A few days ago I got a SR-10 from a friend at my local makerspace. This particular model is a SR-10 Version 2 hardware revision D. I'm not sure where the date code is but this model was first released in 1973.

It hadn't been used in 40+ years and had very exploded batteries. I made the very difficult choice to void my 50 year old expired warranty and fix it up. Thankfully the explosion was almost completely contained in the battery compartment so the cleanup wasn't too difficult. When I put in new batteries it turned on and all the buttons work and feel great! The calculator comes with built-in power saving mode which looks so cool when it activates! https://i.imgur.com/OtfsXkX.mp4


My 50 year old warranty

It's not perfect though. The display has some issues I need help diagnosing.

• The far right digit (used to represent the first digit of scientific notation) only displays a single dot at the top instead of a number (see left pic below).
  
• The second from left digit always mimics the number shown on the fourth digit from the right (see right pic below). If I fill up the screen with numbers, the second from left digit still displays the correct number, it just overlays the correct number with the number it's mimicking.

Thankfully DataMath.org has documentation on not only the display, but the four display drivers.
'Calculator on Chip': http://www.datamath.org/Chips/TMS0120.htm
Display: http://www.datamath.org/Displays/DIS115F.htm
Top 2 display drivers: http://www.datamath.org/Chips/SN75494.htm
Bottom 2 display drivers: http://www.datamath.org/Chips/SN75493.htm
DataMath states: "The two SN75493 Segment Drivers for four segments, each and the two SN75494 Digit Drivers for six digits..."

Datamath insides vs my calc's insides:

(click for full size)

So if I understand this right:

• Each of the SN75494 drivers handles 6 of the 12 digits on the display. It handles storing which number goes in which position?
  
• Each of the SN75493 handles 4 of the 8 segments on each digit.

Is my understanding correct? What do you guys think could be the issue?

• For the first issue I suspect the whole digit is broken? Since none of the other digits are like this, I don't think it's a driver issue. Is it safe to apply power to the individual segments myself to test them?
  
• For the second issue, my thought is either one of the SN75494 drivers is broken, or one of the components between the driver and the display is broken. I'm not sure what component could break that could cause this issue though.

I would really appreciate any guidance!

I don't have much experience with these things but it looks like you can still get those IC's on ebay?

I doubt caps since there's only a few of them it's not a widespread problem across the display, still never hurts to replace. Most of the parts look pretty straightforward to replace which is good.

It could be display related though and could be anything from corrosion or a deteriorated solder joint?

Perhaps go over some of the solder work again to see if it improves anything. Also with cleaning were you using isopropyl alcohol? Best to clean the entire board.

Good to know I can get those IC if needed!

Unfortunately TI made this very difficult to disassemble. See the many strips of metal connecting the motherboard to the display board? There's another one of those connecting the motherboard to the keypad. I'll have to desolder a lot of joints if I want to remove the motherboard and replace components or clean it throughly.

I was using ~91% isopropyl alcohol but I didn't really touch the motherboard. I was focused on the battery compartment.

After some probing the only bit of corrosion that might have broken a compent is in the top right. I'm not sure if it's a diode or resistor but I can't get any continuity across it no matter which way I probe. It's transparent and which I haven't seen a clear resistor before so I don't know what it is? https://imgur.com/a/RdKI9op

That component does appear to be a diode. It's a bit hard to tell from the picture: what colors are the three bands around it?

I was hoping those IC part numbers meant there were modern 74HC or 74LS equivalents you could substitute, but alas not. I find Datamath's documentation to be quite poor, but luckily the datasheets are out there. They act as the row and column drivers of the LED displays, where the two hex digit drivers can therefore drive up to 12 digits, and the two segment drivers can drive up to 8 segments (7 digit segments + the decimal point). Like any LED array, you set the digit you want to control using the digit driver (SN75494), set which segments should light up with the segment driver (SN75493), and then move on. It's common to repeatedly sequentially scan over the "columns" (the digits, here), while setting the relevant "rows" (segments) as you go. Persistence of vision takes care of the rest.

Therefore, the "ghosting" behavior you're seeing suggests aliasing of the input to (or output from) one of the digit drivers. I agree with your suspicion that the driver itself is unlikely to have gone bad - it's a pretty simple Darlington arrangement for each pin to turn a little current into more current. You should be safe to drive those LED pins individually, but the symptoms suggest that it will be more illuminating to figure out if there's a short between some of the digit driver input pins (which is what it sounds like to me) - try an all-to-all test with a multimeter.

For the digit that only shows a single dot, that sounds like an open circuit either to a digit driver input or from a digit driver output to the LED display. If you can figure out which digit driver input/output matches each digit, you should be able to trace continuity there with your meter.

Thank you for your help!

I went through and documented which pins on the display went to which driver. I started running out of unique colors so it's a little messy. However, it seems like all the numbers are handled by the lower SN75493 drivers while all the decimal points are handled by the upper SN75494 drivers. The only pin connected to the TMS0120 is pin 1 which handles the mantissa sign and error notice.


(click for full size)

A question I have with this is: shouldn't there be a ground connection on the display? I probed all the pins but nothing connects directly to ground.

After probing I put the calculator back together and for some reason the ghosting issue just disappeared?? I have no idea what I did to fix it but I'm not complaining.


I haven't applied voltage to the broken digit (far right digit in the image above) yet but after probing, the digit appears to be getting the same voltages as the digit right next. I fear the LEDs in that digit might be broken. Am I overlooking something?

No. It's an LED array, so the cathodes are connected to one set of drivers, and the anodes are connected to another set of drivers.