Trying to revive an HP-25C
Message #1 Posted by Mark Henderson on 24 May 2010, 4:38 p.m.
I bought a non-working 25C from TAS and am trying to fix it. My first HP was a 25 and I've always wanted a 25C. I've got a good amount of experience now fixing Classics and recently my Spice (34C). The 25C is in good cosmetic condition, keyboard is great, display lens and digits clean. I've read Katie's article on repairing Woodstocks, and every other 25/25C article I can find. The symptoms I have are pretty heavily loaded toward a dead ACT and/or RAM in the traditional Woodstock manner, but not everything follows the patterns I've seen described. Here are my symptoms:
As received:
- Calc would not show/do anything on power up (good battery from 25)
- Seeing some of the posts, I left the calc on for a few minutes, and sure enough, after off-on the display would show "0000000000000"
- Calc non-responsive to keys or switches, once "0000.." showing it would stay on display
So I was pretty sure the ACT was dead. I opened the calc and everything inside looked good, except for some blue-ish corrosion on the battery contacts, the traces above the ACT and generally around that end of the battery bay. Couldn't see anything eaten away, though.
Based on Katie's article I put a resistor from ACT pin 11 to ground (12). Effect of this was that the calc stopped showing anything on the display, even after a "warm up" of several minutes. Removed the resistor and the original behavior returned. Hmmm.
Next I gave the main board a dilute warm water/vinegar bath and gently cleaned off the corrosion after soaking for a while. There is discoloration to many of the traces where the corrosion was worst, but no breaks visible.
Behavior after this operation is subtly different: no display when cold, after warm up there is a display after power on - but instead of solid "00000000.." I now get assortment of random and rapidly changing digits across entire display, which then disappear after <1 second. Sometimes a couple of DPs will be left on, otherwise blank. Katie's resistor is not in-circuit anymore.
Any thoughts? I'm keeping optimistic that the fault might be a broken trace or other corrosion damage, not a blown chip (yes, I know this might sound desperate) - and since the behavior changed quite noticeably just from a cleaning...
Next step will be to power up the scope and do some probing around. I can use my 25 as a reference, too. I'm pretty sure I'll need an ACT in the near future...?!
Cheers
Mark
Edited: 24 May 2010, 4:39 p.m.
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