2015's calculator anniversary - the SR-52

[jebem]

Another small update on my SR-52 restoration saga.

1. The failed electronics issue
After swapping around the IC's from three different calculators, I managed to have one good working calculator.
Once again, thank you Etienne for your "spares" supply !

As I see it, these electronic failures can be caused by over-voltage, as a result of applying energy from the external power adapter without having good Ni-Cd batteries installed (or having corroded or broken battery terminals for that matter).
On top of that, most of the times the battery electrolytes leakage has spread all over the PCB components, and applying energy to the machine will not help at all.


Besides that, there are a few more identified potential issues:

2.- The "flat cable" Keyboard PCA - Main PCA interconnect.
This "16-wires cable" breaks easily, because it uses extremely rigid zinc alloy strips working as unifilar "wires".
This results in multiple key register failures.

I really am annoyed with myself, as I knew about this issue and still, due to careless handling, I manage to break a few of these brass strips.
The cure is simple but delicate to implement. I'm planning to just solder the broken brass legs once I have all the other issues fixed. This will be the last procedure just before installing the PCA in the front cover.


3.- Main PCA broken/corroded zinc alloy battery contacts
This is another very common issue with vintage Texas machines, mostly resulting from battery leaking corrosion.

In order to make new spring lever contacts, I found an excellent German supplier at TAS, offering high quality brass sheet in thickness as thin as 0.1mm.
This material can be cut with common scissors and it is easy to solder.


4.- The motor rubber driver wheel going gummy.
This is a endemic issue on calculators from the 70's from any brand.

As a mechanical component failure is not my area of expertise, I have been learning from good people's work published here and there concerning possible rubber wheel replacement solutions.

Like Etienne's 8.2mm diameter wheel, I find that a 8mm outer diameter should be good enough to drag the magnetic card at a even speed.

To test it, I ordered some silicone hose tubing from TAS.
The inner diameter is 4mm and the outer diameter is 8mm. Not ideal, but very close to the required specs.

I have cut a small slice of this silicone tubing and inserted it into the motor shaft.

Because this motor shaft doesn't have an adjustment system to set the pressure against the pinch roller, I used some extra foam inserted between the motor body and its case, to reinforce the pressure from the original Texas foam.

Using an external power supply, have applied 3Volt to the motor and inserted a previously used card.
The card was dragged at uniform speed, at least from what I can see on my oscilloscope connected to the magnetic head coils.
(I have these old used cards that have some data written by the previous owners, probably more than 30 years ago).
The existing written data generates a stable signal with an estimate of about 6mVpp amplitude (2.1mVrms). I can't be more precise here as my old vintage oscilloscope can only resolve up to 10mV per division.


More to come one of these weeks (I hope).

Right now I'm quite busy on one of our datacenters in Spain with so many virtual appliances to create and validate. Customers comes first. Calculators are a hobby and shall wait.
But of course the goodies found in the Cataluña restaurants like Jamón and vino tinto are really comforting me for having to work so hard.