2015's calculator anniversary - the SR-52

[jebem]

Small update.
OK. Time for a deeper dismantling here.

Keyboard removal.
Found two corroded wires of the "flat cable" assembly, and one third broke when removing it. Not sure if it was broken already, but my guess is that it was about to break.

These corroded solder joints are the most difficult to desolder, as the iron heat is not easily transferred to the joint to melt it in the required in short time. The longer the iron stays on the joint, the bigger the danger of damaging the copper traces and components.



I did check the power supply lines for noise. Zero instability or noise.
Even so, I removed the caps for a off circuit checking.
These vintage Siemens electrolytic capacitors are of the best quality at the time.
All of them are fine. And all made between 7602 and 7604 (1976 week 02 and 04).

So this is curious: An American brand, USA made machine, using Swiss precision motors and German Siemens capacitors.



And then it was the turn for the IC's extraction.
The task is easy although laborious and delicate.
This time I had to extract the chips without destroying them and keep the PCB copper pads in good shape as well, for the next chip insertion, only because I do not have spares and I need to reuse the chips.
Otherwise I would just cut out the chips and desolder the legs remains from the PCB.

So each IC took me about 5 minutes to extract. As I said, this is a delicate job, considering the vintage state of the PCB and its, let us say, average quality.



The two piggyback PRAM chips looks a little suspicious.
One leg is missing there. Or maybe not.
That's why I need the service guide (or another SR-52 to compare with).



The bigger DIP28 SCOM and ARITH chips are easier to extract.
I used a very old dog trick from the 70's, when we had to extract the chips to be tested off circuit: After desoldering the pad, used a small screwdriver to carefully rock the chip legs until they get loose in the pad hole.
In this way the chip comes out easily.




I decided to extract the card reader controller as well.



Partial PCA dismantling general overview.
I left the power supply oscillator and the 4 NAND chips in the circuit, as they don't seems to be related to this calculator problem.




Next steps:

I found a couple of solder joints with marks of battery corrosion at the keyboard flat cable and on the ARITH chip, so the new soldering will fix it.

I'm planing to install DIP sockets and see if the extracted chips can work in this way.
The issue here is that the chips legs were cut off at the manufacturing plant and I still have to check if they can be used in a socket.