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2015's calculator anniversary - the SR-52
01-30-2016, 11:26 AM
Post: #13
RE: 2015's calculator anniversary - the SR-52
Saturday morning, 6h00 in the morning.

Time to reinsert the IC's to their original positions.
While I'm at it, have re-soldered most of the PCB pad joints. Some had marks of battery electrolytic corrosion.

Then have checked all resistors, capacitors, diodes, transistors and coils.
All were tested good, with exception of the battery charging limiter resistor, and that doesn't contribute to the current calculator fault.

This limiting resistor has signals of past overheating and its value is way out of the tolerance. It is marked Brown/Orange/Black /Gold (13Ohm 5%) at 1/2W, and the current value is 17.2Ohm.
Despite its value is not that critical, it must be stable. So I replaced it with a 15Ohm that I had at hand.

[Image: texas_sr-52_022.jpg]


Details of the Card Reader photocoupler sensor.
Contrary to the so many bad critics found in the forums on this card reader system (which I respect) I like its simplicity and easy of access.
I'm not saying it will be easy to replace the motor roller wheel, though.
That is the weak part of this design imho. The roller pressure is not adjustable (as we find on the HP card readers) and this alone creates a huge issue when restoring this mechanism.

So the photocoupler is a classic diode / photo-transistor combo. Both are in good condition.
The photo-transistor presents about 56KOhm under fluorescent light and about 190KOhm in the dark (measured in circuit).

[Image: texas_sr-52_023.jpg] [Image: texas_sr-52_024.jpg]


14-digits 7-segment LED display details.
Printed labels: "UNIV SR" "EL SALVADOR 7551" "234 G"
The date code suggested it was made in 1975 week 51

[Image: texas_sr-52_025.jpg] [Image: texas_sr-52_026.jpg]

Jose Mesquita
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RE: 2015's calculator anniversary - the SR-52 - jebem - 01-30-2016 11:26 AM



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