the PSU (2 screws and it slides out) and it 'looks' OK - at least there are no bulging caps. The original power cord has been cut off, but once I've fitted a new one I'll try it out with an incandescent bulb as Chuck suggested.

I think my main area of concern is some rodent damage. :mad: There are several strands of wire that appear to have been chewed and in some places missing completely. I cannot see any chewed components, but I'm going to have to strip it down and check thoroughly.

Looks like this'll be a slow one. I haven't even got a keyboard or any OS diskettes for it yet. At least the guy I got it from said he'd make some copies when he goes back to Poland in a month or so.

Good luck!
Click to expand...
Thanks.
 
MikeS said:
Aren't there quite a few NECs with the APC designation? I take it this is an 8" drive model?
Click to expand...
The guy I got it from thinks it's a model I with a colour screen - s'pose I'll find out when I examine the boards a little more closely. And yes, it has 8" drives.
 
Yes, it was voted "Australian Personal Computer of the Year" for 1983 by a panel of experts in that country.

NEC made a big thing of this in their advertisements.

Tez
 
Chuck(G) said:
Unplug the power supply from the main board and check for shorts with a meter. Beyond that, I'd probably power the thing up with a 100 watt incandescent lamp in series with the machine.
Click to expand...

Right... the PSU simply unscrews and slides out. Once I've reattached a power cord, I assume I just hook up an incandescent lamp to the AC OUT connections on the lower right socket in the picture?

NECAPC_PSU1.JPG
 
SteveH said:
Right... the PSU simply unscrews and slides out. Once I've reattached a power cord, I assume I just hook up an incandescent lamp to the AC OUT connections on the lower right socket in the picture?
Click to expand...

Yes, but put the lamp in series with the AC supply, so that a short at the power supply does little more than illuminate the lamp. It's also important that it's an incandescent and not a compact fluorescent lamp. If you apply power and the lamp glows brightly, remove power as you probably have a short. If, on the other hand, the lamp glows dimly or not at all, you're probably fine.

If you do much of this, it's probably a good idea to wire up a "cheater" that has a plug on one end and a socket on the other, with a lamp in series with one leg. It'll come in handy when you need to do this again.
 
Plugging in old switching power supplies is always more fun when you live in a 230 VAC country. If the NEC can take 110 V trying it with a stepdown transformer may be less dramatic.

I have been working on my mother's old Hoover vacuum cleaner and the EMI capacitor network blew up like a roman candle when I plugged it in. Good thing it was outside the house.

Aside from degradation of 30 yr old caps, I've noticed the suppression caps I've secured as a replacement are rated to a slighty-less dodgy 270 VAC instead of 250 VAC.
 
Chuck(G) / barythrin,

Cheers guys, I understand now. :lightbulb: I'll make up a simple series lamp limiter circuit and plug the PSU into that.

lamp-limiter-circuit.gif

(picture from www.vintage-radio.com)

That vintage radio site is certainly a mine of information - I reckon a lot of the repair techniques mentioned there can also be applied to old computer kit.
 
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Hi Guys,

I made up a series lamp limiter circuit, wired up a new power cord to the PSU and turned on the juice. The lamp momentarily glowed and after about a second something sounding like a solenoid clicked. Rather an anticlimax.:rolleyes:

I've checked the outputs with a multimeter and they appear to be about the right voltage levels (fortunately the voltage levels are printed on the circuit board). I say the voltage levels are "about" right, because I see the voltage level rise but then drop off again once the whatever-it-is clicks.

Is the PSU just cutting out because it has no load attached? Or could it be some other problem?

Cheers,
Steve
 
Last edited:
The click is probably the result of power supply protection cutting the current.

There are two reasons this happens. The first is that the system hooked to the power supply is drawing too much current. The other is that the system is not drawing enough current--and the protection circuit cuts the power because the supply cannot regulate at low loads.

So, the question is: "Is there anything hooked as a load to the power supply?" If not, you have your answer (second case). Otherwise, it's probably the first situation and is going to take some probing to find the cause.
 
Chuck(G) said:
The other [second] is that the system is not drawing enough current--and the protection circuit cuts the power because the supply cannot regulate at low loads.

So, the question is: "Is there anything hooked as a load to the power supply?" If not, you have your answer (second case).
Click to expand...

Just what I thought - there's no load attached, so the PSU cuts out.

Next step, to repair the rodent damage to the video circuitry wiring, re-install the PSU and then try powering up certain parts of the system.

Cheers,
Steve
 
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