INSPIRON 1501 CHARGING/POWER PROBLEM

I recognise through browsing forums that the problem I describe is not wholly new but  I have tried some of the suggested solutions without success.

I have two identical Inspiron 1501 laptops which have failed in exactly the same manner within 1 week of each other.

The basic problem is that the laptop will no longer recognise the AC power adaptor and hence will neither power the PC nor charge the battery. For both PCs the failure mode was pretty well identical as follows:

1)       The power adaptor shows a green light and a multimeter shows DC power being output

2)       I have previously repaired the power adaptor at the connector end where flexing had broken the connections ( a common problem of poor design). It had subsequently worked OK charging the batteries in both machines for about a month

3)       The laptops then intermittently showed the battery green light as charging then not charging. This appeared to be solved by fiddling with the connector but eventually this failed to get any indication on the laptop light.

4)       One laptop stopped seeing the charger whilst the other one would still see the same charger for about another week

5)       With a charged battery the laptops appear to function perfectly in all other respects

6)       Both batteries were in good condition and would charge normally and hold charge for several hours

7)       I have had the sockets on both motherboards professionally replaced as I suspected a poor connection but still get no indication of power to the laptop

8)       As suggested in one forum I have updated the BIOS successfully on one laptop but this had no effect

Having read many forums they tend to suggest replacing the motherboard but this is an expensive option I am not willing to take until I understand the failure mechanism. If a device has died on the motherboard I would have expected an instantaneous failure mode not an intermittent one. I could buy a new AC adapter but again I don’t understand why this should work. Can anyone please give me a view on the failure mechanism or I could end up wasting a lot of money!