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Compaq Evo malfunctioning on stock 18,5V voltage

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First post, by Ahrle

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Picked up this P4 laptop with a really weird behavior.

Came with no charger to begin with. Bottom sticker says 18.5V.

Using a universal adapter at 18.5V, it shows a mixed behaviour. From not booting at all, or completing POST and freezing with a blank screen, to booting fully into Windows and operating perfectly. No charge LED ever lights up, and the last 2 times it popped "103 Charge circuit failure".

Using the same adapter set to 19V, 103 error disappeared. Charge LED still off.

Then I have a mismatched voltage (19V) PSU used for an LTE 5400. That one works perfect every time, and unlike the general purpose adapter, charging LED lights up too.

Questions:
Could there be a problem with the general purpose AC adapter?
Why does it work so bad with its original voltage?
Is it harmful to use the 19V adapter as 18,5V appears not enough?

Thanks

Reply 2 of 3, by EduBat

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I have a P4 compaq evo. It has a weird circuit to detect if it is connected to a car adapter (which should give it about 14V) and if it thinks it is then it will not charge the battery. This is by design.
I can only assume that 18.5V is just on the edge of confusing that circuit. 19V is safe.

Reply 3 of 3, by Ahrle

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EduBat wrote on 2024-05-09, 22:55:

I have a P4 compaq evo. It has a weird circuit to detect if it is connected to a car adapter (which should give it about 14V) and if it thinks it is then it will not charge the battery. This is by design.
I can only assume that 18.5V is just on the edge of confusing that circuit. 19V is safe.

Thank you, found the problem. Did some measuring and figured the universal adapter only outputs 12,48V regardless what it is set to. Probably too much resistance in those terminal blocks after a redneck repair last summer.

Edit: Also solved why an old Asus laptop didn't start last winter...

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