Re: Dead HP-67 Message #5 Posted by Tony Duell on 14 May 2004, 6:59 a.m., in response to message #1 by Imran Farooq
Your fault could be one of many things, some easily repairable, others would need parts from another HP calculator.
Here's what I would do. Open up the machine. To do this, take out the battery pack, flick out the 2 plasitic plugs near the charger connector, and peel up the rear (battery pack end) corners of the back label. Take out the 6 screws (2 under the label, 2 under the plugs, 2 under the front feet which can be lifted to get to them). Take off the bottom case.
Now power it up from a bench PSU clipped onto the battery contacts. I am pretty sure the rear one, nearest the display/charger connector, is -ve, but you can check by looking for continuity between the battery contacts and the middle pin of the charger connector. The one that's directly connected to the middle pin is certainly -ve. Use about 3.75V to 4V.
Now check the power converter outputs. The easiest place to find these is on the pins of the ACT (CPU) chip. This is the 22 pin, 0.4" wide chip on the logic PCB. The common reference point for all measurements is the -ve battery terminal. Anyway, pin 1 of the ACT (Vss) should be at about 6.2V, pin 2 (Vgg) at about -12V. Check these, and if you have a 'scope, look for ripple too. They should be stradyt DC voltages.
If those are right, the PSU is working. If not, then it's not. In the latter case it can most likely be repaired (FWIW, 2N3094 transistors are suitable replacements for the ones in the power converter).
If the PSU is fine, I'd then look at the 2-phase clock outputs from the ACT. These, IIRC, are pins 16 and 17 of the ACT. If they're missing, then most likely the ACT is dead, but check the LC tank components connected to pins 13 and 14 of the ACT.
If you've got this far, then most likely you have either a dead ACT (HP custom, you can get a replacement from an HP21, 22, 25, 25C, 29C, 19C, 91, 92, 97 or 97S -- NOT a 27 though), or ROM0 (the 18 pin chip on the logic board, and this has to come from another 67).
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