06-08-2017, 08:31 AM
HP-21/25 ACTs can repair a defective HP-29C
HP used the so called ACT chip (Arithmetic Control Timing) in all Woodstock calculators HP21/22/25/25C/27/29C and later also in the HP-19C/67/91/92/95C/97. Finally they developed a new Chip for the next generation, the "Spice" calculators HP-31/32/33/34C/37/38, which made the ACT chip obsolete. From the first HP-21 to the last HP-97 this chip evolved and got some internal enhancements in several steps, whereas the details are not known. Basically the later ACTs needed to communicate with the PIK I/O chip and had a few more instructions for printer and magnetic card reader handling.
The HP-29C used the later 1820-1596 or 1820-1741 ACTs, whereas the HP-21 and HP-25 used the earlier 1820-1396 1820-1523.
Although I made a couple of repairs of "Woodstock" calculators by replacing their defective ACT chip with the new ACT chip, I never tried to run an HP-29C calculator with an original HP-25 or HP-21 ACT. Yesterday I made this experiment and put an HP-21 and HP-25 ACT into my HP-29C. I tried nearly every function and tested progamming and the "Continuous Memory". It works with both ACTs without any issues. I assume, that the later enhanced ACTs were used in all models by HP, but only the high end calculators with printer and card reader used the enhanced features. The HP-29C however did not make use of them and therefore the earlier ACTs can be taken as an replacement.
I hope you will not transplant an ACT from a working HP-21 into an HP-29C and throw away the HP-21, which would be a shame. But if for some reason you have an HP-21/22/25 ACT as spare, then you could repair an HP-29C with it.
Here a view to my little box with (mostly defective) "Woodstock" calculator chips, the remains of some "Woodstock" repairs.
Bernhard
HP used the so called ACT chip (Arithmetic Control Timing) in all Woodstock calculators HP21/22/25/25C/27/29C and later also in the HP-19C/67/91/92/95C/97. Finally they developed a new Chip for the next generation, the "Spice" calculators HP-31/32/33/34C/37/38, which made the ACT chip obsolete. From the first HP-21 to the last HP-97 this chip evolved and got some internal enhancements in several steps, whereas the details are not known. Basically the later ACTs needed to communicate with the PIK I/O chip and had a few more instructions for printer and magnetic card reader handling.
The HP-29C used the later 1820-1596 or 1820-1741 ACTs, whereas the HP-21 and HP-25 used the earlier 1820-1396 1820-1523.
Although I made a couple of repairs of "Woodstock" calculators by replacing their defective ACT chip with the new ACT chip, I never tried to run an HP-29C calculator with an original HP-25 or HP-21 ACT. Yesterday I made this experiment and put an HP-21 and HP-25 ACT into my HP-29C. I tried nearly every function and tested progamming and the "Continuous Memory". It works with both ACTs without any issues. I assume, that the later enhanced ACTs were used in all models by HP, but only the high end calculators with printer and card reader used the enhanced features. The HP-29C however did not make use of them and therefore the earlier ACTs can be taken as an replacement.
I hope you will not transplant an ACT from a working HP-21 into an HP-29C and throw away the HP-21, which would be a shame. But if for some reason you have an HP-21/22/25 ACT as spare, then you could repair an HP-29C with it.
Here a view to my little box with (mostly defective) "Woodstock" calculator chips, the remains of some "Woodstock" repairs.
Bernhard