|Re: HP 25-C Repairs|
Message #4 Posted by Viktor Toth on 10 June 2001, 7:44 a.m.,
in response to message #3 by Tony Duell
Well, if you're making an external charger, you might as well do it right and build a "proper" charger with regulated charging current: it's very easy to do, you can build a simple circuit using a common 7805 voltage regulator chip, for instance (all you need is that 3-pin chip, a suitable resistor, and a capacitor.) Or better yet, if you've already opened your Woodstock battery pack to replace dead cells, you might as well make it so that you can remove the cells anytime, and just charge them in an "AA" NiCd battery charger. That would also solve the problem of connecting to the battery pack.
I don't know why it's the ROM that fails, but I've seen something like half a dozen or more dead "C" Woodstocks, and it was always one or more ROM chip that has failed. One possibility (I am just guessing here of course) is that, since the ROM and RAM chips share part of the bus, when the RAM receives excess voltage, while it itself might survive, it puts out something excessive on the bus that kills the ROMs?
That said, I have also encountered fried RAM chips occasionally, but in those cases, the calculator generally remained functional (or it was possible to bring it back to life by removing the RAM), it's just that its registers or program memory wasn't working properly.