This is a ti89 that was donated from a friend which didn't work due to corrosion... from both battery sets ( AAA and backup battery...

There was a lot of corrosion near the battery terminals and a lot of cleaning brought the calculator to life... partially because the backup battery circuit did not work, so unfortunally only using AAA. Now it looks like this:

- Images:

Recently the calculator was "eating" more batteries than usual and so it was openend again for repairs... Both capacitors C11 and C18 read bad value capacitance (less than 1uF), none related to the marking and C18 has a resistance of 20KOhm and dropping.


- Can C11 and C18 be replaced with tantalum capacitors?
- What does it mean its marking , eg C18 100 25n corresponds to 100uF 25v?
Alright, I haven't messed with tantalum caps too much, mostly since they're surface-mounted parts and I don't have a sophisticated-enough workshop for cleanly soldering them. C11 and C18 appear to be tantalums (although I couldn't see C11, where is it in your pictures? I've always been bad a seek-'n-find puzzles Razz). The 100 on C18 would mean it can take 100 volts, if I recall, and the 25n means 25 nano-farads (10^-9). Normally I'd just stick a wire to replace the caps if they're that small, since that would basically have the same effect; but since this is a tiny computer I assume the voltage would be important so don't listen to my bad science. You could replace the tantalums with ceramics with the same values, but I'm pretty sure you won't need capacitors capable of taking 100 volts, you would have the same effects with 25 volt capacitors. Good job fixing up the calc, by the way. Always recycle everything, so that it may live an unnaturally long life and serve you well!
Thanks for the reply.

The capacitors refered can be seen on the third image.

Location of the components :

- C11 is on the upper corner to the right ( above the flex cable labeled U2)
- C18 is on the downer corner to the right (Above a bunch of 5 pin IC's )

However if are tantalum capacitors , a quick search on mouser electronics reveal values above the 25nF value and not much for 100V parts... not cheap either..
Inline images for easy reference:

C11 near the LCD column driver, and C18 down by some discrete semiconductor parts (diodes?). C11 appears to be marked "10 25n" to me.

C18 looks to me like a simple ceramic decoupling cap- 0.1 uF or 1 uF at maybe 10V ought to be fine there.

Markings on tantalum caps (which C11 does appear to be) vary somewhat, so I did a quick survey of case markings as done by various manufacturers.

AVX's C or N case has the correct text orientation, but I'm guessing the package in question is 1206 which would be an A case. These markings are pretty obvious and don't seem to apply.

Vishay simply mark the capacitance in uF and working voltage, with additional letters for feature codes. Text orientation still not correct.

Rohm use an alphabetic code.

Panasonic use alphabetic voltage codes and resistor-like capacitance codes.

And abracon's data sheet doesn't say anything about the package markings.

So my guess given the circuit this is in (it looks like bulk capacitance for the LCD driver) and the markings is that C11 is 10 uF, 25V. This is a reasonable amount of capacitance and sensibly-specced working voltage.

Looking at how common given capacitance values are in 1206 tantalum capacitors on digikey, 10 uF is a very common value:

Also keeping in mind that on an 89 it would easily be using parts that are 10 years behind the state of the art now, I don't think it would be any higher than 10 uF.
Well then, I may not be needed anymore (amazing job Tari, I'd only just begun understanding these markings, thanks for making this clear), but to answer Malagas' question on where to buy parts, mouser does not have too many caps in the nanofarad range that are cheap (or even moderately priced); Tari mentioned that you could use .1 uf, which I searched myself on mouser with some better results. I agree that your caps should be within the 10-volt tolerance range, a greater value won't have any effect. An example would be a little led-flasher project I made, which had bad capacitors with 10v tolerance, which I swapped out for ones with 100v tolerance. The circuit still functions the same, it can just survive harsher conditions (although with a 9-volt battery, I shouldn't need to worry).

Side question: you mentioned that the capacitors had bad values, I assume you checked with a voltmeter, but how did you know those values were off? Do you have a TI-89 schematic that you're checking? I've googled for one but have yet to find a full board schematic; did you just look at the capacitor values? Hope I helped to clear something up, although Tari has done a better job than me!
Thank you a lot, thats a very usefull information, unfortunally my scavanged parts are rated only for 10V and i rather not risk those. I'll place an order.

What i'll do is to place a capacitor at least 2x times rated the voltage of the AAA batteries 1.5V alkaline.. 6V so 25V falls on this criteria.

Tari thats not C18 it is C10 ...

Let me enhance :

- C10:

- C18:
Okay, so what I thought was C18 is in fact C10. I was correct about what C11 is.

The actual C18 given the same thoughts looks like it's 100 uF, 10V. Given the placement I think it's bulk capacitance on the battery inputs: the collection of D* parts might be related to battery selection (using the backup battery for memory retention when main batteries are absent) and/or trickle charging the backup battery.
Thank you again.

looks like C18 positive pad follows to IC's related to backup battery. very good

Later will remove faulty capacitors, re-check them, check for any possible shorts on the C18 and C10 pads and put maybe some existent THT just to do a sanity check before ordering the SMD parts.
Its been a while since i've posted something related to ti89 and got time to make some repairs, lacking the smd capacitors and bodged some THT ( Though-Hole Technology) instead :

Before soldering the capacitor a continuity test was done on the pads left and no short-circuits were present and then soldering in a way that capacitors would easly let the case sit above them:

Close the case, insert some almost fully charged NIMH batteries ant It worked.. for a few minutes, then presented a Address Error... A combo key was applied to restore its operation but then power went off a few seconds later and never got on again... Maybe it gave up, but at least we got something:

Maybe it gave up???
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