HP-29C ACT compatibility

[biogon]

Finally I decided to buy a cheap and untested HP-21 as donor.
The HP-21 didn´t work at all. But all of the ACT signals were present.
As the HP-29C PS voltages and start circuit were OK...
I removed the 1820-1523 HP-21 ACT and replaced the damaged 1820-1596 HP-29c with it using pin strips.
It was still not working. But all the 1523 ACT and anode driver signals were present.
I removed the HP-21 cathode driver and replaced the HP-29c device with the same reference.
It was still not working...
As experienced in other HP models I decided to remove the PS circuit and replace all the tantalum and electrolytic capacitors.
It was still not working. But sometimes, touching some ACT pins the display shown some numbers and the keyboard responded sometimes.
I found that the sensitiv ACT pins were nº10 (Ia/Is) and nº11 (DATA).
The Ia/Is signal in the oscilloscope had good 6V (1) level pulses, some less than 1V (0) level pulses, but someone had a variable level between 1 and 3 volts. The same in the DATA pin.
I connected a variable limited 50K resistors in both pins to ground (pin 12), and reduced the resistance value just until the 0 pulses were all less than 1 volt. The resistors were 12KOhms.
I tried and tested all the main features of the HP-29C and the result was OK!
But ERROR was displayed in the starting up, cleared with CLR, and the GTO 00 displayed also ERROR.
I decided to assemble the calculator again in order to use the original batteries.
As result no more ERROR displayed switching ON the calculator, and the GTO instructión needs a .00 to work, not like the HP-25. The repaired HP-29C is full functional now, including the cmos memory retention!

Some pictures of the reparation.

   
   
   

In these pictures you can see the Ia/Is and DATA signals before and after the modification grounding them with 12KOhms.