SMPS: Substituting SBD/Power Diodes 1A for 1.5A and Filter Cap for +20% safe?

[throbscottle]

I wouldn't read too much into a 1v difference. It's possible that your meter is off by that much, or it could be due to ripple. Better to open the laptop case, connect the power supply and check the voltage at the main board. If it drops significantly at the power connector (maybe when you press the power button) it would indicate a faulty psu. If it stays the same it's more likely to be the laptop that's at fault.
The cpu voltage (and other voltages) is derived from an on-board power converter so the exact voltage coming out of the power pack shouldn't make too much of a difference.
The power pack actually has a 3 wire output, the 3rd wire being a 1-wire protocol connection that tells the BIOS if it's the correct power pack to use. I think it's the fine pin in the middle of the plug, with the power being delivered to the outer and inner rings.
I have a Dell Inspiron laptop from maybe 2008 which died. I re-flowed the main board, particularly the power converter area, using a heat-gun and it worked again, at least long enough for me to get my stuff off it and onto a replacement. You have to be very careful not to melt plastic bits if doing this, get the board out of the laptop casing, remove everything you can from it, and loosely wrap aluminium foil round any plastic bits.

 

[spec]

is 18.5V without load a sign that it's definitely gone?

Almost certainly 18.5v does not imply a faulty power supply.

If you can find how much current the power supply is supposed to provide, just load the power supply for that current with a resistor and see if the voltage holds up.

The other thing you can do is to make sure every thing is electrically sound: oxidized pins on mains plug, fuse in plug gone high resistance etc etc.

Also if the unit has batteries are the batteries faulty?

spec
(crossed posts TS )

 

[throbscottle]

Also don't trust cheap clone no-name laptop PSU's. I know from experience...
Used proper ones are cheap enough to pick up. Also any Dell laptop power pack will power any Dell laptop as long as it's rated for high enough power and the connector is the same.
Spec has a point - try it without the battery. Although it's rare for them to be sufficiently bad to stop it from turning on.