Woodstock HP-27 ACT woes...

[John Garza (3665)]

I'm working on an HP-27.

I've read the old articles on Woodstock ACT chip problems. There are obvious differences in pinouts/etc. so you have to take those old articles with a grain of salt. But the theory seems sound - for some reason, internal electrical noise renders the machine useless. Has anyone found the root cause of this? Is this some sort of internal chip degradation, or maybe external capacitors, micro-cracks, etc ? And is there an accepted 'standard' fix? The old articles mention adding resistors and capacitors to thwart some of the noise - but I think that may have limited effect. If the chip continues to degrade, it may reach a point where filtering won't work.

This particular machine appears to have 2 different clock signals, both with noise. On a scope, the noise looks like 3 or 4 different signals with different amplitudes and phases superimposed on the main signal. The machine was completely dead - no display. But by putting a 25K resistor across one clock line to ground allowed me to use the keyboard, stack, arithmetic (I presume the ACT chip). However trig functions, stats, etc. are still a no go (I presume the ROM/RAM chip for the CORDIC tables, etc. ?)

-John