HP-67 problem

[Achim]

Recently I received a HP-67 which shows the following failure: when switched on (in run-mode) the display shows "00000.0". Numeric key entry gives nothing but math functions seem to work. E.g. enterting "h 2" displays pi as "31415.9". All calculations seem to work fine, only the diplay is weird. Values between 0 and 9.999E19 give a wrong display, while values greater than 1E20 are displayed correctly. Pressing one of the definable keys causes the calculator to hang up.
When switching to PRGM the display constantly shows "992 31 25 24". No keystroke changes this display.
Any idea of what is defective? Maybe memory?
Thanks, Achim

[Tony Duell]

My first guess would be one or more of the 8 pin ROM/RAM ICs (all HP-custom, of course), but don't ask me which one. It could be just about any IC on the board, though.

[David Smith]

My first guess whenever I see a display with lots of zeroes and no keyboard inputs is the ACT chip. It is the 22 pin, 0.4 inch wide chip. By far this chip is the most common to fail in any Woodstock series machine (which the '67 and '97 are internally).

[Tony Duell]

Odd... I generally find ROM0 (The 18 pin chip) to be the normal cause of a 67 that ignores the keyboard and has odd displays. I have had ACT failure too, though.

[Achim]

Is there any possibility to get these chips other than fragmenting another HP-67?

[David Smith]

You can get the ACT chip from any Woodstock series machine (except possibly the HP27). I know HP21 and HP25 chips work just fine. The ACT chip is unique to the HP67 as it contains a ROM with cpu instructions in it.

[Tony Duell]

Both chips are HP-custom parts so you can't go and get them from the local electronics shop. The ROM0 is unique to the HP67. Although other machines (Woodstocks, Topcats) use a similar combined ROM0 and display anode decoder, they contain different code, so can't be used in a 67.
The ACT chip can be obtained from a number of machines, though. I think the one from any Woodstock _apart from the HP27_ or any Topcat is worth trying. I've swaped them round in the past and had the machines work correctly. It certainly won't damage anything to try it.
The HP27 is strange, the chips are NMOS, not PMOS (as in all the other Woodstocks, 67, Topcats), so the power supply polartity is different too. Don't try that chip in a 67.