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Author Topic: Calculator Ribbon Cable Repair  (Read 2694 times)

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Offline CVSoft

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Calculator Ribbon Cable Repair
« on: November 16, 2015, 05:25:00 am »
Introduction
A while back I acquired a TI-81 showing ribbon cable failure between the LCD board and the mainboard. I thought I had fixed it, but recently it started failing again. After popping it open, I found that two more connections on the cable have failed and needed replacement. I soldered some 30 gauge wrapping wire across the ribbon cable in the failed locations, and functionality is restored. I'll explain how I did this in this topic.
   
The leftmost picture shows the broken test pattern;  the center picture shows the post-repair ribbon cable;  the rightmost picture shows the correct test pattern.

Symptoms
The specific symptoms vary from calculator to calculator. On all of them, contrast can be erratic, or the calculator may not turn on. TI-81 in light blue housing, TI-85, and TI-86 manifest cable failure in garbled and/or misplaced text. TI-81 in dark blue housing, TI-82, and TI-83 series will also show garbled and/or misplaced text, but can also show extra or missing pixels in characters, have an entirely black screen, or enter LCD test mode (a blue or purple line on the screen). Screenshots will also show the erroneous display. The dark blue TI-81 also frequently shows missing rows on the display, not to be mistaken with failure of the LCD <--> display driver cable; pixels in the affected region should be connected but not [properly] addressed, appearing slightly darker than the LCD's border areas.

Failure in the light blue TI-81, TI-85, and TI-86 is uncommon for some reason. In cases where the calculator does not turn on, the ribbon cable should be tested.

Repair Preparation
On a clean and generally dust-free desk, disassemble the calculator, removing the PCBs from the housing. With a multimeter, check the resistance across the ribbon cable at the PCB pads or the row of test points above the LCD board connection. Resistances above 20 ohms are potentially failing, and anything over 35 ohms should be repaired. Note that on older port-mapped models (TI-81 dark blue, TI-82), the fourth pin from the left is not connected.

Repair
When doing a full repair of a ribbon cable (recommended if several connections are failing), peel the clear tape on the front of the PCB off both the LCD board and mainboard, then flip the calculator back to the side with ICs and peel the ribbon cable off. When doing a partial repair, use a razor blade to cut notches into the ribbon cable on both sides of the high-resistance connections, then peel/push back the plastic film of the ribbon cable to expose the pad on the PCB. Be careful not to take out any innocent bystanders. Repeat for the other side of the ribbon cable, then check the resistances of the low-resistance cable connections again now that the high-resistance connections' pads are exposed. Repeat as necessary.

Now that the affected cable connections' pads are exposed, apply a little flux to them and tin them with a soldering iron. Take about 3cm of 28 gauge or smaller wire (the length should be a bit longer than the ribbon cable, strip and tin both ends, and put a slight bend into it so it lies flat against both PCBs. Solder the wire to the corresponding pads, being sure to solder to the correct pad and not create any solder bridges. Repeat until all necessary connections have been made.

Post-Repair
Clean up the area you soldered; loose solder can bridge connections that shouldn't be bridged, and leftover flux can corrode contacts and traces. Check the resistances with a multimeter again to make sure all connections are solid and any existing ones have not been damaged. Wipe any debris off of the front of the LCD and inside of the LCD cover, if present (this region scratches VERY easily!), and reassemble the calculator. Be careful to not bend the wires too much, as they may break. Run self-test to make sure the calculator is functioning, and the calculator has been successfully repaired!


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Offline Streetwalrus

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Re: Calculator Ribbon Cable Repair
« Reply #1 on: November 16, 2015, 07:29:44 pm »
Nice write up, it's good to see retro calcs being fixed. :)
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Offline alexgt

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Re: Calculator Ribbon Cable Repair
« Reply #2 on: November 16, 2015, 09:30:07 pm »
Good tutorial, how it helps some people ;)
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Offline DJ Omnimaga

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Re: Calculator Ribbon Cable Repair
« Reply #3 on: November 16, 2015, 10:39:08 pm »
Nice tutorial CVSoft. I also learned new stuff it seems. I was sure that misplaced text was due to corrupted ROM, not failing ribbon cable. Perhaps that could be why my TI-80 has misplaced text?

I admit that the thought of doing stuff with my ribbon cables scares me, though. Can't we permanently damage the ribbon connection by accidentally cutting the ribbon cable with a blade or un-stickying it from the mobo?
  • Calculators owned: TI-73, TI-80 (broken), TI-81, TI-82, TI-83, TI-83+ (broken), TI-83+ (broken), TI-83+SE (broken), TI-84+, TI-84+CSE, TI-84+CE, TI-85, TI-86, TI-89T, TI-92, TI-Nspire, TI-Nspire CX, HP 39gII, HP Prime, Casio fx-7000G, fx-7400G+, fx-7700GE, fx-9750G+, fx-9750GII, fx-9860G, cfx-9850G, FX-1.0+, fx-CG10, fx-CP400
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Offline Caleb Hansberry

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Re: Calculator Ribbon Cable Repair
« Reply #4 on: November 16, 2015, 10:48:12 pm »
I think you missed the easiest was to repair the wires, with an old IDE cable. It was pretty easy that way!
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Offline CVSoft

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Re: Calculator Ribbon Cable Repair
« Reply #5 on: November 17, 2015, 01:49:48 am »
Nice tutorial CVSoft. I also learned new stuff it seems. I was sure that misplaced text was due to corrupted ROM, not failing ribbon cable. Perhaps that could be why my TI-80 has misplaced text?
Yes and no. The TI-80 uses a similar setup for its LCD driver ribbon cable, but it has a greater number of pins (about 30?) spaced tighter together. It's possible, but it won't be fun.

I admit that the thought of doing stuff with my ribbon cables scares me, though. Can't we permanently damage the ribbon connection by accidentally cutting the ribbon cable with a blade or un-stickying it from the mobo?
Just be careful when working around it if doing a partial repair. If doing a full repair, you're removing the ribbon cable altogether, so there isn't any need to preserve it.
  • Calculators owned: TI-73, TI-81, TI-81, TI-81, TI-81, TI-81, TI-81, TI-82, TI-82, TI-82, TI-82, TI-82, TI-82, TI-82, TI-82, TI-82, TI-82, TI-82, TI-83, TI-83, TI-83 Plus, TI-84 Plus, TI-85, TI-86, TI-89 Titanium

Offline DJ Omnimaga

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Re: Calculator Ribbon Cable Repair
« Reply #6 on: December 02, 2015, 02:31:38 am »
Ah I see then. I was always in the impression in the past that in most cases, once the ribbon cable was either cut or unstickied, then it was game over.
  • Calculators owned: TI-73, TI-80 (broken), TI-81, TI-82, TI-83, TI-83+ (broken), TI-83+ (broken), TI-83+SE (broken), TI-84+, TI-84+CSE, TI-84+CE, TI-85, TI-86, TI-89T, TI-92, TI-Nspire, TI-Nspire CX, HP 39gII, HP Prime, Casio fx-7000G, fx-7400G+, fx-7700GE, fx-9750G+, fx-9750GII, fx-9860G, cfx-9850G, FX-1.0+, fx-CG10, fx-CP400
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